ext: Move to pkg/sentry/fsimpl.

fsimpl is the keeper of all filesystem implementations in VFS2.

PiperOrigin-RevId: 262617869

48 files changed, 6287 insertions, 0 deletions

diff --git a/pkg/sentry/fsimpl/ext/BUILD b/pkg/sentry/fsimpl/ext/BUILD
new file mode 100644
index 000000000..a41101339
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/BUILD
@@ -0,0 +1,86 @@
+package(licenses = ["notice"])
+
+load("//tools/go_stateify:defs.bzl", "go_library", "go_test")
+load("//tools/go_generics:defs.bzl", "go_template_instance")
+
+go_template_instance(
+    name = "dirent_list",
+    out = "dirent_list.go",
+    package = "ext",
+    prefix = "dirent",
+    template = "//pkg/ilist:generic_list",
+    types = {
+        "Element": "*dirent",
+        "Linker": "*dirent",
+    },
+)
+
+go_library(
+    name = "ext",
+    srcs = [
+        "block_map_file.go",
+        "dentry.go",
+        "directory.go",
+        "dirent_list.go",
+        "ext.go",
+        "extent_file.go",
+        "file_description.go",
+        "filesystem.go",
+        "inode.go",
+        "regular_file.go",
+        "symlink.go",
+        "utils.go",
+    ],
+    importpath = "gvisor.dev/gvisor/pkg/sentry/fsimpl/ext",
+    visibility = ["//pkg/sentry:internal"],
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/binary",
+        "//pkg/fd",
+        "//pkg/log",
+        "//pkg/sentry/arch",
+        "//pkg/sentry/context",
+        "//pkg/sentry/fs",
+        "//pkg/sentry/fsimpl/ext/disklayout",
+        "//pkg/sentry/kernel/auth",
+        "//pkg/sentry/memmap",
+        "//pkg/sentry/safemem",
+        "//pkg/sentry/syscalls/linux",
+        "//pkg/sentry/usermem",
+        "//pkg/sentry/vfs",
+        "//pkg/syserror",
+        "//pkg/waiter",
+    ],
+)
+
+go_test(
+    name = "ext_test",
+    size = "small",
+    srcs = [
+        "block_map_test.go",
+        "ext_test.go",
+        "extent_test.go",
+    ],
+    data = [
+        "//pkg/sentry/fsimpl/ext:assets/bigfile.txt",
+        "//pkg/sentry/fsimpl/ext:assets/file.txt",
+        "//pkg/sentry/fsimpl/ext:assets/tiny.ext2",
+        "//pkg/sentry/fsimpl/ext:assets/tiny.ext3",
+        "//pkg/sentry/fsimpl/ext:assets/tiny.ext4",
+    ],
+    embed = [":ext"],
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/binary",
+        "//pkg/sentry/context",
+        "//pkg/sentry/context/contexttest",
+        "//pkg/sentry/fsimpl/ext/disklayout",
+        "//pkg/sentry/kernel/auth",
+        "//pkg/sentry/usermem",
+        "//pkg/sentry/vfs",
+        "//pkg/syserror",
+        "//runsc/test/testutil",
+        "@com_github_google_go-cmp//cmp:go_default_library",
+        "@com_github_google_go-cmp//cmp/cmpopts:go_default_library",
+    ],
+)
diff --git a/pkg/sentry/fsimpl/ext/README.md b/pkg/sentry/fsimpl/ext/README.md
new file mode 100644
index 000000000..af00cfda8
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/README.md
@@ -0,0 +1,117 @@
+## EXT(2/3/4) File System
+
+This is a filesystem driver which supports ext2, ext3 and ext4 filesystems.
+Linux has specialized drivers for each variant but none which supports all. This
+library takes advantage of ext's backward compatibility and understands the
+internal organization of on-disk structures to support all variants.
+
+This driver implementation diverges from the Linux implementations in being more
+forgiving about versioning. For instance, if a filesystem contains both extent
+based inodes and classical block map based inodes, this driver will not complain
+and interpret them both correctly. While in Linux this would be an issue. This
+blurs the line between the three ext fs variants.
+
+Ext2 is considered deprecated as of Red Hat Enterprise Linux 7, and ext3 has
+been superseded by ext4 by large performance gains. Thus it is recommended to
+upgrade older filesystem images to ext4 using e2fsprogs for better performance.
+
+### Read Only
+
+This driver currently only allows read only operations. A lot of the design
+decisions are based on this feature. There are plans to implement write (the
+process for which is documented in the future work section).
+
+### Performance
+
+One of the biggest wins about this driver is that it directly talks to the
+underlying block device (or whatever persistent storage is being used), instead
+of making expensive RPCs to a gofer.
+
+Another advantage is that ext fs supports fast concurrent reads. Currently the
+device is represented using a `io.ReaderAt` which allows for concurrent reads.
+All reads are directly passed to the device driver which intelligently serves
+the read requests in the optimal order. There is no congestion due to locking
+while reading in the filesystem level.
+
+Reads are optimized further in the way file data is transferred over to user
+memory. Ext fs directly copies over file data from disk into user memory with no
+additional allocations on the way. We can only get faster by preloading file
+data into memory (see future work section).
+
+The internal structures used to represent files, inodes and file descriptors use
+a lot of inheritance. With the level of indirection that an interface adds with
+an internal pointer, it can quickly fragment a structure across memory. As this
+runs along side a full blown kernel (which is memory intensive), having a
+fragmented struct might hurt performance. Hence these internal structures,
+though interfaced, are tightly packed in memory using the same inheritance
+pattern that pkg/sentry/vfs uses. The pkg/sentry/fsimpl/ext/disklayout package
+makes an execption to this pattern for reasons documented in the package.
+
+### Security
+
+This driver also intends to help sandbox the container better by reducing the
+surface of the host kernel that the application touches. It prevents the
+application from exploiting vulnerabilities in the host filesystem driver. All
+`io.ReaderAt.ReadAt()` calls are translated to `pread(2)` which are directly
+passed to the device driver in the kernel. Hence this reduces the surface for
+attack.
+
+The application can not affect any host filesystems other than the one passed
+via block device by the user.
+
+### Future Work
+
+#### Write
+
+To support write operations we would need to modify the block device underneath.
+Currently, the driver does not modify the device at all, not even for updating
+the access times for reads. Modifying the filesystem incorrectly can corrupt it
+and render it unreadable for other correct ext(x) drivers. Hence caution must be
+maintained while modifying metadata structures.
+
+Ext4 specifically is built for performance and has added a lot of complexity as
+to how metadata structures are modified. For instance, files that are organized
+via an extent tree which must be balanced and file data blocks must be placed in
+the same extent as much as possible to increase locality. Such properties must
+be maintained while modifying the tree.
+
+Ext filesystems boast a lot about locality, which plays a big role in them being
+performant. The block allocation algorithm in Linux does a good job in keeping
+related data together. This behavior must be maintained as much as possible,
+else we might end up degrading the filesystem performance over time.
+
+Ext4 also supports a wide variety of features which are specialized for varying
+use cases. Implementing all of them can get difficult very quickly.
+
+Ext(x) checksums all its metadata structures to check for corruption, so
+modification of any metadata struct must correspond with re-checksumming the
+struct. Linux filesystem drivers also order on-disk updates intelligently to not
+corrupt the filesystem and also remain performant. The in-memory metadata
+structures must be kept in sync with what is on disk.
+
+There is also replication of some important structures across the filesystem.
+All replicas must be updated when their original copy is updated. There is also
+provisioning for snapshotting which must be kept in mind, although it should not
+affect this implementation unless we allow users to create filesystem snapshots.
+
+Ext4 also introduced journaling (jbd2). The journal must be updated
+appropriately.
+
+#### Performance
+
+To improve performance we should implement a buffer cache, and optionally, read
+ahead for small files. While doing so we must also keep in mind the memory usage
+and have a reasonable cap on how much file data we want to hold in memory.
+
+#### Features
+
+Our current implementation will work with most ext4 filesystems for readonly
+purposed. However, the following features are not supported yet:
+
+-   Journal
+-   Snapshotting
+-   Extended Attributes
+-   Hash Tree Directories
+-   Meta Block Groups
+-   Multiple Mount Protection
+-   Bigalloc
diff --git a/pkg/sentry/fsimpl/ext/assets/README.md b/pkg/sentry/fsimpl/ext/assets/README.md
new file mode 100644
index 000000000..6f1e81b3a
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/assets/README.md
@@ -0,0 +1,36 @@
+### Tiny Ext(2/3/4) Images
+
+The images are of size 64Kb which supports 64 1k blocks and 16 inodes. This is
+the smallest size mkfs.ext(2/3/4) works with.
+
+These images were generated using the following commands.
+
+```bash
+fallocate -l 64K tiny.ext$VERSION
+mkfs.ext$VERSION -j tiny.ext$VERSION
+```
+
+where `VERSION` is `2`, `3` or `4`.
+
+You can mount it using:
+
+```bash
+sudo mount -o loop tiny.ext$VERSION $MOUNTPOINT
+```
+
+`file.txt`, `bigfile.txt` and `symlink.txt` were added to this image by just
+mounting it and copying (while preserving links) those files to the mountpoint
+directory using:
+
+```bash
+sudo cp -P {file.txt,symlink.txt,bigfile.txt} $MOUNTPOINT
+```
+
+The files in this directory mirror the contents and organisation of the files
+stored in the image.
+
+You can umount the filesystem using:
+
+```bash
+sudo umount $MOUNTPOINT
+```
diff --git a/pkg/sentry/fsimpl/ext/assets/bigfile.txt b/pkg/sentry/fsimpl/ext/assets/bigfile.txt
new file mode 100644
index 000000000..3857cf516
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/assets/bigfile.txt
@@ -0,0 +1,41 @@
+Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus faucibus eleifend orci, ut ornare nibh faucibus eu. Cras at condimentum massa. Nullam luctus, elit non porttitor congue, sapien diam feugiat sapien, sed eleifend nulla mauris non arcu. Sed lacinia mauris magna, eu mollis libero varius sit amet. Donec mollis, quam convallis commodo posuere, dolor nisi placerat nisi, in faucibus augue mi eu lorem. In pharetra consectetur faucibus. Ut euismod ex efficitur egestas tincidunt. Maecenas condimentum ut ante in rutrum. Vivamus sed arcu tempor, faucibus turpis et, lacinia diam.
+
+Sed in lacus vel nisl interdum bibendum in sed justo. Nunc tellus risus, molestie vitae arcu sed, molestie tempus ligula. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Nunc risus neque, volutpat et ante non, ullamcorper condimentum ante. Aliquam sed metus in urna condimentum convallis. Vivamus ut libero mauris. Proin mollis posuere consequat. Vestibulum placerat mollis est et pulvinar.
+
+Donec rutrum odio ac diam pharetra, id fermentum magna cursus. Pellentesque in dapibus elit, et condimentum orci. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Suspendisse euismod dapibus est, id vestibulum mauris. Nulla facilisi. Nulla cursus gravida nisi. Phasellus vestibulum rutrum lectus, a dignissim mauris hendrerit vitae. In at elementum mauris. Integer vel efficitur velit. Nullam fringilla sapien mi, quis luctus neque efficitur ac. Aenean nec quam dapibus nunc commodo pharetra. Proin sapien mi, fermentum aliquet vulputate non, aliquet porttitor diam. Quisque lacinia, urna et finibus fermentum, nunc lacus vehicula ex, sed congue metus lectus ac quam. Aliquam erat volutpat. Suspendisse sodales, dolor ut tincidunt finibus, augue erat varius tellus, a interdum erat sem at nunc. Vestibulum cursus iaculis sapien, vitae feugiat dui auctor quis.
+
+Pellentesque nec maximus nulla, eu blandit diam. Maecenas quis arcu ornare, congue ante at, vehicula ipsum. Praesent feugiat mauris rutrum sem fermentum, nec luctus ipsum placerat. Pellentesque placerat ipsum at dignissim fringilla. Vivamus et posuere sem, eget hendrerit felis. Aenean vulputate, augue vel mollis feugiat, justo ipsum mollis dolor, eu mollis elit neque ut ipsum. Orci varius natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Fusce bibendum sem quam, vulputate laoreet mi dapibus imperdiet. Sed a purus non nibh pretium aliquet. Integer eget luctus augue, vitae tincidunt magna. Ut eros enim, egestas eu nulla et, lobortis egestas arcu. Cras id ipsum ac justo lacinia rutrum. Vivamus lectus leo, ultricies sed justo at, pellentesque feugiat magna. Ut sollicitudin neque elit, vel ornare mauris commodo id.
+
+Duis dapibus orci et sapien finibus finibus. Mauris eleifend, lacus at vestibulum maximus, quam ligula pharetra erat, sit amet dapibus neque elit vitae neque. In bibendum sollicitudin erat, eget ultricies tortor malesuada at. Sed sit amet orci turpis. Donec feugiat ligula nibh, molestie tincidunt lectus elementum id. Donec volutpat maximus nibh, in vulputate felis posuere eu. Cras tincidunt ullamcorper lacus. Phasellus porta lorem auctor, congue magna a, commodo elit.
+
+Etiam auctor mi quis elit sodales, eu pulvinar arcu condimentum. Aenean imperdiet risus et dapibus tincidunt. Nullam tincidunt dictum dui, sed commodo urna rutrum id. Ut mollis libero vel elit laoreet bibendum. Quisque arcu arcu, tincidunt at ultricies id, vulputate nec metus. In tristique posuere quam sit amet volutpat. Vivamus scelerisque et nunc at dapibus. Fusce finibus libero ut ligula pretium rhoncus. Mauris non elit in arcu finibus imperdiet. Pellentesque nec massa odio. Proin rutrum mauris non sagittis efficitur. Aliquam auctor quam at dignissim faucibus. Ut eget ligula in magna posuere ultricies vitae sit amet turpis. Duis maximus odio nulla. Donec gravida sem tristique tempus scelerisque.
+
+Interdum et malesuada fames ac ante ipsum primis in faucibus. Fusce pharetra magna vulputate aliquet tempus. Duis id hendrerit arcu. Quisque ut ex elit. Integer velit orci, venenatis ut sapien ac, placerat porttitor dui. Interdum et malesuada fames ac ante ipsum primis in faucibus. Nunc hendrerit cursus diam, hendrerit finibus ipsum scelerisque ut. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos.
+
+Nulla non euismod neque. Phasellus vel sapien eu metus pulvinar rhoncus. Suspendisse eu mollis tellus, quis vestibulum tortor. Maecenas interdum dolor sed nulla fermentum maximus. Donec imperdiet ullamcorper condimentum. Nam quis nibh ante. Praesent quis tellus ut tortor pulvinar blandit sit amet ut sapien. Vestibulum est orci, pellentesque vitae tristique sit amet, tristique non felis.
+
+Vivamus sodales pellentesque varius. Sed vel tempus ligula. Nulla tristique nisl vel dui facilisis, ac sodales augue hendrerit. Proin augue nisi, vestibulum quis augue nec, sagittis tincidunt velit. Vestibulum euismod, nulla nec sodales faucibus, urna sapien vulputate magna, id varius metus sapien ut neque. Duis in mollis urna, in scelerisque enim. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Nunc condimentum dictum turpis, et egestas neque dapibus eget. Quisque fringilla, dui eu venenatis eleifend, erat nibh lacinia urna, at lacinia lacus sapien eu dui. Duis eu erat ut mi lacinia convallis a sed ex.
+
+Fusce elit metus, tincidunt nec eleifend a, hendrerit nec ligula. Duis placerat finibus sollicitudin. In euismod porta tellus, in luctus justo bibendum bibendum. Maecenas at magna eleifend lectus tincidunt suscipit ut a ligula. Nulla tempor accumsan felis, fermentum dapibus est eleifend vitae. Mauris urna sem, fringilla at ultricies non, ultrices in arcu. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Nam vehicula nunc at laoreet imperdiet. Nunc tristique ut risus id aliquet. Integer eleifend massa orci.
+
+Vestibulum sed ante sollicitudin nisi fringilla bibendum nec vel quam. Sed pretium augue eu ligula congue pulvinar. Donec vitae magna tincidunt, pharetra lacus id, convallis nulla. Cras viverra nisl nisl, varius convallis leo vulputate nec. Morbi at consequat dui, sed aliquet metus. Sed suscipit fermentum mollis. Maecenas nec mi sodales, tincidunt purus in, tristique mauris. Orci varius natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Donec interdum mi in velit efficitur, quis ultrices ex imperdiet. Sed vestibulum, magna ut tristique pretium, mi ipsum placerat tellus, non tempor enim augue et ex. Pellentesque eget felis quis ante sodales viverra ac sed lacus. Donec suscipit tempus massa, eget laoreet massa molestie at.
+
+Aenean fringilla dui non aliquet consectetur. Fusce cursus quam nec orci hendrerit faucibus. Donec consequat suscipit enim, non volutpat lectus auctor interdum. Proin lorem purus, maximus vel orci vitae, suscipit egestas turpis. Donec risus urna, congue a sem eu, aliquet placerat odio. Morbi gravida tristique turpis, quis efficitur enim. Nunc interdum gravida ipsum vel facilisis. Nunc congue finibus sollicitudin. Quisque euismod aliquet lectus et tincidunt. Curabitur ultrices sem ut mi fringilla fermentum. Morbi pretium, nisi sit amet dapibus congue, dolor enim consectetur risus, a interdum ligula odio sed odio. Quisque facilisis, mi at suscipit gravida, nunc sapien cursus justo, ut luctus odio nulla quis leo. Integer condimentum lobortis mauris, non egestas tellus lobortis sit amet.
+
+In sollicitudin velit ac ante vehicula, vitae varius tortor mollis. In hac habitasse platea dictumst. Quisque et orci lorem. Integer malesuada fringilla luctus. Pellentesque malesuada, mi non lobortis porttitor, ante ligula vulputate ante, nec dictum risus eros sit amet sapien. Nulla aliquam lorem libero, ac varius nulla tristique eget. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Ut pellentesque mauris orci, vel consequat mi varius a. Ut sit amet elit vulputate, lacinia metus non, fermentum nisl. Pellentesque eu nisi sed quam egestas blandit. Duis sit amet lobortis dolor. Donec consectetur sem interdum, tristique elit sit amet, sodales lacus. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Fusce id aliquam augue. Sed pretium congue risus vitae lacinia. Vestibulum non vulputate risus, ut malesuada justo.
+
+Sed odio elit, consectetur ac mauris quis, consequat commodo libero. Fusce sodales velit vulputate pulvinar fermentum. Donec iaculis nec nisl eget faucibus. Mauris at dictum velit. Donec fermentum lectus eu viverra volutpat. Aliquam consequat facilisis lorem, cursus consequat dui bibendum ullamcorper. Pellentesque nulla magna, imperdiet at magna et, cursus egestas enim. Nullam semper molestie lectus sit amet semper. Duis eget tincidunt est. Integer id neque risus. Integer ultricies hendrerit vestibulum. Donec blandit blandit sagittis. Nunc consectetur vitae nisi consectetur volutpat.
+
+Nulla id lorem fermentum, efficitur magna a, hendrerit dui. Vivamus sagittis orci gravida, bibendum quam eget, molestie est. Phasellus nec enim tincidunt, volutpat sapien non, laoreet diam. Nulla posuere enim nec porttitor lobortis. Donec auctor odio ut orci eleifend, ut eleifend purus convallis. Interdum et malesuada fames ac ante ipsum primis in faucibus. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut hendrerit, purus eget viverra tincidunt, sem magna imperdiet libero, et aliquam turpis neque vitae elit. Maecenas semper varius iaculis. Cras non lorem quis quam bibendum eleifend in et libero. Curabitur at purus mauris. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus porta diam sed elit eleifend gravida.
+
+Nulla facilisi. Ut ultricies diam vel diam consectetur, vel porta augue molestie. Fusce interdum sapien et metus facilisis pellentesque. Nulla convallis sem at nunc vehicula facilisis. Nam ac rutrum purus. Nunc bibendum, dolor sit amet tempus ullamcorper, lorem leo tempor sem, id fringilla nunc augue scelerisque augue. Nullam sit amet rutrum nisl. Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia Curae; Donec sed mauris gravida eros vehicula sagittis at eget orci. Cras elementum, eros at accumsan bibendum, libero neque blandit purus, vitae vestibulum libero massa ac nibh. Integer at placerat nulla. Mauris eu eleifend orci. Aliquam consequat ligula vitae erat porta lobortis. Duis fermentum elit ac aliquet ornare.
+
+Mauris eget cursus tellus, eget sodales purus. Aliquam malesuada, augue id vulputate finibus, nisi ex bibendum nisl, sit amet laoreet quam urna a dolor. Nullam ultricies, sapien eu laoreet consequat, erat eros dignissim diam, ultrices sodales lectus mauris et leo. Morbi lacinia eu ante at tempus. Sed iaculis finibus magna malesuada efficitur. Donec faucibus erat sit amet elementum feugiat. Praesent a placerat nisi. Etiam lacinia gravida diam, et sollicitudin sapien tincidunt ut.
+
+Maecenas felis quam, tincidunt vitae venenatis scelerisque, viverra vitae odio. Phasellus enim neque, ultricies suscipit malesuada sit amet, vehicula sit amet purus. Nulla placerat sit amet dui vel tincidunt. Nam quis neque vel magna commodo egestas. Vestibulum sagittis rutrum lorem ut congue. Maecenas vel ultrices tellus. Donec efficitur, urna ac consequat iaculis, lorem felis pharetra eros, eget faucibus orci lectus sit amet arcu.
+
+Ut a tempus nisi. Nulla facilisi. Praesent vulputate maximus mi et dapibus. Sed sit amet libero ac augue hendrerit efficitur in a sapien. Mauris placerat velit sit amet tellus sollicitudin faucibus. Donec egestas a magna ac suscipit. Duis enim sapien, mollis sed egestas et, vestibulum vel leo.
+
+Proin quis dapibus dui. Donec eu tincidunt nunc. Vivamus eget purus consectetur, maximus ante vitae, tincidunt elit. Aenean mattis dolor a gravida aliquam. Praesent quis tellus id sem maximus vulputate nec sed nulla. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Curabitur metus nulla, volutpat volutpat est eu, hendrerit congue erat. Aliquam sollicitudin augue ante. Sed sollicitudin, magna eu consequat elementum, mi augue ullamcorper felis, molestie imperdiet erat metus iaculis est. Proin ac tortor nisi. Pellentesque quis nisi risus. Integer enim sapien, tincidunt quis tortor id, accumsan venenatis mi. Nulla facilisi.
+
+Cras pretium sit amet quam congue maximus. Morbi lacus libero, imperdiet commodo massa sed, scelerisque placerat libero. Cras nisl nisi, consectetur sed bibendum eu, venenatis at enim. Proin sodales justo at quam aliquam, a consectetur mi ornare. Donec porta ac est sit amet efficitur. Suspendisse vestibulum tortor id neque imperdiet, id lacinia risus vehicula. Phasellus ac eleifend purus. Mauris vel gravida ante. Aliquam vitae lobortis risus. Sed vehicula consectetur tincidunt. Nam et justo vitae purus molestie consequat. Pellentesque ipsum ex, convallis quis blandit non, gravida et urna. Donec diam ligula amet.
diff --git a/pkg/sentry/fsimpl/ext/assets/file.txt b/pkg/sentry/fsimpl/ext/assets/file.txt
new file mode 100644
index 000000000..980a0d5f1
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/assets/file.txt
@@ -0,0 +1 @@
+Hello World!
diff --git a/pkg/sentry/fsimpl/ext/assets/symlink.txt b/pkg/sentry/fsimpl/ext/assets/symlink.txt
new file mode 120000
index 000000000..4c330738c
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/assets/symlink.txt
@@ -0,0 +1 @@
+file.txt
\ No newline at end of file
diff --git a/pkg/sentry/fsimpl/ext/assets/tiny.ext2 b/pkg/sentry/fsimpl/ext/assets/tiny.ext2
new file mode 100644
index 000000000..381ade9bf
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/assets/tiny.ext2
diff --git a/pkg/sentry/fsimpl/ext/assets/tiny.ext3 b/pkg/sentry/fsimpl/ext/assets/tiny.ext3
new file mode 100644
index 000000000..0e97a324c
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/assets/tiny.ext3
diff --git a/pkg/sentry/fsimpl/ext/assets/tiny.ext4 b/pkg/sentry/fsimpl/ext/assets/tiny.ext4
new file mode 100644
index 000000000..a6859736d
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/assets/tiny.ext4
diff --git a/pkg/sentry/fsimpl/ext/benchmark/BUILD b/pkg/sentry/fsimpl/ext/benchmark/BUILD
new file mode 100644
index 000000000..9fddb4c4c
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/benchmark/BUILD
@@ -0,0 +1,16 @@
+load("//tools/go_stateify:defs.bzl", "go_test")
+
+package(licenses = ["notice"])
+
+go_test(
+    name = "benchmark_test",
+    size = "small",
+    srcs = ["benchmark_test.go"],
+    deps = [
+        "//pkg/sentry/context",
+        "//pkg/sentry/context/contexttest",
+        "//pkg/sentry/fsimpl/ext",
+        "//pkg/sentry/kernel/auth",
+        "//pkg/sentry/vfs",
+    ],
+)
diff --git a/pkg/sentry/fsimpl/ext/benchmark/benchmark_test.go b/pkg/sentry/fsimpl/ext/benchmark/benchmark_test.go
new file mode 100644
index 000000000..10a8083a0
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/benchmark/benchmark_test.go
@@ -0,0 +1,193 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// These benchmarks emulate memfs benchmarks. Ext4 images must be created
+// before this benchmark is run using the `make_deep_ext4.sh` script at
+// /tmp/image-{depth}.ext4 for all the depths tested below.
+package benchmark_test
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+	"strings"
+	"testing"
+
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/context/contexttest"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/ext"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+)
+
+var depths = []int{1, 2, 3, 8, 64, 100}
+
+const filename = "file.txt"
+
+// setUp opens imagePath as an ext Filesystem and returns all necessary
+// elements required to run tests. If error is nil, it also returns a tear
+// down function which must be called after the test is run for clean up.
+func setUp(b *testing.B, imagePath string) (context.Context, *vfs.VirtualFilesystem, *vfs.VirtualDentry, func(), error) {
+	f, err := os.Open(imagePath)
+	if err != nil {
+		return nil, nil, nil, nil, err
+	}
+
+	ctx := contexttest.Context(b)
+	creds := auth.CredentialsFromContext(ctx)
+
+	// Create VFS.
+	vfsObj := vfs.New()
+	vfsObj.MustRegisterFilesystemType("extfs", ext.FilesystemType{})
+	mntns, err := vfsObj.NewMountNamespace(ctx, creds, imagePath, "extfs", &vfs.NewFilesystemOptions{InternalData: int(f.Fd())})
+	if err != nil {
+		f.Close()
+		return nil, nil, nil, nil, err
+	}
+
+	root := mntns.Root()
+
+	tearDown := func() {
+		root.DecRef()
+
+		if err := f.Close(); err != nil {
+			b.Fatalf("tearDown failed: %v", err)
+		}
+	}
+	return ctx, vfsObj, &root, tearDown, nil
+}
+
+// mount mounts extfs at the path operation passed. Returns a tear down
+// function which must be called after the test is run for clean up.
+func mount(b *testing.B, imagePath string, vfsfs *vfs.VirtualFilesystem, pop *vfs.PathOperation) func() {
+	b.Helper()
+
+	f, err := os.Open(imagePath)
+	if err != nil {
+		b.Fatalf("could not open image at %s: %v", imagePath, err)
+	}
+
+	ctx := contexttest.Context(b)
+	creds := auth.CredentialsFromContext(ctx)
+
+	if err := vfsfs.NewMount(ctx, creds, imagePath, pop, "extfs", &vfs.NewFilesystemOptions{InternalData: int(f.Fd())}); err != nil {
+		b.Fatalf("failed to mount tmpfs submount: %v", err)
+	}
+	return func() {
+		if err := f.Close(); err != nil {
+			b.Fatalf("tearDown failed: %v", err)
+		}
+	}
+}
+
+// BenchmarkVFS2Ext4fsStat emulates BenchmarkVFS2MemfsStat.
+func BenchmarkVFS2Ext4fsStat(b *testing.B) {
+	for _, depth := range depths {
+		b.Run(fmt.Sprintf("%d", depth), func(b *testing.B) {
+			ctx, vfsfs, root, tearDown, err := setUp(b, fmt.Sprintf("/tmp/image-%d.ext4", depth))
+			if err != nil {
+				b.Fatalf("setUp failed: %v", err)
+			}
+			defer tearDown()
+
+			creds := auth.CredentialsFromContext(ctx)
+			var filePathBuilder strings.Builder
+			filePathBuilder.WriteByte('/')
+			for i := 1; i <= depth; i++ {
+				filePathBuilder.WriteString(fmt.Sprintf("%d", i))
+				filePathBuilder.WriteByte('/')
+			}
+			filePathBuilder.WriteString(filename)
+			filePath := filePathBuilder.String()
+
+			runtime.GC()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				stat, err := vfsfs.StatAt(ctx, creds, &vfs.PathOperation{
+					Root:               *root,
+					Start:              *root,
+					Pathname:           filePath,
+					FollowFinalSymlink: true,
+				}, &vfs.StatOptions{})
+				if err != nil {
+					b.Fatalf("stat(%q) failed: %v", filePath, err)
+				}
+				// Sanity check.
+				if stat.Size > 0 {
+					b.Fatalf("got wrong file size (%d)", stat.Size)
+				}
+			}
+		})
+	}
+}
+
+// BenchmarkVFS2ExtfsMountStat emulates BenchmarkVFS2MemfsMountStat.
+func BenchmarkVFS2ExtfsMountStat(b *testing.B) {
+	for _, depth := range depths {
+		b.Run(fmt.Sprintf("%d", depth), func(b *testing.B) {
+			// Create root extfs with depth 1 so we can mount extfs again at /1/.
+			ctx, vfsfs, root, tearDown, err := setUp(b, fmt.Sprintf("/tmp/image-%d.ext4", 1))
+			if err != nil {
+				b.Fatalf("setUp failed: %v", err)
+			}
+			defer tearDown()
+
+			creds := auth.CredentialsFromContext(ctx)
+			mountPointName := "/1/"
+			pop := vfs.PathOperation{
+				Root:     *root,
+				Start:    *root,
+				Pathname: mountPointName,
+			}
+
+			// Save the mount point for later use.
+			mountPoint, err := vfsfs.GetDentryAt(ctx, creds, &pop, &vfs.GetDentryOptions{})
+			if err != nil {
+				b.Fatalf("failed to walk to mount point: %v", err)
+			}
+			defer mountPoint.DecRef()
+
+			// Create extfs submount.
+			mountTearDown := mount(b, fmt.Sprintf("/tmp/image-%d.ext4", depth), vfsfs, &pop)
+			defer mountTearDown()
+
+			var filePathBuilder strings.Builder
+			filePathBuilder.WriteString(mountPointName)
+			for i := 1; i <= depth; i++ {
+				filePathBuilder.WriteString(fmt.Sprintf("%d", i))
+				filePathBuilder.WriteByte('/')
+			}
+			filePathBuilder.WriteString(filename)
+			filePath := filePathBuilder.String()
+
+			runtime.GC()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				stat, err := vfsfs.StatAt(ctx, creds, &vfs.PathOperation{
+					Root:               *root,
+					Start:              *root,
+					Pathname:           filePath,
+					FollowFinalSymlink: true,
+				}, &vfs.StatOptions{})
+				if err != nil {
+					b.Fatalf("stat(%q) failed: %v", filePath, err)
+				}
+				// Sanity check. touch(1) always creates files of size 0 (empty).
+				if stat.Size > 0 {
+					b.Fatalf("got wrong file size (%d)", stat.Size)
+				}
+			}
+		})
+	}
+}
diff --git a/pkg/sentry/fsimpl/ext/benchmark/make_deep_ext4.sh b/pkg/sentry/fsimpl/ext/benchmark/make_deep_ext4.sh
new file mode 100755
index 000000000..d0910da1f
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/benchmark/make_deep_ext4.sh
@@ -0,0 +1,72 @@
+#!/bin/bash
+
+# Copyright 2019 The gVisor Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# This script creates an ext4 image with $1 depth of directories and a file in
+# the inner most directory. The created file is at path /1/2/.../depth/file.txt.
+# The ext4 image is written to $2. The image is temporarily mounted at
+# /tmp/mountpoint. This script must be run with sudo privileges.
+
+# Usage:
+# sudo bash make_deep_ext4.sh {depth} {output path}
+
+# Check positional arguments.
+if [ "$#" -ne 2 ]; then
+    echo "Usage: sudo bash make_deep_ext4.sh {depth} {output path}"
+    exit 1
+fi
+
+# Make sure depth is a non-negative number.
+if ! [[ "$1" =~ ^[0-9]+$ ]]; then
+        echo "Depth must be a non-negative number."
+        exit 1
+fi
+
+# Create a 1 MB filesystem image at the requested output path.
+rm -f $2
+fallocate -l 1M $2
+if [ $? -ne 0 ]; then
+    echo "fallocate failed"
+    exit $?
+fi
+
+# Convert that blank into an ext4 image.
+mkfs.ext4 -j $2
+if [ $? -ne 0 ]; then
+    echo "mkfs.ext4 failed"
+    exit $?
+fi
+
+# Mount the image.
+MOUNTPOINT=/tmp/mountpoint
+mkdir -p $MOUNTPOINT
+mount -o loop $2 $MOUNTPOINT
+if [ $? -ne 0 ]; then
+    echo "mount failed"
+    exit $?
+fi
+
+# Create nested directories and the file.
+if [ "$1" -eq 0 ]; then
+   FILEPATH=$MOUNTPOINT/file.txt
+else
+   FILEPATH=$MOUNTPOINT/$(seq -s '/' 1 $1)/file.txt
+fi
+mkdir -p $(dirname $FILEPATH) || exit
+touch $FILEPATH
+
+# Clean up.
+umount $MOUNTPOINT
+rm -rf $MOUNTPOINT
diff --git a/pkg/sentry/fsimpl/ext/block_map_file.go b/pkg/sentry/fsimpl/ext/block_map_file.go
new file mode 100644
index 000000000..cea89bcd9
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/block_map_file.go
@@ -0,0 +1,200 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ext
+
+import (
+	"io"
+	"math"
+
+	"gvisor.dev/gvisor/pkg/binary"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+const (
+	// numDirectBlks is the number of direct blocks in ext block map inodes.
+	numDirectBlks = 12
+)
+
+// blockMapFile is a type of regular file which uses direct/indirect block
+// addressing to store file data. This was deprecated in ext4.
+type blockMapFile struct {
+	regFile regularFile
+
+	// directBlks are the direct blocks numbers. The physical blocks pointed by
+	// these holds file data. Contains file blocks 0 to 11.
+	directBlks [numDirectBlks]uint32
+
+	// indirectBlk is the physical block which contains (blkSize/4) direct block
+	// numbers (as uint32 integers).
+	indirectBlk uint32
+
+	// doubleIndirectBlk is the physical block which contains (blkSize/4) indirect
+	// block numbers (as uint32 integers).
+	doubleIndirectBlk uint32
+
+	// tripleIndirectBlk is the physical block which contains (blkSize/4) doubly
+	// indirect block numbers (as uint32 integers).
+	tripleIndirectBlk uint32
+
+	// coverage at (i)th index indicates the amount of file data a node at
+	// height (i) covers. Height 0 is the direct block.
+	coverage [4]uint64
+}
+
+// Compiles only if blockMapFile implements io.ReaderAt.
+var _ io.ReaderAt = (*blockMapFile)(nil)
+
+// newBlockMapFile is the blockMapFile constructor. It initializes the file to
+// physical blocks map with (at most) the first 12 (direct) blocks.
+func newBlockMapFile(regFile regularFile) (*blockMapFile, error) {
+	file := &blockMapFile{regFile: regFile}
+	file.regFile.impl = file
+
+	for i := uint(0); i < 4; i++ {
+		file.coverage[i] = getCoverage(regFile.inode.blkSize, i)
+	}
+
+	blkMap := regFile.inode.diskInode.Data()
+	binary.Unmarshal(blkMap[:numDirectBlks*4], binary.LittleEndian, &file.directBlks)
+	binary.Unmarshal(blkMap[numDirectBlks*4:(numDirectBlks+1)*4], binary.LittleEndian, &file.indirectBlk)
+	binary.Unmarshal(blkMap[(numDirectBlks+1)*4:(numDirectBlks+2)*4], binary.LittleEndian, &file.doubleIndirectBlk)
+	binary.Unmarshal(blkMap[(numDirectBlks+2)*4:(numDirectBlks+3)*4], binary.LittleEndian, &file.tripleIndirectBlk)
+	return file, nil
+}
+
+// ReadAt implements io.ReaderAt.ReadAt.
+func (f *blockMapFile) ReadAt(dst []byte, off int64) (int, error) {
+	if len(dst) == 0 {
+		return 0, nil
+	}
+
+	if off < 0 {
+		return 0, syserror.EINVAL
+	}
+
+	offset := uint64(off)
+	size := f.regFile.inode.diskInode.Size()
+	if offset >= size {
+		return 0, io.EOF
+	}
+
+	// dirBlksEnd is the file offset until which direct blocks cover file data.
+	// Direct blocks cover 0 <= file offset < dirBlksEnd.
+	dirBlksEnd := numDirectBlks * f.coverage[0]
+
+	// indirBlkEnd is the file offset until which the indirect block covers file
+	// data. The indirect block covers dirBlksEnd <= file offset < indirBlkEnd.
+	indirBlkEnd := dirBlksEnd + f.coverage[1]
+
+	// doubIndirBlkEnd is the file offset until which the double indirect block
+	// covers file data. The double indirect block covers the range
+	// indirBlkEnd <= file offset < doubIndirBlkEnd.
+	doubIndirBlkEnd := indirBlkEnd + f.coverage[2]
+
+	read := 0
+	toRead := len(dst)
+	if uint64(toRead)+offset > size {
+		toRead = int(size - offset)
+	}
+	for read < toRead {
+		var err error
+		var curR int
+
+		// Figure out which block to delegate the read to.
+		switch {
+		case offset < dirBlksEnd:
+			// Direct block.
+			curR, err = f.read(f.directBlks[offset/f.regFile.inode.blkSize], offset%f.regFile.inode.blkSize, 0, dst[read:])
+		case offset < indirBlkEnd:
+			// Indirect block.
+			curR, err = f.read(f.indirectBlk, offset-dirBlksEnd, 1, dst[read:])
+		case offset < doubIndirBlkEnd:
+			// Doubly indirect block.
+			curR, err = f.read(f.doubleIndirectBlk, offset-indirBlkEnd, 2, dst[read:])
+		default:
+			// Triply indirect block.
+			curR, err = f.read(f.tripleIndirectBlk, offset-doubIndirBlkEnd, 3, dst[read:])
+		}
+
+		read += curR
+		offset += uint64(curR)
+		if err != nil {
+			return read, err
+		}
+	}
+
+	if read < len(dst) {
+		return read, io.EOF
+	}
+	return read, nil
+}
+
+// read is the recursive step of the ReadAt function. It relies on knowing the
+// current node's location on disk (curPhyBlk) and its height in the block map
+// tree. A height of 0 shows that the current node is actually holding file
+// data. relFileOff tells the offset from which we need to start to reading
+// under the current node. It is completely relative to the current node.
+func (f *blockMapFile) read(curPhyBlk uint32, relFileOff uint64, height uint, dst []byte) (int, error) {
+	curPhyBlkOff := int64(curPhyBlk) * int64(f.regFile.inode.blkSize)
+	if height == 0 {
+		toRead := int(f.regFile.inode.blkSize - relFileOff)
+		if len(dst) < toRead {
+			toRead = len(dst)
+		}
+
+		n, _ := f.regFile.inode.dev.ReadAt(dst[:toRead], curPhyBlkOff+int64(relFileOff))
+		if n < toRead {
+			return n, syserror.EIO
+		}
+		return n, nil
+	}
+
+	childCov := f.coverage[height-1]
+	startIdx := relFileOff / childCov
+	endIdx := f.regFile.inode.blkSize / 4 // This is exclusive.
+	wantEndIdx := (relFileOff + uint64(len(dst))) / childCov
+	wantEndIdx++ // Make this exclusive.
+	if wantEndIdx < endIdx {
+		endIdx = wantEndIdx
+	}
+
+	read := 0
+	curChildOff := relFileOff % childCov
+	for i := startIdx; i < endIdx; i++ {
+		var childPhyBlk uint32
+		err := readFromDisk(f.regFile.inode.dev, curPhyBlkOff+int64(i*4), &childPhyBlk)
+		if err != nil {
+			return read, err
+		}
+
+		n, err := f.read(childPhyBlk, curChildOff, height-1, dst[read:])
+		read += n
+		if err != nil {
+			return read, err
+		}
+
+		curChildOff = 0
+	}
+
+	return read, nil
+}
+
+// getCoverage returns the number of bytes a node at the given height covers.
+// Height 0 is the file data block itself. Height 1 is the indirect block.
+//
+// Formula: blkSize * ((blkSize / 4)^height)
+func getCoverage(blkSize uint64, height uint) uint64 {
+	return blkSize * uint64(math.Pow(float64(blkSize/4), float64(height)))
+}
diff --git a/pkg/sentry/fsimpl/ext/block_map_test.go b/pkg/sentry/fsimpl/ext/block_map_test.go
new file mode 100644
index 000000000..213aa3919
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/block_map_test.go
@@ -0,0 +1,157 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ext
+
+import (
+	"bytes"
+	"math/rand"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"gvisor.dev/gvisor/pkg/binary"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/ext/disklayout"
+)
+
+// These consts are for mocking the block map tree.
+const (
+	mockBMBlkSize  = uint32(16)
+	mockBMDiskSize = 2500
+)
+
+// TestBlockMapReader stress tests block map reader functionality. It performs
+// random length reads from all possible positions in the block map structure.
+func TestBlockMapReader(t *testing.T) {
+	mockBMFile, want := blockMapSetUp(t)
+	n := len(want)
+
+	for from := 0; from < n; from++ {
+		got := make([]byte, n-from)
+
+		if read, err := mockBMFile.ReadAt(got, int64(from)); err != nil {
+			t.Fatalf("file read operation from offset %d to %d only read %d bytes: %v", from, n, read, err)
+		}
+
+		if diff := cmp.Diff(got, want[from:]); diff != "" {
+			t.Fatalf("file data from offset %d to %d mismatched (-want +got):\n%s", from, n, diff)
+		}
+	}
+}
+
+// blkNumGen is a number generator which gives block numbers for building the
+// block map file on disk. It gives unique numbers in a random order which
+// facilitates in creating an extremely fragmented filesystem.
+type blkNumGen struct {
+	nums []uint32
+}
+
+// newBlkNumGen is the blkNumGen constructor.
+func newBlkNumGen() *blkNumGen {
+	blkNums := &blkNumGen{}
+	lim := mockBMDiskSize / mockBMBlkSize
+	blkNums.nums = make([]uint32, lim)
+	for i := uint32(0); i < lim; i++ {
+		blkNums.nums[i] = i
+	}
+
+	rand.Shuffle(int(lim), func(i, j int) {
+		blkNums.nums[i], blkNums.nums[j] = blkNums.nums[j], blkNums.nums[i]
+	})
+	return blkNums
+}
+
+// next returns the next random block number.
+func (n *blkNumGen) next() uint32 {
+	ret := n.nums[0]
+	n.nums = n.nums[1:]
+	return ret
+}
+
+// blockMapSetUp creates a mock disk and a block map file. It initializes the
+// block map file with 12 direct block, 1 indirect block, 1 double indirect
+// block and 1 triple indirect block (basically fill it till the rim). It
+// initializes the disk to reflect the inode. Also returns the file data that
+// the inode covers and that is written to disk.
+func blockMapSetUp(t *testing.T) (*blockMapFile, []byte) {
+	mockDisk := make([]byte, mockBMDiskSize)
+	regFile := regularFile{
+		inode: inode{
+			diskInode: &disklayout.InodeNew{
+				InodeOld: disklayout.InodeOld{
+					SizeLo: getMockBMFileFize(),
+				},
+			},
+			dev:     bytes.NewReader(mockDisk),
+			blkSize: uint64(mockBMBlkSize),
+		},
+	}
+
+	var fileData []byte
+	blkNums := newBlkNumGen()
+	var data []byte
+
+	// Write the direct blocks.
+	for i := 0; i < numDirectBlks; i++ {
+		curBlkNum := blkNums.next()
+		data = binary.Marshal(data, binary.LittleEndian, curBlkNum)
+		fileData = append(fileData, writeFileDataToBlock(mockDisk, curBlkNum, 0, blkNums)...)
+	}
+
+	// Write to indirect block.
+	indirectBlk := blkNums.next()
+	data = binary.Marshal(data, binary.LittleEndian, indirectBlk)
+	fileData = append(fileData, writeFileDataToBlock(mockDisk, indirectBlk, 1, blkNums)...)
+
+	// Write to indirect block.
+	doublyIndirectBlk := blkNums.next()
+	data = binary.Marshal(data, binary.LittleEndian, doublyIndirectBlk)
+	fileData = append(fileData, writeFileDataToBlock(mockDisk, doublyIndirectBlk, 2, blkNums)...)
+
+	// Write to indirect block.
+	triplyIndirectBlk := blkNums.next()
+	data = binary.Marshal(data, binary.LittleEndian, triplyIndirectBlk)
+	fileData = append(fileData, writeFileDataToBlock(mockDisk, triplyIndirectBlk, 3, blkNums)...)
+
+	copy(regFile.inode.diskInode.Data(), data)
+
+	mockFile, err := newBlockMapFile(regFile)
+	if err != nil {
+		t.Fatalf("newBlockMapFile failed: %v", err)
+	}
+	return mockFile, fileData
+}
+
+// writeFileDataToBlock writes random bytes to the block on disk.
+func writeFileDataToBlock(disk []byte, blkNum uint32, height uint, blkNums *blkNumGen) []byte {
+	if height == 0 {
+		start := blkNum * mockBMBlkSize
+		end := start + mockBMBlkSize
+		rand.Read(disk[start:end])
+		return disk[start:end]
+	}
+
+	var fileData []byte
+	for off := blkNum * mockBMBlkSize; off < (blkNum+1)*mockBMBlkSize; off += 4 {
+		curBlkNum := blkNums.next()
+		copy(disk[off:off+4], binary.Marshal(nil, binary.LittleEndian, curBlkNum))
+		fileData = append(fileData, writeFileDataToBlock(disk, curBlkNum, height-1, blkNums)...)
+	}
+	return fileData
+}
+
+// getMockBMFileFize gets the size of the mock block map file which is used for
+// testing.
+func getMockBMFileFize() uint32 {
+	return uint32(numDirectBlks*getCoverage(uint64(mockBMBlkSize), 0) + getCoverage(uint64(mockBMBlkSize), 1) + getCoverage(uint64(mockBMBlkSize), 2) + getCoverage(uint64(mockBMBlkSize), 3))
+}
diff --git a/pkg/sentry/fsimpl/ext/dentry.go b/pkg/sentry/fsimpl/ext/dentry.go
new file mode 100644
index 000000000..054fb42b6
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/dentry.go
@@ -0,0 +1,56 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ext
+
+import (
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+)
+
+// dentry implements vfs.DentryImpl.
+type dentry struct {
+	vfsd vfs.Dentry
+
+	// inode is the inode represented by this dentry. Multiple Dentries may
+	// share a single non-directory Inode (with hard links). inode is
+	// immutable.
+	inode *inode
+}
+
+// Compiles only if dentry implements vfs.DentryImpl.
+var _ vfs.DentryImpl = (*dentry)(nil)
+
+// newDentry is the dentry constructor.
+func newDentry(in *inode) *dentry {
+	d := &dentry{
+		inode: in,
+	}
+	d.vfsd.Init(d)
+	return d
+}
+
+// IncRef implements vfs.DentryImpl.IncRef.
+func (d *dentry) IncRef(vfsfs *vfs.Filesystem) {
+	d.inode.incRef()
+}
+
+// TryIncRef implements vfs.DentryImpl.TryIncRef.
+func (d *dentry) TryIncRef(vfsfs *vfs.Filesystem) bool {
+	return d.inode.tryIncRef()
+}
+
+// DecRef implements vfs.DentryImpl.DecRef.
+func (d *dentry) DecRef(vfsfs *vfs.Filesystem) {
+	d.inode.decRef(vfsfs.Impl().(*filesystem))
+}
diff --git a/pkg/sentry/fsimpl/ext/directory.go b/pkg/sentry/fsimpl/ext/directory.go
new file mode 100644
index 000000000..b51f3e18d
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/directory.go
@@ -0,0 +1,308 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ext
+
+import (
+	"sync"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/binary"
+	"gvisor.dev/gvisor/pkg/log"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/fs"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/ext/disklayout"
+	"gvisor.dev/gvisor/pkg/sentry/memmap"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// directory represents a directory inode. It holds the childList in memory.
+type directory struct {
+	inode inode
+
+	// mu serializes the changes to childList.
+	// Lock Order (outermost locks must be taken first):
+	//   directory.mu
+	//     filesystem.mu
+	mu sync.Mutex
+
+	// childList is a list containing (1) child dirents and (2) fake dirents
+	// (with diskDirent == nil) that represent the iteration position of
+	// directoryFDs. childList is used to support directoryFD.IterDirents()
+	// efficiently. childList is protected by mu.
+	childList direntList
+
+	// childMap maps the child's filename to the dirent structure stored in
+	// childList. This adds some data replication but helps in faster path
+	// traversal. For consistency, key == childMap[key].diskDirent.FileName().
+	// Immutable.
+	childMap map[string]*dirent
+}
+
+// newDirectroy is the directory constructor.
+func newDirectroy(inode inode, newDirent bool) (*directory, error) {
+	file := &directory{inode: inode, childMap: make(map[string]*dirent)}
+	file.inode.impl = file
+
+	// Initialize childList by reading dirents from the underlying file.
+	if inode.diskInode.Flags().Index {
+		// TODO(b/134676337): Support hash tree directories. Currently only the '.'
+		// and '..' entries are read in.
+
+		// Users cannot navigate this hash tree directory yet.
+		log.Warningf("hash tree directory being used which is unsupported")
+		return file, nil
+	}
+
+	// The dirents are organized in a linear array in the file data.
+	// Extract the file data and decode the dirents.
+	regFile, err := newRegularFile(inode)
+	if err != nil {
+		return nil, err
+	}
+
+	// buf is used as scratch space for reading in dirents from disk and
+	// unmarshalling them into dirent structs.
+	buf := make([]byte, disklayout.DirentSize)
+	size := inode.diskInode.Size()
+	for off, inc := uint64(0), uint64(0); off < size; off += inc {
+		toRead := size - off
+		if toRead > disklayout.DirentSize {
+			toRead = disklayout.DirentSize
+		}
+		if n, err := regFile.impl.ReadAt(buf[:toRead], int64(off)); uint64(n) < toRead {
+			return nil, err
+		}
+
+		var curDirent dirent
+		if newDirent {
+			curDirent.diskDirent = &disklayout.DirentNew{}
+		} else {
+			curDirent.diskDirent = &disklayout.DirentOld{}
+		}
+		binary.Unmarshal(buf, binary.LittleEndian, curDirent.diskDirent)
+
+		if curDirent.diskDirent.Inode() != 0 && len(curDirent.diskDirent.FileName()) != 0 {
+			// Inode number and name length fields being set to 0 is used to indicate
+			// an unused dirent.
+			file.childList.PushBack(&curDirent)
+			file.childMap[curDirent.diskDirent.FileName()] = &curDirent
+		}
+
+		// The next dirent is placed exactly after this dirent record on disk.
+		inc = uint64(curDirent.diskDirent.RecordSize())
+	}
+
+	return file, nil
+}
+
+func (i *inode) isDir() bool {
+	_, ok := i.impl.(*directory)
+	return ok
+}
+
+// dirent is the directory.childList node.
+type dirent struct {
+	diskDirent disklayout.Dirent
+
+	// direntEntry links dirents into their parent directory.childList.
+	direntEntry
+}
+
+// directoryFD represents a directory file description. It implements
+// vfs.FileDescriptionImpl.
+type directoryFD struct {
+	fileDescription
+	vfs.DirectoryFileDescriptionDefaultImpl
+
+	// Protected by directory.mu.
+	iter *dirent
+	off  int64
+}
+
+// Compiles only if directoryFD implements vfs.FileDescriptionImpl.
+var _ vfs.FileDescriptionImpl = (*directoryFD)(nil)
+
+// Release implements vfs.FileDescriptionImpl.Release.
+func (fd *directoryFD) Release() {
+	if fd.iter == nil {
+		return
+	}
+
+	dir := fd.inode().impl.(*directory)
+	dir.mu.Lock()
+	dir.childList.Remove(fd.iter)
+	dir.mu.Unlock()
+	fd.iter = nil
+}
+
+// IterDirents implements vfs.FileDescriptionImpl.IterDirents.
+func (fd *directoryFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallback) error {
+	extfs := fd.filesystem()
+	dir := fd.inode().impl.(*directory)
+
+	dir.mu.Lock()
+	defer dir.mu.Unlock()
+
+	// Ensure that fd.iter exists and is not linked into dir.childList.
+	var child *dirent
+	if fd.iter == nil {
+		// Start iteration at the beginning of dir.
+		child = dir.childList.Front()
+		fd.iter = &dirent{}
+	} else {
+		// Continue iteration from where we left off.
+		child = fd.iter.Next()
+		dir.childList.Remove(fd.iter)
+	}
+	for ; child != nil; child = child.Next() {
+		// Skip other directoryFD iterators.
+		if child.diskDirent != nil {
+			childType, ok := child.diskDirent.FileType()
+			if !ok {
+				// We will need to read the inode off disk. Do not increment
+				// ref count here because this inode is not being added to the
+				// dentry tree.
+				extfs.mu.Lock()
+				childInode, err := extfs.getOrCreateInodeLocked(child.diskDirent.Inode())
+				extfs.mu.Unlock()
+				if err != nil {
+					// Usage of the file description after the error is
+					// undefined. This implementation would continue reading
+					// from the next dirent.
+					fd.off++
+					dir.childList.InsertAfter(child, fd.iter)
+					return err
+				}
+				childType = fs.ToInodeType(childInode.diskInode.Mode().FileType())
+			}
+
+			if !cb.Handle(vfs.Dirent{
+				Name: child.diskDirent.FileName(),
+				Type: fs.ToDirentType(childType),
+				Ino:  uint64(child.diskDirent.Inode()),
+				Off:  fd.off,
+			}) {
+				dir.childList.InsertBefore(child, fd.iter)
+				return nil
+			}
+			fd.off++
+		}
+	}
+	dir.childList.PushBack(fd.iter)
+	return nil
+}
+
+// Seek implements vfs.FileDescriptionImpl.Seek.
+func (fd *directoryFD) Seek(ctx context.Context, offset int64, whence int32) (int64, error) {
+	if whence != linux.SEEK_SET && whence != linux.SEEK_CUR {
+		return 0, syserror.EINVAL
+	}
+
+	dir := fd.inode().impl.(*directory)
+
+	dir.mu.Lock()
+	defer dir.mu.Unlock()
+
+	// Find resulting offset.
+	if whence == linux.SEEK_CUR {
+		offset += fd.off
+	}
+
+	if offset < 0 {
+		// lseek(2) specifies that EINVAL should be returned if the resulting offset
+		// is negative.
+		return 0, syserror.EINVAL
+	}
+
+	n := int64(len(dir.childMap))
+	realWantOff := offset
+	if realWantOff > n {
+		realWantOff = n
+	}
+	realCurOff := fd.off
+	if realCurOff > n {
+		realCurOff = n
+	}
+
+	// Ensure that fd.iter exists and is linked into dir.childList so we can
+	// intelligently seek from the optimal position.
+	if fd.iter == nil {
+		fd.iter = &dirent{}
+		dir.childList.PushFront(fd.iter)
+	}
+
+	// Guess that iterating from the current position is optimal.
+	child := fd.iter
+	diff := realWantOff - realCurOff // Shows direction and magnitude of travel.
+
+	// See if starting from the beginning or end is better.
+	abDiff := diff
+	if diff < 0 {
+		abDiff = -diff
+	}
+	if abDiff > realWantOff {
+		// Starting from the beginning is best.
+		child = dir.childList.Front()
+		diff = realWantOff
+	} else if abDiff > (n - realWantOff) {
+		// Starting from the end is best.
+		child = dir.childList.Back()
+		// (n - 1) because the last non-nil dirent represents the (n-1)th offset.
+		diff = realWantOff - (n - 1)
+	}
+
+	for child != nil {
+		// Skip other directoryFD iterators.
+		if child.diskDirent != nil {
+			if diff == 0 {
+				if child != fd.iter {
+					dir.childList.Remove(fd.iter)
+					dir.childList.InsertBefore(child, fd.iter)
+				}
+
+				fd.off = offset
+				return offset, nil
+			}
+
+			if diff < 0 {
+				diff++
+				child = child.Prev()
+			} else {
+				diff--
+				child = child.Next()
+			}
+			continue
+		}
+
+		if diff < 0 {
+			child = child.Prev()
+		} else {
+			child = child.Next()
+		}
+	}
+
+	// Reaching here indicates that the offset is beyond the end of the childList.
+	dir.childList.Remove(fd.iter)
+	dir.childList.PushBack(fd.iter)
+	fd.off = offset
+	return offset, nil
+}
+
+// IterDirents implements vfs.FileDescriptionImpl.IterDirents.
+func (fd *directoryFD) ConfigureMMap(ctx context.Context, opts memmap.MMapOpts) error {
+	// mmap(2) specifies that EACCESS should be returned for non-regular file fds.
+	return syserror.EACCES
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/BUILD b/pkg/sentry/fsimpl/ext/disklayout/BUILD
new file mode 100644
index 000000000..907d35b7e
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/BUILD
@@ -0,0 +1,48 @@
+package(licenses = ["notice"])
+
+load("//tools/go_stateify:defs.bzl", "go_library", "go_test")
+
+go_library(
+    name = "disklayout",
+    srcs = [
+        "block_group.go",
+        "block_group_32.go",
+        "block_group_64.go",
+        "dirent.go",
+        "dirent_new.go",
+        "dirent_old.go",
+        "disklayout.go",
+        "extent.go",
+        "inode.go",
+        "inode_new.go",
+        "inode_old.go",
+        "superblock.go",
+        "superblock_32.go",
+        "superblock_64.go",
+        "superblock_old.go",
+        "test_utils.go",
+    ],
+    importpath = "gvisor.dev/gvisor/pkg/sentry/fsimpl/ext/disklayout",
+    visibility = ["//pkg/sentry:internal"],
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/binary",
+        "//pkg/sentry/fs",
+        "//pkg/sentry/kernel/auth",
+        "//pkg/sentry/kernel/time",
+    ],
+)
+
+go_test(
+    name = "disklayout_test",
+    size = "small",
+    srcs = [
+        "block_group_test.go",
+        "dirent_test.go",
+        "extent_test.go",
+        "inode_test.go",
+        "superblock_test.go",
+    ],
+    embed = [":disklayout"],
+    deps = ["//pkg/sentry/kernel/time"],
+)
diff --git a/pkg/sentry/fsimpl/ext/disklayout/block_group.go b/pkg/sentry/fsimpl/ext/disklayout/block_group.go
new file mode 100644
index 000000000..ad6f4fef8
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/block_group.go
@@ -0,0 +1,137 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+// BlockGroup represents a Linux ext block group descriptor. An ext file system
+// is split into a series of block groups. This provides an access layer to
+// information needed to access and use a block group.
+//
+// Location:
+//   - The block group descriptor table is always placed in the blocks
+//     immediately after the block containing the superblock.
+//   - The 1st block group descriptor in the original table is in the
+//      (sb.FirstDataBlock() + 1)th block.
+//   - See SuperBlock docs to see where the block group descriptor table is
+//     replicated.
+//   - sb.BgDescSize() must be used as the block group descriptor entry size
+//     while reading the table from disk.
+//
+// See https://www.kernel.org/doc/html/latest/filesystems/ext4/globals.html#block-group-descriptors.
+type BlockGroup interface {
+	// InodeTable returns the absolute block number of the block containing the
+	// inode table. This points to an array of Inode structs. Inode tables are
+	// statically allocated at mkfs time. The superblock records the number of
+	// inodes per group (length of this table) and the size of each inode struct.
+	InodeTable() uint64
+
+	// BlockBitmap returns the absolute block number of the block containing the
+	// block bitmap. This bitmap tracks the usage of data blocks within this block
+	// group and has its own checksum.
+	BlockBitmap() uint64
+
+	// InodeBitmap returns the absolute block number of the block containing the
+	// inode bitmap. This bitmap tracks the usage of this group's inode table
+	// entries and has its own checksum.
+	InodeBitmap() uint64
+
+	// ExclusionBitmap returns the absolute block number of the snapshot exclusion
+	// bitmap.
+	ExclusionBitmap() uint64
+
+	// FreeBlocksCount returns the number of free blocks in the group.
+	FreeBlocksCount() uint32
+
+	// FreeInodesCount returns the number of free inodes in the group.
+	FreeInodesCount() uint32
+
+	// DirectoryCount returns the number of inodes that represent directories
+	// under this block group.
+	DirectoryCount() uint32
+
+	// UnusedInodeCount returns the number of unused inodes beyond the last used
+	// inode in this group's inode table. As a result, we needn’t scan past the
+	// (InodesPerGroup - UnusedInodeCount())th entry in the inode table.
+	UnusedInodeCount() uint32
+
+	// BlockBitmapChecksum returns the block bitmap checksum. This is calculated
+	// using crc32c(FS UUID + group number + entire bitmap).
+	BlockBitmapChecksum() uint32
+
+	// InodeBitmapChecksum returns the inode bitmap checksum. This is calculated
+	// using crc32c(FS UUID + group number + entire bitmap).
+	InodeBitmapChecksum() uint32
+
+	// Checksum returns this block group's checksum.
+	//
+	// If SbMetadataCsum feature is set:
+	//     - checksum is crc32c(FS UUID + group number + group descriptor
+	//       structure) & 0xFFFF.
+	//
+	// If SbGdtCsum feature is set:
+	//     - checksum is crc16(FS UUID + group number + group descriptor
+	//       structure).
+	//
+	// SbMetadataCsum and SbGdtCsum should not be both set.
+	// If they are, Linux warns and asks to run fsck.
+	Checksum() uint16
+
+	// Flags returns BGFlags which represents the block group flags.
+	Flags() BGFlags
+}
+
+// These are the different block group flags.
+const (
+	// BgInodeUninit indicates that inode table and bitmap are not initialized.
+	BgInodeUninit uint16 = 0x1
+
+	// BgBlockUninit indicates that block bitmap is not initialized.
+	BgBlockUninit uint16 = 0x2
+
+	// BgInodeZeroed indicates that inode table is zeroed.
+	BgInodeZeroed uint16 = 0x4
+)
+
+// BGFlags represents all the different combinations of block group flags.
+type BGFlags struct {
+	InodeUninit bool
+	BlockUninit bool
+	InodeZeroed bool
+}
+
+// ToInt converts a BGFlags struct back to its 16-bit representation.
+func (f BGFlags) ToInt() uint16 {
+	var res uint16
+
+	if f.InodeUninit {
+		res |= BgInodeUninit
+	}
+	if f.BlockUninit {
+		res |= BgBlockUninit
+	}
+	if f.InodeZeroed {
+		res |= BgInodeZeroed
+	}
+
+	return res
+}
+
+// BGFlagsFromInt converts the 16-bit flag representation to a BGFlags struct.
+func BGFlagsFromInt(flags uint16) BGFlags {
+	return BGFlags{
+		InodeUninit: flags&BgInodeUninit > 0,
+		BlockUninit: flags&BgBlockUninit > 0,
+		InodeZeroed: flags&BgInodeZeroed > 0,
+	}
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/block_group_32.go b/pkg/sentry/fsimpl/ext/disklayout/block_group_32.go
new file mode 100644
index 000000000..3e16c76db
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/block_group_32.go
@@ -0,0 +1,72 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+// BlockGroup32Bit emulates the first half of struct ext4_group_desc in
+// fs/ext4/ext4.h. It is the block group descriptor struct for ext2, ext3 and
+// 32-bit ext4 filesystems. It implements BlockGroup interface.
+type BlockGroup32Bit struct {
+	BlockBitmapLo         uint32
+	InodeBitmapLo         uint32
+	InodeTableLo          uint32
+	FreeBlocksCountLo     uint16
+	FreeInodesCountLo     uint16
+	UsedDirsCountLo       uint16
+	FlagsRaw              uint16
+	ExcludeBitmapLo       uint32
+	BlockBitmapChecksumLo uint16
+	InodeBitmapChecksumLo uint16
+	ItableUnusedLo        uint16
+	ChecksumRaw           uint16
+}
+
+// Compiles only if BlockGroup32Bit implements BlockGroup.
+var _ BlockGroup = (*BlockGroup32Bit)(nil)
+
+// InodeTable implements BlockGroup.InodeTable.
+func (bg *BlockGroup32Bit) InodeTable() uint64 { return uint64(bg.InodeTableLo) }
+
+// BlockBitmap implements BlockGroup.BlockBitmap.
+func (bg *BlockGroup32Bit) BlockBitmap() uint64 { return uint64(bg.BlockBitmapLo) }
+
+// InodeBitmap implements BlockGroup.InodeBitmap.
+func (bg *BlockGroup32Bit) InodeBitmap() uint64 { return uint64(bg.InodeBitmapLo) }
+
+// ExclusionBitmap implements BlockGroup.ExclusionBitmap.
+func (bg *BlockGroup32Bit) ExclusionBitmap() uint64 { return uint64(bg.ExcludeBitmapLo) }
+
+// FreeBlocksCount implements BlockGroup.FreeBlocksCount.
+func (bg *BlockGroup32Bit) FreeBlocksCount() uint32 { return uint32(bg.FreeBlocksCountLo) }
+
+// FreeInodesCount implements BlockGroup.FreeInodesCount.
+func (bg *BlockGroup32Bit) FreeInodesCount() uint32 { return uint32(bg.FreeInodesCountLo) }
+
+// DirectoryCount implements BlockGroup.DirectoryCount.
+func (bg *BlockGroup32Bit) DirectoryCount() uint32 { return uint32(bg.UsedDirsCountLo) }
+
+// UnusedInodeCount implements BlockGroup.UnusedInodeCount.
+func (bg *BlockGroup32Bit) UnusedInodeCount() uint32 { return uint32(bg.ItableUnusedLo) }
+
+// BlockBitmapChecksum implements BlockGroup.BlockBitmapChecksum.
+func (bg *BlockGroup32Bit) BlockBitmapChecksum() uint32 { return uint32(bg.BlockBitmapChecksumLo) }
+
+// InodeBitmapChecksum implements BlockGroup.InodeBitmapChecksum.
+func (bg *BlockGroup32Bit) InodeBitmapChecksum() uint32 { return uint32(bg.InodeBitmapChecksumLo) }
+
+// Checksum implements BlockGroup.Checksum.
+func (bg *BlockGroup32Bit) Checksum() uint16 { return bg.ChecksumRaw }
+
+// Flags implements BlockGroup.Flags.
+func (bg *BlockGroup32Bit) Flags() BGFlags { return BGFlagsFromInt(bg.FlagsRaw) }
diff --git a/pkg/sentry/fsimpl/ext/disklayout/block_group_64.go b/pkg/sentry/fsimpl/ext/disklayout/block_group_64.go
new file mode 100644
index 000000000..9a809197a
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/block_group_64.go
@@ -0,0 +1,93 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+// BlockGroup64Bit emulates struct ext4_group_desc in fs/ext4/ext4.h.
+// It is the block group descriptor struct for 64-bit ext4 filesystems.
+// It implements BlockGroup interface. It is an extension of the 32-bit
+// version of BlockGroup.
+type BlockGroup64Bit struct {
+	// We embed the 32-bit struct here because 64-bit version is just an extension
+	// of the 32-bit version.
+	BlockGroup32Bit
+
+	// 64-bit specific fields.
+	BlockBitmapHi         uint32
+	InodeBitmapHi         uint32
+	InodeTableHi          uint32
+	FreeBlocksCountHi     uint16
+	FreeInodesCountHi     uint16
+	UsedDirsCountHi       uint16
+	ItableUnusedHi        uint16
+	ExcludeBitmapHi       uint32
+	BlockBitmapChecksumHi uint16
+	InodeBitmapChecksumHi uint16
+	_                     uint32 // Padding to 64 bytes.
+}
+
+// Compiles only if BlockGroup64Bit implements BlockGroup.
+var _ BlockGroup = (*BlockGroup64Bit)(nil)
+
+// Methods to override. Checksum() and Flags() are not overridden.
+
+// InodeTable implements BlockGroup.InodeTable.
+func (bg *BlockGroup64Bit) InodeTable() uint64 {
+	return (uint64(bg.InodeTableHi) << 32) | uint64(bg.InodeTableLo)
+}
+
+// BlockBitmap implements BlockGroup.BlockBitmap.
+func (bg *BlockGroup64Bit) BlockBitmap() uint64 {
+	return (uint64(bg.BlockBitmapHi) << 32) | uint64(bg.BlockBitmapLo)
+}
+
+// InodeBitmap implements BlockGroup.InodeBitmap.
+func (bg *BlockGroup64Bit) InodeBitmap() uint64 {
+	return (uint64(bg.InodeBitmapHi) << 32) | uint64(bg.InodeBitmapLo)
+}
+
+// ExclusionBitmap implements BlockGroup.ExclusionBitmap.
+func (bg *BlockGroup64Bit) ExclusionBitmap() uint64 {
+	return (uint64(bg.ExcludeBitmapHi) << 32) | uint64(bg.ExcludeBitmapLo)
+}
+
+// FreeBlocksCount implements BlockGroup.FreeBlocksCount.
+func (bg *BlockGroup64Bit) FreeBlocksCount() uint32 {
+	return (uint32(bg.FreeBlocksCountHi) << 16) | uint32(bg.FreeBlocksCountLo)
+}
+
+// FreeInodesCount implements BlockGroup.FreeInodesCount.
+func (bg *BlockGroup64Bit) FreeInodesCount() uint32 {
+	return (uint32(bg.FreeInodesCountHi) << 16) | uint32(bg.FreeInodesCountLo)
+}
+
+// DirectoryCount implements BlockGroup.DirectoryCount.
+func (bg *BlockGroup64Bit) DirectoryCount() uint32 {
+	return (uint32(bg.UsedDirsCountHi) << 16) | uint32(bg.UsedDirsCountLo)
+}
+
+// UnusedInodeCount implements BlockGroup.UnusedInodeCount.
+func (bg *BlockGroup64Bit) UnusedInodeCount() uint32 {
+	return (uint32(bg.ItableUnusedHi) << 16) | uint32(bg.ItableUnusedLo)
+}
+
+// BlockBitmapChecksum implements BlockGroup.BlockBitmapChecksum.
+func (bg *BlockGroup64Bit) BlockBitmapChecksum() uint32 {
+	return (uint32(bg.BlockBitmapChecksumHi) << 16) | uint32(bg.BlockBitmapChecksumLo)
+}
+
+// InodeBitmapChecksum implements BlockGroup.InodeBitmapChecksum.
+func (bg *BlockGroup64Bit) InodeBitmapChecksum() uint32 {
+	return (uint32(bg.InodeBitmapChecksumHi) << 16) | uint32(bg.InodeBitmapChecksumLo)
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/block_group_test.go b/pkg/sentry/fsimpl/ext/disklayout/block_group_test.go
new file mode 100644
index 000000000..0ef4294c0
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/block_group_test.go
@@ -0,0 +1,26 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+import (
+	"testing"
+)
+
+// TestBlockGroupSize tests that the block group descriptor structs are of the
+// correct size.
+func TestBlockGroupSize(t *testing.T) {
+	assertSize(t, BlockGroup32Bit{}, 32)
+	assertSize(t, BlockGroup64Bit{}, 64)
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/dirent.go b/pkg/sentry/fsimpl/ext/disklayout/dirent.go
new file mode 100644
index 000000000..417b6cf65
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/dirent.go
@@ -0,0 +1,72 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+import (
+	"gvisor.dev/gvisor/pkg/sentry/fs"
+)
+
+const (
+	// MaxFileName is the maximum length of an ext fs file's name.
+	MaxFileName = 255
+
+	// DirentSize is the size of ext dirent structures.
+	DirentSize = 263
+)
+
+var (
+	// inodeTypeByFileType maps ext4 file types to vfs inode types.
+	//
+	// See https://www.kernel.org/doc/html/latest/filesystems/ext4/dynamic.html#ftype.
+	inodeTypeByFileType = map[uint8]fs.InodeType{
+		0: fs.Anonymous,
+		1: fs.RegularFile,
+		2: fs.Directory,
+		3: fs.CharacterDevice,
+		4: fs.BlockDevice,
+		5: fs.Pipe,
+		6: fs.Socket,
+		7: fs.Symlink,
+	}
+)
+
+// The Dirent interface should be implemented by structs representing ext
+// directory entries. These are for the linear classical directories which
+// just store a list of dirent structs. A directory is a series of data blocks
+// where is each data block contains a linear array of dirents. The last entry
+// of the block has a record size that takes it to the end of the block. The
+// end of the directory is when you read dirInode.Size() bytes from the blocks.
+//
+// See https://www.kernel.org/doc/html/latest/filesystems/ext4/dynamic.html#linear-classic-directories.
+type Dirent interface {
+	// Inode returns the absolute inode number of the underlying inode.
+	// Inode number 0 signifies an unused dirent.
+	Inode() uint32
+
+	// RecordSize returns the record length of this dirent on disk. The next
+	// dirent in the dirent list should be read after these many bytes from
+	// the current dirent. Must be a multiple of 4.
+	RecordSize() uint16
+
+	// FileName returns the name of the file. Can be at most 255 is length.
+	FileName() string
+
+	// FileType returns the inode type of the underlying inode. This is a
+	// performance hack so that we do not have to read the underlying inode struct
+	// to know the type of inode. This will only work when the SbDirentFileType
+	// feature is set. If not, the second returned value will be false indicating
+	// that user code has to use the inode mode to extract the file type.
+	FileType() (fs.InodeType, bool)
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/dirent_new.go b/pkg/sentry/fsimpl/ext/disklayout/dirent_new.go
new file mode 100644
index 000000000..29ae4a5c2
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/dirent_new.go
@@ -0,0 +1,61 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+import (
+	"fmt"
+
+	"gvisor.dev/gvisor/pkg/sentry/fs"
+)
+
+// DirentNew represents the ext4 directory entry struct. This emulates Linux's
+// ext4_dir_entry_2 struct. The FileName can not be more than 255 bytes so we
+// only need 8 bits to store the NameLength. As a result, NameLength has been
+// shortened and the other 8 bits are used to encode the file type. Use the
+// FileTypeRaw field only if the SbDirentFileType feature is set.
+//
+// Note: This struct can be of variable size on disk. The one described below
+// is of maximum size and the FileName beyond NameLength bytes might contain
+// garbage.
+type DirentNew struct {
+	InodeNumber  uint32
+	RecordLength uint16
+	NameLength   uint8
+	FileTypeRaw  uint8
+	FileNameRaw  [MaxFileName]byte
+}
+
+// Compiles only if DirentNew implements Dirent.
+var _ Dirent = (*DirentNew)(nil)
+
+// Inode implements Dirent.Inode.
+func (d *DirentNew) Inode() uint32 { return d.InodeNumber }
+
+// RecordSize implements Dirent.RecordSize.
+func (d *DirentNew) RecordSize() uint16 { return d.RecordLength }
+
+// FileName implements Dirent.FileName.
+func (d *DirentNew) FileName() string {
+	return string(d.FileNameRaw[:d.NameLength])
+}
+
+// FileType implements Dirent.FileType.
+func (d *DirentNew) FileType() (fs.InodeType, bool) {
+	if inodeType, ok := inodeTypeByFileType[d.FileTypeRaw]; ok {
+		return inodeType, true
+	}
+
+	panic(fmt.Sprintf("unknown file type %v", d.FileTypeRaw))
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/dirent_old.go b/pkg/sentry/fsimpl/ext/disklayout/dirent_old.go
new file mode 100644
index 000000000..6fff12a6e
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/dirent_old.go
@@ -0,0 +1,49 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+import "gvisor.dev/gvisor/pkg/sentry/fs"
+
+// DirentOld represents the old directory entry struct which does not contain
+// the file type. This emulates Linux's ext4_dir_entry struct.
+//
+// Note: This struct can be of variable size on disk. The one described below
+// is of maximum size and the FileName beyond NameLength bytes might contain
+// garbage.
+type DirentOld struct {
+	InodeNumber  uint32
+	RecordLength uint16
+	NameLength   uint16
+	FileNameRaw  [MaxFileName]byte
+}
+
+// Compiles only if DirentOld implements Dirent.
+var _ Dirent = (*DirentOld)(nil)
+
+// Inode implements Dirent.Inode.
+func (d *DirentOld) Inode() uint32 { return d.InodeNumber }
+
+// RecordSize implements Dirent.RecordSize.
+func (d *DirentOld) RecordSize() uint16 { return d.RecordLength }
+
+// FileName implements Dirent.FileName.
+func (d *DirentOld) FileName() string {
+	return string(d.FileNameRaw[:d.NameLength])
+}
+
+// FileType implements Dirent.FileType.
+func (d *DirentOld) FileType() (fs.InodeType, bool) {
+	return fs.Anonymous, false
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/dirent_test.go b/pkg/sentry/fsimpl/ext/disklayout/dirent_test.go
new file mode 100644
index 000000000..934919f8a
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/dirent_test.go
@@ -0,0 +1,26 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+import (
+	"testing"
+)
+
+// TestDirentSize tests that the dirent structs are of the correct
+// size.
+func TestDirentSize(t *testing.T) {
+	assertSize(t, DirentOld{}, uintptr(DirentSize))
+	assertSize(t, DirentNew{}, uintptr(DirentSize))
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/disklayout.go b/pkg/sentry/fsimpl/ext/disklayout/disklayout.go
new file mode 100644
index 000000000..bdf4e2132
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/disklayout.go
@@ -0,0 +1,50 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package disklayout provides Linux ext file system's disk level structures
+// which can be directly read into from the underlying device. Structs aim to
+// emulate structures `exactly` how they are layed out on disk.
+//
+// This library aims to be compatible with all ext(2/3/4) systems so it
+// provides a generic interface for all major structures and various
+// implementations (for different versions). The user code is responsible for
+// using appropriate implementations based on the underlying device.
+//
+// Interfacing all major structures here serves a few purposes:
+//   - Abstracts away the complexity of the underlying structure from client
+//     code. The client only has to figure out versioning on set up and then
+//     can use these as black boxes and pass it higher up the stack.
+//   - Having pointer receivers forces the user to use pointers to these
+//     heavy structs. Hence, prevents the client code from unintentionally
+//     copying these by value while passing the interface around.
+//   - Version-based implementation selection is resolved on set up hence
+//     avoiding per call overhead of choosing implementation.
+//   - All interface methods are pretty light weight (do not take in any
+//     parameters by design). Passing pointer arguments to interface methods
+//     can lead to heap allocation as the compiler won't be able to perform
+//     escape analysis on an unknown implementation at compile time.
+//
+// Notes:
+//   - All fields in these structs are exported because binary.Read would
+//     panic otherwise.
+//   - All structures on disk are in little-endian order. Only jbd2 (journal)
+//     structures are in big-endian order.
+//   - All OS dependent fields in these structures will be interpretted using
+//     the Linux version of that field.
+//   - The suffix `Lo` in field names stands for lower bits of that field.
+//   - The suffix `Hi` in field names stands for upper bits of that field.
+//   - The suffix `Raw` has been added to indicate that the field is not split
+//     into Lo and Hi fields and also to resolve name collision with the
+//     respective interface.
+package disklayout
diff --git a/pkg/sentry/fsimpl/ext/disklayout/extent.go b/pkg/sentry/fsimpl/ext/disklayout/extent.go
new file mode 100644
index 000000000..567523d32
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/extent.go
@@ -0,0 +1,139 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+// Extents were introduced in ext4 and provide huge performance gains in terms
+// data locality and reduced metadata block usage. Extents are organized in
+// extent trees. The root node is contained in inode.BlocksRaw.
+//
+// Terminology:
+//   - Physical Block:
+//       Filesystem data block which is addressed normally wrt the entire
+//       filesystem (addressed with 48 bits).
+//
+//   - File Block:
+//       Data block containing *only* file data and addressed wrt to the file
+//       with only 32 bits. The (i)th file block contains file data from
+//       byte (i * sb.BlockSize()) to ((i+1) * sb.BlockSize()).
+
+const (
+	// ExtentStructsSize is the size of all the three extent on-disk structs.
+	ExtentStructsSize = 12
+
+	// ExtentMagic is the magic number which must be present in the header.
+	ExtentMagic = 0xf30a
+)
+
+// ExtentEntryPair couples an in-memory ExtendNode with the ExtentEntry that
+// points to it. We want to cache these structs in memory to avoid repeated
+// disk reads.
+//
+// Note: This struct itself does not represent an on-disk struct.
+type ExtentEntryPair struct {
+	// Entry points to the child node on disk.
+	Entry ExtentEntry
+	// Node points to child node in memory. Is nil if the current node is a leaf.
+	Node *ExtentNode
+}
+
+// ExtentNode represents an extent tree node. For internal nodes, all Entries
+// will be ExtendIdxs. For leaf nodes, they will all be Extents.
+//
+// Note: This struct itself does not represent an on-disk struct.
+type ExtentNode struct {
+	Header  ExtentHeader
+	Entries []ExtentEntryPair
+}
+
+// ExtentEntry reprsents an extent tree node entry. The entry can either be
+// an ExtentIdx or Extent itself. This exists to simplify navigation logic.
+type ExtentEntry interface {
+	// FileBlock returns the first file block number covered by this entry.
+	FileBlock() uint32
+
+	// PhysicalBlock returns the child physical block that this entry points to.
+	PhysicalBlock() uint64
+}
+
+// ExtentHeader emulates the ext4_extent_header struct in ext4. Each extent
+// tree node begins with this and is followed by `NumEntries` number of:
+//   - Extent         if `Depth` == 0
+//   - ExtentIdx      otherwise
+type ExtentHeader struct {
+	// Magic in the extent magic number, must be 0xf30a.
+	Magic uint16
+
+	// NumEntries indicates the number of valid entries following the header.
+	NumEntries uint16
+
+	// MaxEntries that could follow the header. Used while adding entries.
+	MaxEntries uint16
+
+	// Height represents the distance of this node from the farthest leaf. Please
+	// note that Linux incorrectly calls this `Depth` (which means the distance
+	// of the node from the root).
+	Height uint16
+	_      uint32
+}
+
+// ExtentIdx emulates the ext4_extent_idx struct in ext4. Only present in
+// internal nodes. Sorted in ascending order based on FirstFileBlock since
+// Linux does a binary search on this. This points to a block containing the
+// child node.
+type ExtentIdx struct {
+	FirstFileBlock uint32
+	ChildBlockLo   uint32
+	ChildBlockHi   uint16
+	_              uint16
+}
+
+// Compiles only if ExtentIdx implements ExtentEntry.
+var _ ExtentEntry = (*ExtentIdx)(nil)
+
+// FileBlock implements ExtentEntry.FileBlock.
+func (ei *ExtentIdx) FileBlock() uint32 {
+	return ei.FirstFileBlock
+}
+
+// PhysicalBlock implements ExtentEntry.PhysicalBlock. It returns the
+// physical block number of the child block.
+func (ei *ExtentIdx) PhysicalBlock() uint64 {
+	return (uint64(ei.ChildBlockHi) << 32) | uint64(ei.ChildBlockLo)
+}
+
+// Extent represents the ext4_extent struct in ext4. Only present in leaf
+// nodes. Sorted in ascending order based on FirstFileBlock since Linux does a
+// binary search on this. This points to an array of data blocks containing the
+// file data. It covers `Length` data blocks starting from `StartBlock`.
+type Extent struct {
+	FirstFileBlock uint32
+	Length         uint16
+	StartBlockHi   uint16
+	StartBlockLo   uint32
+}
+
+// Compiles only if Extent implements ExtentEntry.
+var _ ExtentEntry = (*Extent)(nil)
+
+// FileBlock implements ExtentEntry.FileBlock.
+func (e *Extent) FileBlock() uint32 {
+	return e.FirstFileBlock
+}
+
+// PhysicalBlock implements ExtentEntry.PhysicalBlock. It returns the
+// physical block number of the first data block this extent covers.
+func (e *Extent) PhysicalBlock() uint64 {
+	return (uint64(e.StartBlockHi) << 32) | uint64(e.StartBlockLo)
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/extent_test.go b/pkg/sentry/fsimpl/ext/disklayout/extent_test.go
new file mode 100644
index 000000000..b0fad9b71
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/extent_test.go
@@ -0,0 +1,27 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+import (
+	"testing"
+)
+
+// TestExtentSize tests that the extent structs are of the correct
+// size.
+func TestExtentSize(t *testing.T) {
+	assertSize(t, ExtentHeader{}, ExtentStructsSize)
+	assertSize(t, ExtentIdx{}, ExtentStructsSize)
+	assertSize(t, Extent{}, ExtentStructsSize)
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/inode.go b/pkg/sentry/fsimpl/ext/disklayout/inode.go
new file mode 100644
index 000000000..88ae913f5
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/inode.go
@@ -0,0 +1,274 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/time"
+)
+
+// Special inodes. See https://www.kernel.org/doc/html/latest/filesystems/ext4/overview.html#special-inodes.
+const (
+	// RootDirInode is the inode number of the root directory inode.
+	RootDirInode = 2
+)
+
+// The Inode interface must be implemented by structs representing ext inodes.
+// The inode stores all the metadata pertaining to the file (except for the
+// file name which is held by the directory entry). It does NOT expose all
+// fields and should be extended if need be.
+//
+// Some file systems (e.g. FAT) use the directory entry to store all this
+// information. Ext file systems do not so that they can support hard links.
+// However, ext4 cheats a little bit and duplicates the file type in the
+// directory entry for performance gains.
+//
+// See https://www.kernel.org/doc/html/latest/filesystems/ext4/dynamic.html#index-nodes.
+type Inode interface {
+	// Mode returns the linux file mode which is majorly used to extract
+	// information like:
+	// - File permissions (read/write/execute by user/group/others).
+	// - Sticky, set UID and GID bits.
+	// - File type.
+	//
+	// Masks to extract this information are provided in pkg/abi/linux/file.go.
+	Mode() linux.FileMode
+
+	// UID returns the owner UID.
+	UID() auth.KUID
+
+	// GID returns the owner GID.
+	GID() auth.KGID
+
+	// Size returns the size of the file in bytes.
+	Size() uint64
+
+	// InodeSize returns the size of this inode struct in bytes.
+	// In ext2 and ext3, the inode struct and inode disk record size was fixed at
+	// 128 bytes. Ext4 makes it possible for the inode struct to be bigger.
+	// However, accessing any field beyond the 128 bytes marker must be verified
+	// using this method.
+	InodeSize() uint16
+
+	// AccessTime returns the last access time. Shows when the file was last read.
+	//
+	// If InExtendedAttr is set, then this should NOT be used because the
+	// underlying field is used to store the extended attribute value checksum.
+	AccessTime() time.Time
+
+	// ChangeTime returns the last change time. Shows when the file meta data
+	// (like permissions) was last changed.
+	//
+	// If InExtendedAttr is set, then this should NOT be used because the
+	// underlying field is used to store the lower 32 bits of the attribute
+	// value’s reference count.
+	ChangeTime() time.Time
+
+	// ModificationTime returns the last modification time. Shows when the file
+	// content was last modified.
+	//
+	// If InExtendedAttr is set, then this should NOT be used because
+	// the underlying field contains the number of the inode that owns the
+	// extended attribute.
+	ModificationTime() time.Time
+
+	// DeletionTime returns the deletion time. Inodes are marked as deleted by
+	// writing to the underlying field. FS tools can restore files until they are
+	// actually overwritten.
+	DeletionTime() time.Time
+
+	// LinksCount returns the number of hard links to this inode.
+	//
+	// Normally there is an upper limit on the number of hard links:
+	//   - ext2/ext3 = 32,000
+	//   - ext4      = 65,000
+	//
+	// This implies that an ext4 directory cannot have more than 64,998
+	// subdirectories because each subdirectory will have a hard link to the
+	// directory via the `..` entry. The directory has hard link via the `.` entry
+	// of its own. And finally the inode is initiated with 1 hard link (itself).
+	//
+	// The underlying value is reset to 1 if all the following hold:
+	//     - Inode is a directory.
+	//     - SbDirNlink is enabled.
+	//     - Number of hard links is incremented past 64,999.
+	// Hard link value of 1 for a directory would indicate that the number of hard
+	// links is unknown because a directory can have minimum 2 hard links (itself
+	// and `.` entry).
+	LinksCount() uint16
+
+	// Flags returns InodeFlags which represents the inode flags.
+	Flags() InodeFlags
+
+	// Data returns the underlying inode.i_block array as a slice so it's
+	// modifiable. This field is special and is used to store various kinds of
+	// things depending on the filesystem version and inode type. The underlying
+	// field name in Linux is a little misleading.
+	//   - In ext2/ext3, it contains the block map.
+	//   - In ext4, it contains the extent tree root node.
+	//   - For inline files, it contains the file contents.
+	//   - For symlinks, it contains the link path (if it fits here).
+	//
+	// See https://www.kernel.org/doc/html/latest/filesystems/ext4/dynamic.html#the-contents-of-inode-i-block.
+	Data() []byte
+}
+
+// Inode flags. This is not comprehensive and flags which were not used in
+// the Linux kernel have been excluded.
+const (
+	// InSync indicates that all writes to the file must be synchronous.
+	InSync = 0x8
+
+	// InImmutable indicates that this file is immutable.
+	InImmutable = 0x10
+
+	// InAppend indicates that this file can only be appended to.
+	InAppend = 0x20
+
+	// InNoDump indicates that teh dump(1) utility should not dump this file.
+	InNoDump = 0x40
+
+	// InNoAccessTime indicates that the access time of this inode must not be
+	// updated.
+	InNoAccessTime = 0x80
+
+	// InIndex indicates that this directory has hashed indexes.
+	InIndex = 0x1000
+
+	// InJournalData indicates that file data must always be written through a
+	// journal device.
+	InJournalData = 0x4000
+
+	// InDirSync indicates that all the directory entiry data must be written
+	// synchronously.
+	InDirSync = 0x10000
+
+	// InTopDir indicates that this inode is at the top of the directory hierarchy.
+	InTopDir = 0x20000
+
+	// InHugeFile indicates that this is a huge file.
+	InHugeFile = 0x40000
+
+	// InExtents indicates that this inode uses extents.
+	InExtents = 0x80000
+
+	// InExtendedAttr indicates that this inode stores a large extended attribute
+	// value in its data blocks.
+	InExtendedAttr = 0x200000
+
+	// InInline indicates that this inode has inline data.
+	InInline = 0x10000000
+
+	// InReserved indicates that this inode is reserved for the ext4 library.
+	InReserved = 0x80000000
+)
+
+// InodeFlags represents all possible combinations of inode flags. It aims to
+// cover the bit masks and provide a more user-friendly interface.
+type InodeFlags struct {
+	Sync         bool
+	Immutable    bool
+	Append       bool
+	NoDump       bool
+	NoAccessTime bool
+	Index        bool
+	JournalData  bool
+	DirSync      bool
+	TopDir       bool
+	HugeFile     bool
+	Extents      bool
+	ExtendedAttr bool
+	Inline       bool
+	Reserved     bool
+}
+
+// ToInt converts inode flags back to its 32-bit rep.
+func (f InodeFlags) ToInt() uint32 {
+	var res uint32
+
+	if f.Sync {
+		res |= InSync
+	}
+	if f.Immutable {
+		res |= InImmutable
+	}
+	if f.Append {
+		res |= InAppend
+	}
+	if f.NoDump {
+		res |= InNoDump
+	}
+	if f.NoAccessTime {
+		res |= InNoAccessTime
+	}
+	if f.Index {
+		res |= InIndex
+	}
+	if f.JournalData {
+		res |= InJournalData
+	}
+	if f.DirSync {
+		res |= InDirSync
+	}
+	if f.TopDir {
+		res |= InTopDir
+	}
+	if f.HugeFile {
+		res |= InHugeFile
+	}
+	if f.Extents {
+		res |= InExtents
+	}
+	if f.ExtendedAttr {
+		res |= InExtendedAttr
+	}
+	if f.Inline {
+		res |= InInline
+	}
+	if f.Reserved {
+		res |= InReserved
+	}
+
+	return res
+}
+
+// InodeFlagsFromInt converts the integer representation of inode flags to
+// a InodeFlags struct.
+func InodeFlagsFromInt(f uint32) InodeFlags {
+	return InodeFlags{
+		Sync:         f&InSync > 0,
+		Immutable:    f&InImmutable > 0,
+		Append:       f&InAppend > 0,
+		NoDump:       f&InNoDump > 0,
+		NoAccessTime: f&InNoAccessTime > 0,
+		Index:        f&InIndex > 0,
+		JournalData:  f&InJournalData > 0,
+		DirSync:      f&InDirSync > 0,
+		TopDir:       f&InTopDir > 0,
+		HugeFile:     f&InHugeFile > 0,
+		Extents:      f&InExtents > 0,
+		ExtendedAttr: f&InExtendedAttr > 0,
+		Inline:       f&InInline > 0,
+		Reserved:     f&InReserved > 0,
+	}
+}
+
+// These masks define how users can view/modify inode flags. The rest of the
+// flags are for internal kernel usage only.
+const (
+	InUserReadFlagMask  = 0x4BDFFF
+	InUserWriteFlagMask = 0x4B80FF
+)
diff --git a/pkg/sentry/fsimpl/ext/disklayout/inode_new.go b/pkg/sentry/fsimpl/ext/disklayout/inode_new.go
new file mode 100644
index 000000000..8f9f574ce
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/inode_new.go
@@ -0,0 +1,96 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+import "gvisor.dev/gvisor/pkg/sentry/kernel/time"
+
+// InodeNew represents ext4 inode structure which can be bigger than
+// OldInodeSize. The actual size of this struct should be determined using
+// inode.ExtraInodeSize. Accessing any field here should be verified with the
+// actual size. The extra space between the end of the inode struct and end of
+// the inode record can be used to store extended attr.
+//
+// If the TimeExtra fields are in scope, the lower 2 bits of those are used
+// to extend their counter part to be 34 bits wide; the rest (upper) 30 bits
+// are used to provide nanoscond precision. Hence, these timestamps will now
+// overflow in May 2446.
+// See https://www.kernel.org/doc/html/latest/filesystems/ext4/dynamic.html#inode-timestamps.
+type InodeNew struct {
+	InodeOld
+
+	ExtraInodeSize        uint16
+	ChecksumHi            uint16
+	ChangeTimeExtra       uint32
+	ModificationTimeExtra uint32
+	AccessTimeExtra       uint32
+	CreationTime          uint32
+	CreationTimeExtra     uint32
+	VersionHi             uint32
+	ProjectID             uint32
+}
+
+// Compiles only if InodeNew implements Inode.
+var _ Inode = (*InodeNew)(nil)
+
+// fromExtraTime decodes the extra time and constructs the kernel time struct
+// with nanosecond precision.
+func fromExtraTime(lo int32, extra uint32) time.Time {
+	// See description above InodeNew for format.
+	seconds := (int64(extra&0x3) << 32) + int64(lo)
+	nanoseconds := int64(extra >> 2)
+	return time.FromUnix(seconds, nanoseconds)
+}
+
+// Only override methods which change due to ext4 specific fields.
+
+// Size implements Inode.Size.
+func (in *InodeNew) Size() uint64 {
+	return (uint64(in.SizeHi) << 32) | uint64(in.SizeLo)
+}
+
+// InodeSize implements Inode.InodeSize.
+func (in *InodeNew) InodeSize() uint16 {
+	return OldInodeSize + in.ExtraInodeSize
+}
+
+// ChangeTime implements Inode.ChangeTime.
+func (in *InodeNew) ChangeTime() time.Time {
+	// Apply new timestamp logic if inode.ChangeTimeExtra is in scope.
+	if in.ExtraInodeSize >= 8 {
+		return fromExtraTime(in.ChangeTimeRaw, in.ChangeTimeExtra)
+	}
+
+	return in.InodeOld.ChangeTime()
+}
+
+// ModificationTime implements Inode.ModificationTime.
+func (in *InodeNew) ModificationTime() time.Time {
+	// Apply new timestamp logic if inode.ModificationTimeExtra is in scope.
+	if in.ExtraInodeSize >= 12 {
+		return fromExtraTime(in.ModificationTimeRaw, in.ModificationTimeExtra)
+	}
+
+	return in.InodeOld.ModificationTime()
+}
+
+// AccessTime implements Inode.AccessTime.
+func (in *InodeNew) AccessTime() time.Time {
+	// Apply new timestamp logic if inode.AccessTimeExtra is in scope.
+	if in.ExtraInodeSize >= 16 {
+		return fromExtraTime(in.AccessTimeRaw, in.AccessTimeExtra)
+	}
+
+	return in.InodeOld.AccessTime()
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/inode_old.go b/pkg/sentry/fsimpl/ext/disklayout/inode_old.go
new file mode 100644
index 000000000..db25b11b6
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/inode_old.go
@@ -0,0 +1,117 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/time"
+)
+
+const (
+	// OldInodeSize is the inode size in ext2/ext3.
+	OldInodeSize = 128
+)
+
+// InodeOld implements Inode interface. It emulates ext2/ext3 inode struct.
+// Inode struct size and record size are both 128 bytes for this.
+//
+// All fields representing time are in seconds since the epoch. Which means that
+// they will overflow in January 2038.
+type InodeOld struct {
+	ModeRaw uint16
+	UIDLo   uint16
+	SizeLo  uint32
+
+	// The time fields are signed integers because they could be negative to
+	// represent time before the epoch.
+	AccessTimeRaw       int32
+	ChangeTimeRaw       int32
+	ModificationTimeRaw int32
+	DeletionTimeRaw     int32
+
+	GIDLo         uint16
+	LinksCountRaw uint16
+	BlocksCountLo uint32
+	FlagsRaw      uint32
+	VersionLo     uint32 // This is OS dependent.
+	DataRaw       [60]byte
+	Generation    uint32
+	FileACLLo     uint32
+	SizeHi        uint32
+	ObsoFaddr     uint32
+
+	// OS dependent fields have been inlined here.
+	BlocksCountHi uint16
+	FileACLHi     uint16
+	UIDHi         uint16
+	GIDHi         uint16
+	ChecksumLo    uint16
+	_             uint16
+}
+
+// Compiles only if InodeOld implements Inode.
+var _ Inode = (*InodeOld)(nil)
+
+// Mode implements Inode.Mode.
+func (in *InodeOld) Mode() linux.FileMode { return linux.FileMode(in.ModeRaw) }
+
+// UID implements Inode.UID.
+func (in *InodeOld) UID() auth.KUID {
+	return auth.KUID((uint32(in.UIDHi) << 16) | uint32(in.UIDLo))
+}
+
+// GID implements Inode.GID.
+func (in *InodeOld) GID() auth.KGID {
+	return auth.KGID((uint32(in.GIDHi) << 16) | uint32(in.GIDLo))
+}
+
+// Size implements Inode.Size.
+func (in *InodeOld) Size() uint64 {
+	// In ext2/ext3, in.SizeHi did not exist, it was instead named in.DirACL.
+	return uint64(in.SizeLo)
+}
+
+// InodeSize implements Inode.InodeSize.
+func (in *InodeOld) InodeSize() uint16 { return OldInodeSize }
+
+// AccessTime implements Inode.AccessTime.
+func (in *InodeOld) AccessTime() time.Time {
+	return time.FromUnix(int64(in.AccessTimeRaw), 0)
+}
+
+// ChangeTime implements Inode.ChangeTime.
+func (in *InodeOld) ChangeTime() time.Time {
+	return time.FromUnix(int64(in.ChangeTimeRaw), 0)
+}
+
+// ModificationTime implements Inode.ModificationTime.
+func (in *InodeOld) ModificationTime() time.Time {
+	return time.FromUnix(int64(in.ModificationTimeRaw), 0)
+}
+
+// DeletionTime implements Inode.DeletionTime.
+func (in *InodeOld) DeletionTime() time.Time {
+	return time.FromUnix(int64(in.DeletionTimeRaw), 0)
+}
+
+// LinksCount implements Inode.LinksCount.
+func (in *InodeOld) LinksCount() uint16 { return in.LinksCountRaw }
+
+// Flags implements Inode.Flags.
+func (in *InodeOld) Flags() InodeFlags { return InodeFlagsFromInt(in.FlagsRaw) }
+
+// Data implements Inode.Data.
+func (in *InodeOld) Data() []byte { return in.DataRaw[:] }
diff --git a/pkg/sentry/fsimpl/ext/disklayout/inode_test.go b/pkg/sentry/fsimpl/ext/disklayout/inode_test.go
new file mode 100644
index 000000000..dd03ee50e
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/inode_test.go
@@ -0,0 +1,222 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+import (
+	"fmt"
+	"strconv"
+	"testing"
+
+	"gvisor.dev/gvisor/pkg/sentry/kernel/time"
+)
+
+// TestInodeSize tests that the inode structs are of the correct size.
+func TestInodeSize(t *testing.T) {
+	assertSize(t, InodeOld{}, OldInodeSize)
+
+	// This was updated from 156 bytes to 160 bytes in Oct 2015.
+	assertSize(t, InodeNew{}, 160)
+}
+
+// TestTimestampSeconds tests that the seconds part of [a/c/m] timestamps in
+// ext4 inode structs are decoded correctly.
+//
+// These tests are derived from the table under https://www.kernel.org/doc/html/latest/filesystems/ext4/dynamic.html#inode-timestamps.
+func TestTimestampSeconds(t *testing.T) {
+	type timestampTest struct {
+		// msbSet tells if the most significant bit of InodeOld.[X]TimeRaw is set.
+		// If this is set then the 32-bit time is negative.
+		msbSet bool
+
+		// lowerBound tells if we should take the lowest possible value of
+		// InodeOld.[X]TimeRaw while satisfying test.msbSet condition. If set to
+		// false it tells to take the highest possible value.
+		lowerBound bool
+
+		// extraBits is InodeNew.[X]TimeExtra.
+		extraBits uint32
+
+		// want is the kernel time struct that is expected.
+		want time.Time
+	}
+
+	tests := []timestampTest{
+		// 1901-12-13
+		{
+			msbSet:     true,
+			lowerBound: true,
+			extraBits:  0,
+			want:       time.FromUnix(int64(-0x80000000), 0),
+		},
+
+		// 1969-12-31
+		{
+			msbSet:     true,
+			lowerBound: false,
+			extraBits:  0,
+			want:       time.FromUnix(int64(-1), 0),
+		},
+
+		// 1970-01-01
+		{
+			msbSet:     false,
+			lowerBound: true,
+			extraBits:  0,
+			want:       time.FromUnix(int64(0), 0),
+		},
+
+		// 2038-01-19
+		{
+			msbSet:     false,
+			lowerBound: false,
+			extraBits:  0,
+			want:       time.FromUnix(int64(0x7fffffff), 0),
+		},
+
+		// 2038-01-19
+		{
+			msbSet:     true,
+			lowerBound: true,
+			extraBits:  1,
+			want:       time.FromUnix(int64(0x80000000), 0),
+		},
+
+		// 2106-02-07
+		{
+			msbSet:     true,
+			lowerBound: false,
+			extraBits:  1,
+			want:       time.FromUnix(int64(0xffffffff), 0),
+		},
+
+		// 2106-02-07
+		{
+			msbSet:     false,
+			lowerBound: true,
+			extraBits:  1,
+			want:       time.FromUnix(int64(0x100000000), 0),
+		},
+
+		// 2174-02-25
+		{
+			msbSet:     false,
+			lowerBound: false,
+			extraBits:  1,
+			want:       time.FromUnix(int64(0x17fffffff), 0),
+		},
+
+		// 2174-02-25
+		{
+			msbSet:     true,
+			lowerBound: true,
+			extraBits:  2,
+			want:       time.FromUnix(int64(0x180000000), 0),
+		},
+
+		// 2242-03-16
+		{
+			msbSet:     true,
+			lowerBound: false,
+			extraBits:  2,
+			want:       time.FromUnix(int64(0x1ffffffff), 0),
+		},
+
+		// 2242-03-16
+		{
+			msbSet:     false,
+			lowerBound: true,
+			extraBits:  2,
+			want:       time.FromUnix(int64(0x200000000), 0),
+		},
+
+		// 2310-04-04
+		{
+			msbSet:     false,
+			lowerBound: false,
+			extraBits:  2,
+			want:       time.FromUnix(int64(0x27fffffff), 0),
+		},
+
+		// 2310-04-04
+		{
+			msbSet:     true,
+			lowerBound: true,
+			extraBits:  3,
+			want:       time.FromUnix(int64(0x280000000), 0),
+		},
+
+		// 2378-04-22
+		{
+			msbSet:     true,
+			lowerBound: false,
+			extraBits:  3,
+			want:       time.FromUnix(int64(0x2ffffffff), 0),
+		},
+
+		// 2378-04-22
+		{
+			msbSet:     false,
+			lowerBound: true,
+			extraBits:  3,
+			want:       time.FromUnix(int64(0x300000000), 0),
+		},
+
+		// 2446-05-10
+		{
+			msbSet:     false,
+			lowerBound: false,
+			extraBits:  3,
+			want:       time.FromUnix(int64(0x37fffffff), 0),
+		},
+	}
+
+	lowerMSB0 := int32(0)           // binary: 00000000 00000000 00000000 00000000
+	upperMSB0 := int32(0x7fffffff)  // binary: 01111111 11111111 11111111 11111111
+	lowerMSB1 := int32(-0x80000000) // binary: 10000000 00000000 00000000 00000000
+	upperMSB1 := int32(-1)          // binary: 11111111 11111111 11111111 11111111
+
+	get32BitTime := func(test timestampTest) int32 {
+		if test.msbSet {
+			if test.lowerBound {
+				return lowerMSB1
+			}
+
+			return upperMSB1
+		}
+
+		if test.lowerBound {
+			return lowerMSB0
+		}
+
+		return upperMSB0
+	}
+
+	getTestName := func(test timestampTest) string {
+		return fmt.Sprintf(
+			"Tests time decoding with epoch bits 0b%s and 32-bit raw time: MSB set=%t, lower bound=%t",
+			strconv.FormatInt(int64(test.extraBits), 2),
+			test.msbSet,
+			test.lowerBound,
+		)
+	}
+
+	for _, test := range tests {
+		t.Run(getTestName(test), func(t *testing.T) {
+			if got := fromExtraTime(get32BitTime(test), test.extraBits); got != test.want {
+				t.Errorf("Expected: %v, Got: %v", test.want, got)
+			}
+		})
+	}
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/superblock.go b/pkg/sentry/fsimpl/ext/disklayout/superblock.go
new file mode 100644
index 000000000..8bb327006
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/superblock.go
@@ -0,0 +1,471 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+const (
+	// SbOffset is the absolute offset at which the superblock is placed.
+	SbOffset = 1024
+)
+
+// SuperBlock should be implemented by structs representing the ext superblock.
+// The superblock holds a lot of information about the enclosing filesystem.
+// This interface aims to provide access methods to important information held
+// by the superblock. It does NOT expose all fields of the superblock, only the
+// ones necessary. This can be expanded when need be.
+//
+// Location and replication:
+//     - The superblock is located at offset 1024 in block group 0.
+//     - Redundant copies of the superblock and group descriptors are kept in
+//       all groups if SbSparse feature flag is NOT set. If it is set, the
+//       replicas only exist in groups whose group number is either 0 or a
+//       power of 3, 5, or 7.
+//     - There is also a sparse superblock feature v2 in which there are just
+//       two replicas saved in the block groups pointed by sb.s_backup_bgs.
+//
+// Replicas should eventually be updated if the superblock is updated.
+//
+// See https://www.kernel.org/doc/html/latest/filesystems/ext4/globals.html#super-block.
+type SuperBlock interface {
+	// InodesCount returns the total number of inodes in this filesystem.
+	InodesCount() uint32
+
+	// BlocksCount returns the total number of data blocks in this filesystem.
+	BlocksCount() uint64
+
+	// FreeBlocksCount returns the number of free blocks in this filesystem.
+	FreeBlocksCount() uint64
+
+	// FreeInodesCount returns the number of free inodes in this filesystem.
+	FreeInodesCount() uint32
+
+	// MountCount returns the number of mounts since the last fsck.
+	MountCount() uint16
+
+	// MaxMountCount returns the number of mounts allowed beyond which a fsck is
+	// needed.
+	MaxMountCount() uint16
+
+	// FirstDataBlock returns the absolute block number of the first data block,
+	// which contains the super block itself.
+	//
+	// If the filesystem has 1kb data blocks then this should return 1. For all
+	// other configurations, this typically returns 0.
+	FirstDataBlock() uint32
+
+	// BlockSize returns the size of one data block in this filesystem.
+	// This can be calculated by 2^(10 + sb.s_log_block_size). This ensures that
+	// the smallest block size is 1kb.
+	BlockSize() uint64
+
+	// BlocksPerGroup returns the number of data blocks in a block group.
+	BlocksPerGroup() uint32
+
+	// ClusterSize returns block cluster size (set during mkfs time by admin).
+	// This can be calculated by 2^(10 + sb.s_log_cluster_size). This ensures that
+	// the smallest cluster size is 1kb.
+	//
+	// sb.s_log_cluster_size must equal sb.s_log_block_size if bigalloc feature
+	// is NOT set and consequently BlockSize() = ClusterSize() in that case.
+	ClusterSize() uint64
+
+	// ClustersPerGroup returns:
+	//     - number of clusters per group        if bigalloc is enabled.
+	//     - BlocksPerGroup()                    otherwise.
+	ClustersPerGroup() uint32
+
+	// InodeSize returns the size of the inode disk record size in bytes. Use this
+	// to iterate over inode arrays on disk.
+	//
+	// In ext2 and ext3:
+	//     - Each inode had a disk record of 128 bytes.
+	//     - The inode struct size was fixed at 128 bytes.
+	//
+	// In ext4 its possible to allocate larger on-disk inodes:
+	//     - Inode disk record size = sb.s_inode_size (function return value).
+	//                              = 256 (default)
+	//     - Inode struct size = 128 + inode.i_extra_isize.
+	//                         = 128 + 32 = 160 (default)
+	InodeSize() uint16
+
+	// InodesPerGroup returns the number of inodes in a block group.
+	InodesPerGroup() uint32
+
+	// BgDescSize returns the size of the block group descriptor struct.
+	//
+	// In ext2, ext3, ext4 (without 64-bit feature), the block group descriptor
+	// is only 32 bytes long.
+	// In ext4 with 64-bit feature, the block group descriptor expands to AT LEAST
+	// 64 bytes. It might be bigger than that.
+	BgDescSize() uint16
+
+	// CompatibleFeatures returns the CompatFeatures struct which holds all the
+	// compatible features this fs supports.
+	CompatibleFeatures() CompatFeatures
+
+	// IncompatibleFeatures returns the CompatFeatures struct which holds all the
+	// incompatible features this fs supports.
+	IncompatibleFeatures() IncompatFeatures
+
+	// ReadOnlyCompatibleFeatures returns the CompatFeatures struct which holds all the
+	// readonly compatible features this fs supports.
+	ReadOnlyCompatibleFeatures() RoCompatFeatures
+
+	// Magic() returns the magic signature which must be 0xef53.
+	Magic() uint16
+
+	// Revision returns the superblock revision. Superblock struct fields from
+	// offset 0x54 till 0x150 should only be used if superblock has DynamicRev.
+	Revision() SbRevision
+}
+
+// SbRevision is the type for superblock revisions.
+type SbRevision uint32
+
+// Super block revisions.
+const (
+	// OldRev is the good old (original) format.
+	OldRev SbRevision = 0
+
+	// DynamicRev is v2 format w/ dynamic inode sizes.
+	DynamicRev SbRevision = 1
+)
+
+// Superblock compatible features.
+// This is not exhaustive, unused features are not listed.
+const (
+	// SbDirPrealloc indicates directory preallocation.
+	SbDirPrealloc = 0x1
+
+	// SbHasJournal indicates the presence of a journal. jbd2 should only work
+	// with this being set.
+	SbHasJournal = 0x4
+
+	// SbExtAttr indicates extended attributes support.
+	SbExtAttr = 0x8
+
+	// SbResizeInode indicates that the fs has reserved GDT blocks (right after
+	// group descriptors) for fs expansion.
+	SbResizeInode = 0x10
+
+	// SbDirIndex indicates that the fs has directory indices.
+	SbDirIndex = 0x20
+
+	// SbSparseV2 stands for Sparse superblock version 2.
+	SbSparseV2 = 0x200
+)
+
+// CompatFeatures represents a superblock's compatible feature set. If the
+// kernel does not understand any of these feature, it can still read/write
+// to this fs.
+type CompatFeatures struct {
+	DirPrealloc bool
+	HasJournal  bool
+	ExtAttr     bool
+	ResizeInode bool
+	DirIndex    bool
+	SparseV2    bool
+}
+
+// ToInt converts superblock compatible features back to its 32-bit rep.
+func (f CompatFeatures) ToInt() uint32 {
+	var res uint32
+
+	if f.DirPrealloc {
+		res |= SbDirPrealloc
+	}
+	if f.HasJournal {
+		res |= SbHasJournal
+	}
+	if f.ExtAttr {
+		res |= SbExtAttr
+	}
+	if f.ResizeInode {
+		res |= SbResizeInode
+	}
+	if f.DirIndex {
+		res |= SbDirIndex
+	}
+	if f.SparseV2 {
+		res |= SbSparseV2
+	}
+
+	return res
+}
+
+// CompatFeaturesFromInt converts the integer representation of superblock
+// compatible features to CompatFeatures struct.
+func CompatFeaturesFromInt(f uint32) CompatFeatures {
+	return CompatFeatures{
+		DirPrealloc: f&SbDirPrealloc > 0,
+		HasJournal:  f&SbHasJournal > 0,
+		ExtAttr:     f&SbExtAttr > 0,
+		ResizeInode: f&SbResizeInode > 0,
+		DirIndex:    f&SbDirIndex > 0,
+		SparseV2:    f&SbSparseV2 > 0,
+	}
+}
+
+// Superblock incompatible features.
+// This is not exhaustive, unused features are not listed.
+const (
+	// SbDirentFileType indicates that directory entries record the file type.
+	// We should use struct DirentNew for dirents then.
+	SbDirentFileType = 0x2
+
+	// SbRecovery indicates that the filesystem needs recovery.
+	SbRecovery = 0x4
+
+	// SbJournalDev indicates that the filesystem has a separate journal device.
+	SbJournalDev = 0x8
+
+	// SbMetaBG indicates that the filesystem is using Meta block groups. Moves
+	// the group descriptors from the congested first block group into the first
+	// group of each metablock group to increase the maximum block groups limit
+	// and hence support much larger filesystems.
+	//
+	// See https://www.kernel.org/doc/html/latest/filesystems/ext4/overview.html#meta-block-groups.
+	SbMetaBG = 0x10
+
+	// SbExtents indicates that the filesystem uses extents. Must be set in ext4
+	// filesystems.
+	SbExtents = 0x40
+
+	// SbIs64Bit indicates that this filesystem addresses blocks with 64-bits.
+	// Hence can support 2^64 data blocks.
+	SbIs64Bit = 0x80
+
+	// SbMMP indicates that this filesystem has multiple mount protection.
+	//
+	// See https://www.kernel.org/doc/html/latest/filesystems/ext4/globals.html#multiple-mount-protection.
+	SbMMP = 0x100
+
+	// SbFlexBg indicates that this filesystem has flexible block groups. Several
+	// block groups are tied into one logical block group so that all the metadata
+	// for the block groups (bitmaps and inode tables) are close together for
+	// faster loading. Consequently, large files will be continuous on disk.
+	// However, this does not affect the placement of redundant superblocks and
+	// group descriptors.
+	//
+	// See https://www.kernel.org/doc/html/latest/filesystems/ext4/overview.html#flexible-block-groups.
+	SbFlexBg = 0x200
+
+	// SbLargeDir shows that large directory enabled. Directory htree can be 3
+	// levels deep. Directory htrees are allowed to be 2 levels deep otherwise.
+	SbLargeDir = 0x4000
+
+	// SbInlineData allows inline data in inodes for really small files.
+	SbInlineData = 0x8000
+
+	// SbEncrypted indicates that this fs contains encrypted inodes.
+	SbEncrypted = 0x10000
+)
+
+// IncompatFeatures represents a superblock's incompatible feature set. If the
+// kernel does not understand any of these feature, it should refuse to mount.
+type IncompatFeatures struct {
+	DirentFileType bool
+	Recovery       bool
+	JournalDev     bool
+	MetaBG         bool
+	Extents        bool
+	Is64Bit        bool
+	MMP            bool
+	FlexBg         bool
+	LargeDir       bool
+	InlineData     bool
+	Encrypted      bool
+}
+
+// ToInt converts superblock incompatible features back to its 32-bit rep.
+func (f IncompatFeatures) ToInt() uint32 {
+	var res uint32
+
+	if f.DirentFileType {
+		res |= SbDirentFileType
+	}
+	if f.Recovery {
+		res |= SbRecovery
+	}
+	if f.JournalDev {
+		res |= SbJournalDev
+	}
+	if f.MetaBG {
+		res |= SbMetaBG
+	}
+	if f.Extents {
+		res |= SbExtents
+	}
+	if f.Is64Bit {
+		res |= SbIs64Bit
+	}
+	if f.MMP {
+		res |= SbMMP
+	}
+	if f.FlexBg {
+		res |= SbFlexBg
+	}
+	if f.LargeDir {
+		res |= SbLargeDir
+	}
+	if f.InlineData {
+		res |= SbInlineData
+	}
+	if f.Encrypted {
+		res |= SbEncrypted
+	}
+
+	return res
+}
+
+// IncompatFeaturesFromInt converts the integer representation of superblock
+// incompatible features to IncompatFeatures struct.
+func IncompatFeaturesFromInt(f uint32) IncompatFeatures {
+	return IncompatFeatures{
+		DirentFileType: f&SbDirentFileType > 0,
+		Recovery:       f&SbRecovery > 0,
+		JournalDev:     f&SbJournalDev > 0,
+		MetaBG:         f&SbMetaBG > 0,
+		Extents:        f&SbExtents > 0,
+		Is64Bit:        f&SbIs64Bit > 0,
+		MMP:            f&SbMMP > 0,
+		FlexBg:         f&SbFlexBg > 0,
+		LargeDir:       f&SbLargeDir > 0,
+		InlineData:     f&SbInlineData > 0,
+		Encrypted:      f&SbEncrypted > 0,
+	}
+}
+
+// Superblock readonly compatible features.
+// This is not exhaustive, unused features are not listed.
+const (
+	// SbSparse indicates sparse superblocks. Only groups with number either 0 or
+	// a power of 3, 5, or 7 will have redundant copies of the superblock and
+	// block descriptors.
+	SbSparse = 0x1
+
+	// SbLargeFile indicates that this fs has been used to store a file >= 2GiB.
+	SbLargeFile = 0x2
+
+	// SbHugeFile indicates that this fs contains files whose sizes are
+	// represented in units of logicals blocks, not 512-byte sectors.
+	SbHugeFile = 0x8
+
+	// SbGdtCsum indicates that group descriptors have checksums.
+	SbGdtCsum = 0x10
+
+	// SbDirNlink indicates that the new subdirectory limit is 64,999. Ext3 has a
+	// 32,000 subdirectory limit.
+	SbDirNlink = 0x20
+
+	// SbExtraIsize indicates that large inodes exist on this filesystem.
+	SbExtraIsize = 0x40
+
+	// SbHasSnapshot indicates the existence of a snapshot.
+	SbHasSnapshot = 0x80
+
+	// SbQuota enables usage tracking for all quota types.
+	SbQuota = 0x100
+
+	// SbBigalloc maps to the bigalloc feature. When set, the minimum allocation
+	// unit becomes a cluster rather than a data block. Then block bitmaps track
+	// clusters, not data blocks.
+	//
+	// See https://www.kernel.org/doc/html/latest/filesystems/ext4/overview.html#bigalloc.
+	SbBigalloc = 0x200
+
+	// SbMetadataCsum indicates that the fs supports metadata checksumming.
+	SbMetadataCsum = 0x400
+
+	// SbReadOnly marks this filesystem as readonly. Should refuse to mount in
+	// read/write mode.
+	SbReadOnly = 0x1000
+)
+
+// RoCompatFeatures represents a superblock's readonly compatible feature set.
+// If the kernel does not understand any of these feature, it can still mount
+// readonly. But if the user wants to mount read/write, the kernel should
+// refuse to mount.
+type RoCompatFeatures struct {
+	Sparse       bool
+	LargeFile    bool
+	HugeFile     bool
+	GdtCsum      bool
+	DirNlink     bool
+	ExtraIsize   bool
+	HasSnapshot  bool
+	Quota        bool
+	Bigalloc     bool
+	MetadataCsum bool
+	ReadOnly     bool
+}
+
+// ToInt converts superblock readonly compatible features to its 32-bit rep.
+func (f RoCompatFeatures) ToInt() uint32 {
+	var res uint32
+
+	if f.Sparse {
+		res |= SbSparse
+	}
+	if f.LargeFile {
+		res |= SbLargeFile
+	}
+	if f.HugeFile {
+		res |= SbHugeFile
+	}
+	if f.GdtCsum {
+		res |= SbGdtCsum
+	}
+	if f.DirNlink {
+		res |= SbDirNlink
+	}
+	if f.ExtraIsize {
+		res |= SbExtraIsize
+	}
+	if f.HasSnapshot {
+		res |= SbHasSnapshot
+	}
+	if f.Quota {
+		res |= SbQuota
+	}
+	if f.Bigalloc {
+		res |= SbBigalloc
+	}
+	if f.MetadataCsum {
+		res |= SbMetadataCsum
+	}
+	if f.ReadOnly {
+		res |= SbReadOnly
+	}
+
+	return res
+}
+
+// RoCompatFeaturesFromInt converts the integer representation of superblock
+// readonly compatible features to RoCompatFeatures struct.
+func RoCompatFeaturesFromInt(f uint32) RoCompatFeatures {
+	return RoCompatFeatures{
+		Sparse:       f&SbSparse > 0,
+		LargeFile:    f&SbLargeFile > 0,
+		HugeFile:     f&SbHugeFile > 0,
+		GdtCsum:      f&SbGdtCsum > 0,
+		DirNlink:     f&SbDirNlink > 0,
+		ExtraIsize:   f&SbExtraIsize > 0,
+		HasSnapshot:  f&SbHasSnapshot > 0,
+		Quota:        f&SbQuota > 0,
+		Bigalloc:     f&SbBigalloc > 0,
+		MetadataCsum: f&SbMetadataCsum > 0,
+		ReadOnly:     f&SbReadOnly > 0,
+	}
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/superblock_32.go b/pkg/sentry/fsimpl/ext/disklayout/superblock_32.go
new file mode 100644
index 000000000..53e515fd3
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/superblock_32.go
@@ -0,0 +1,76 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+// SuperBlock32Bit implements SuperBlock and represents the 32-bit version of
+// the ext4_super_block struct in fs/ext4/ext4.h. Should be used only if
+// RevLevel = DynamicRev and 64-bit feature is disabled.
+type SuperBlock32Bit struct {
+	// We embed the old superblock struct here because the 32-bit version is just
+	// an extension of the old version.
+	SuperBlockOld
+
+	FirstInode         uint32
+	InodeSizeRaw       uint16
+	BlockGroupNumber   uint16
+	FeatureCompat      uint32
+	FeatureIncompat    uint32
+	FeatureRoCompat    uint32
+	UUID               [16]byte
+	VolumeName         [16]byte
+	LastMounted        [64]byte
+	AlgoUsageBitmap    uint32
+	PreallocBlocks     uint8
+	PreallocDirBlocks  uint8
+	ReservedGdtBlocks  uint16
+	JournalUUID        [16]byte
+	JournalInum        uint32
+	JournalDev         uint32
+	LastOrphan         uint32
+	HashSeed           [4]uint32
+	DefaultHashVersion uint8
+	JnlBackupType      uint8
+	BgDescSizeRaw      uint16
+	DefaultMountOpts   uint32
+	FirstMetaBg        uint32
+	MkfsTime           uint32
+	JnlBlocks          [17]uint32
+}
+
+// Compiles only if SuperBlock32Bit implements SuperBlock.
+var _ SuperBlock = (*SuperBlock32Bit)(nil)
+
+// Only override methods which change based on the additional fields above.
+// Not overriding SuperBlock.BgDescSize because it would still return 32 here.
+
+// InodeSize implements SuperBlock.InodeSize.
+func (sb *SuperBlock32Bit) InodeSize() uint16 {
+	return sb.InodeSizeRaw
+}
+
+// CompatibleFeatures implements SuperBlock.CompatibleFeatures.
+func (sb *SuperBlock32Bit) CompatibleFeatures() CompatFeatures {
+	return CompatFeaturesFromInt(sb.FeatureCompat)
+}
+
+// IncompatibleFeatures implements SuperBlock.IncompatibleFeatures.
+func (sb *SuperBlock32Bit) IncompatibleFeatures() IncompatFeatures {
+	return IncompatFeaturesFromInt(sb.FeatureIncompat)
+}
+
+// ReadOnlyCompatibleFeatures implements SuperBlock.ReadOnlyCompatibleFeatures.
+func (sb *SuperBlock32Bit) ReadOnlyCompatibleFeatures() RoCompatFeatures {
+	return RoCompatFeaturesFromInt(sb.FeatureRoCompat)
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/superblock_64.go b/pkg/sentry/fsimpl/ext/disklayout/superblock_64.go
new file mode 100644
index 000000000..7c1053fb4
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/superblock_64.go
@@ -0,0 +1,95 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+// SuperBlock64Bit implements SuperBlock and represents the 64-bit version of
+// the ext4_super_block struct in fs/ext4/ext4.h. This sums up to be exactly
+// 1024 bytes (smallest possible block size) and hence the superblock always
+// fits in no more than one data block. Should only be used when the 64-bit
+// feature is set.
+type SuperBlock64Bit struct {
+	// We embed the 32-bit struct here because 64-bit version is just an extension
+	// of the 32-bit version.
+	SuperBlock32Bit
+
+	BlocksCountHi           uint32
+	ReservedBlocksCountHi   uint32
+	FreeBlocksCountHi       uint32
+	MinInodeSize            uint16
+	WantInodeSize           uint16
+	Flags                   uint32
+	RaidStride              uint16
+	MmpInterval             uint16
+	MmpBlock                uint64
+	RaidStripeWidth         uint32
+	LogGroupsPerFlex        uint8
+	ChecksumType            uint8
+	_                       uint16
+	KbytesWritten           uint64
+	SnapshotInum            uint32
+	SnapshotID              uint32
+	SnapshotRsrvBlocksCount uint64
+	SnapshotList            uint32
+	ErrorCount              uint32
+	FirstErrorTime          uint32
+	FirstErrorInode         uint32
+	FirstErrorBlock         uint64
+	FirstErrorFunction      [32]byte
+	FirstErrorLine          uint32
+	LastErrorTime           uint32
+	LastErrorInode          uint32
+	LastErrorLine           uint32
+	LastErrorBlock          uint64
+	LastErrorFunction       [32]byte
+	MountOpts               [64]byte
+	UserQuotaInum           uint32
+	GroupQuotaInum          uint32
+	OverheadBlocks          uint32
+	BackupBgs               [2]uint32
+	EncryptAlgos            [4]uint8
+	EncryptPwSalt           [16]uint8
+	LostFoundInode          uint32
+	ProjectQuotaInode       uint32
+	ChecksumSeed            uint32
+	WtimeHi                 uint8
+	MtimeHi                 uint8
+	MkfsTimeHi              uint8
+	LastCheckHi             uint8
+	FirstErrorTimeHi        uint8
+	LastErrorTimeHi         uint8
+	_                       [2]uint8
+	Encoding                uint16
+	EncodingFlags           uint16
+	_                       [95]uint32
+	Checksum                uint32
+}
+
+// Compiles only if SuperBlock64Bit implements SuperBlock.
+var _ SuperBlock = (*SuperBlock64Bit)(nil)
+
+// Only override methods which change based on the 64-bit feature.
+
+// BlocksCount implements SuperBlock.BlocksCount.
+func (sb *SuperBlock64Bit) BlocksCount() uint64 {
+	return (uint64(sb.BlocksCountHi) << 32) | uint64(sb.BlocksCountLo)
+}
+
+// FreeBlocksCount implements SuperBlock.FreeBlocksCount.
+func (sb *SuperBlock64Bit) FreeBlocksCount() uint64 {
+	return (uint64(sb.FreeBlocksCountHi) << 32) | uint64(sb.FreeBlocksCountLo)
+}
+
+// BgDescSize implements SuperBlock.BgDescSize.
+func (sb *SuperBlock64Bit) BgDescSize() uint16 { return sb.BgDescSizeRaw }
diff --git a/pkg/sentry/fsimpl/ext/disklayout/superblock_old.go b/pkg/sentry/fsimpl/ext/disklayout/superblock_old.go
new file mode 100644
index 000000000..9221e0251
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/superblock_old.go
@@ -0,0 +1,105 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+// SuperBlockOld implements SuperBlock and represents the old version of the
+// superblock struct. Should be used only if RevLevel = OldRev.
+type SuperBlockOld struct {
+	InodesCountRaw      uint32
+	BlocksCountLo       uint32
+	ReservedBlocksCount uint32
+	FreeBlocksCountLo   uint32
+	FreeInodesCountRaw  uint32
+	FirstDataBlockRaw   uint32
+	LogBlockSize        uint32
+	LogClusterSize      uint32
+	BlocksPerGroupRaw   uint32
+	ClustersPerGroupRaw uint32
+	InodesPerGroupRaw   uint32
+	Mtime               uint32
+	Wtime               uint32
+	MountCountRaw       uint16
+	MaxMountCountRaw    uint16
+	MagicRaw            uint16
+	State               uint16
+	Errors              uint16
+	MinorRevLevel       uint16
+	LastCheck           uint32
+	CheckInterval       uint32
+	CreatorOS           uint32
+	RevLevel            uint32
+	DefResUID           uint16
+	DefResGID           uint16
+}
+
+// Compiles only if SuperBlockOld implements SuperBlock.
+var _ SuperBlock = (*SuperBlockOld)(nil)
+
+// InodesCount implements SuperBlock.InodesCount.
+func (sb *SuperBlockOld) InodesCount() uint32 { return sb.InodesCountRaw }
+
+// BlocksCount implements SuperBlock.BlocksCount.
+func (sb *SuperBlockOld) BlocksCount() uint64 { return uint64(sb.BlocksCountLo) }
+
+// FreeBlocksCount implements SuperBlock.FreeBlocksCount.
+func (sb *SuperBlockOld) FreeBlocksCount() uint64 { return uint64(sb.FreeBlocksCountLo) }
+
+// FreeInodesCount implements SuperBlock.FreeInodesCount.
+func (sb *SuperBlockOld) FreeInodesCount() uint32 { return sb.FreeInodesCountRaw }
+
+// MountCount implements SuperBlock.MountCount.
+func (sb *SuperBlockOld) MountCount() uint16 { return sb.MountCountRaw }
+
+// MaxMountCount implements SuperBlock.MaxMountCount.
+func (sb *SuperBlockOld) MaxMountCount() uint16 { return sb.MaxMountCountRaw }
+
+// FirstDataBlock implements SuperBlock.FirstDataBlock.
+func (sb *SuperBlockOld) FirstDataBlock() uint32 { return sb.FirstDataBlockRaw }
+
+// BlockSize implements SuperBlock.BlockSize.
+func (sb *SuperBlockOld) BlockSize() uint64 { return 1 << (10 + sb.LogBlockSize) }
+
+// BlocksPerGroup implements SuperBlock.BlocksPerGroup.
+func (sb *SuperBlockOld) BlocksPerGroup() uint32 { return sb.BlocksPerGroupRaw }
+
+// ClusterSize implements SuperBlock.ClusterSize.
+func (sb *SuperBlockOld) ClusterSize() uint64 { return 1 << (10 + sb.LogClusterSize) }
+
+// ClustersPerGroup implements SuperBlock.ClustersPerGroup.
+func (sb *SuperBlockOld) ClustersPerGroup() uint32 { return sb.ClustersPerGroupRaw }
+
+// InodeSize implements SuperBlock.InodeSize.
+func (sb *SuperBlockOld) InodeSize() uint16 { return OldInodeSize }
+
+// InodesPerGroup implements SuperBlock.InodesPerGroup.
+func (sb *SuperBlockOld) InodesPerGroup() uint32 { return sb.InodesPerGroupRaw }
+
+// BgDescSize implements SuperBlock.BgDescSize.
+func (sb *SuperBlockOld) BgDescSize() uint16 { return 32 }
+
+// CompatibleFeatures implements SuperBlock.CompatibleFeatures.
+func (sb *SuperBlockOld) CompatibleFeatures() CompatFeatures { return CompatFeatures{} }
+
+// IncompatibleFeatures implements SuperBlock.IncompatibleFeatures.
+func (sb *SuperBlockOld) IncompatibleFeatures() IncompatFeatures { return IncompatFeatures{} }
+
+// ReadOnlyCompatibleFeatures implements SuperBlock.ReadOnlyCompatibleFeatures.
+func (sb *SuperBlockOld) ReadOnlyCompatibleFeatures() RoCompatFeatures { return RoCompatFeatures{} }
+
+// Magic implements SuperBlock.Magic.
+func (sb *SuperBlockOld) Magic() uint16 { return sb.MagicRaw }
+
+// Revision implements SuperBlock.Revision.
+func (sb *SuperBlockOld) Revision() SbRevision { return SbRevision(sb.RevLevel) }
diff --git a/pkg/sentry/fsimpl/ext/disklayout/superblock_test.go b/pkg/sentry/fsimpl/ext/disklayout/superblock_test.go
new file mode 100644
index 000000000..463b5ba21
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/superblock_test.go
@@ -0,0 +1,27 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+import (
+	"testing"
+)
+
+// TestSuperBlockSize tests that the superblock structs are of the correct
+// size.
+func TestSuperBlockSize(t *testing.T) {
+	assertSize(t, SuperBlockOld{}, 84)
+	assertSize(t, SuperBlock32Bit{}, 336)
+	assertSize(t, SuperBlock64Bit{}, 1024)
+}
diff --git a/pkg/sentry/fsimpl/ext/disklayout/test_utils.go b/pkg/sentry/fsimpl/ext/disklayout/test_utils.go
new file mode 100644
index 000000000..9c63f04c0
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/disklayout/test_utils.go
@@ -0,0 +1,30 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package disklayout
+
+import (
+	"reflect"
+	"testing"
+
+	"gvisor.dev/gvisor/pkg/binary"
+)
+
+func assertSize(t *testing.T, v interface{}, want uintptr) {
+	t.Helper()
+
+	if got := binary.Size(v); got != want {
+		t.Errorf("struct %s should be exactly %d bytes but is %d bytes", reflect.TypeOf(v).Name(), want, got)
+	}
+}
diff --git a/pkg/sentry/fsimpl/ext/ext.go b/pkg/sentry/fsimpl/ext/ext.go
new file mode 100644
index 000000000..f10accafc
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/ext.go
@@ -0,0 +1,135 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package ext implements readonly ext(2/3/4) filesystems.
+package ext
+
+import (
+	"errors"
+	"fmt"
+	"io"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/fd"
+	"gvisor.dev/gvisor/pkg/log"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/ext/disklayout"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// FilesystemType implements vfs.FilesystemType.
+type FilesystemType struct{}
+
+// Compiles only if FilesystemType implements vfs.FilesystemType.
+var _ vfs.FilesystemType = (*FilesystemType)(nil)
+
+// getDeviceFd returns an io.ReaderAt to the underlying device.
+// Currently there are two ways of mounting an ext(2/3/4) fs:
+//   1. Specify a mount with our internal special MountType in the OCI spec.
+//   2. Expose the device to the container and mount it from application layer.
+func getDeviceFd(source string, opts vfs.NewFilesystemOptions) (io.ReaderAt, error) {
+	if opts.InternalData == nil {
+		// User mount call.
+		// TODO(b/134676337): Open the device specified by `source` and return that.
+		panic("unimplemented")
+	}
+
+	// NewFilesystem call originated from within the sentry.
+	devFd, ok := opts.InternalData.(int)
+	if !ok {
+		return nil, errors.New("internal data for ext fs must be an int containing the file descriptor to device")
+	}
+
+	if devFd < 0 {
+		return nil, fmt.Errorf("ext device file descriptor is not valid: %d", devFd)
+	}
+
+	// The fd.ReadWriter returned from fd.NewReadWriter() does not take ownership
+	// of the file descriptor and hence will not close it when it is garbage
+	// collected.
+	return fd.NewReadWriter(devFd), nil
+}
+
+// isCompatible checks if the superblock has feature sets which are compatible.
+// We only need to check the superblock incompatible feature set since we are
+// mounting readonly. We will also need to check readonly compatible feature
+// set when mounting for read/write.
+func isCompatible(sb disklayout.SuperBlock) bool {
+	// Please note that what is being checked is limited based on the fact that we
+	// are mounting readonly and that we are not journaling. When mounting
+	// read/write or with a journal, this must be reevaluated.
+	incompatFeatures := sb.IncompatibleFeatures()
+	if incompatFeatures.MetaBG {
+		log.Warningf("ext fs: meta block groups are not supported")
+		return false
+	}
+	if incompatFeatures.MMP {
+		log.Warningf("ext fs: multiple mount protection is not supported")
+		return false
+	}
+	if incompatFeatures.Encrypted {
+		log.Warningf("ext fs: encrypted inodes not supported")
+		return false
+	}
+	if incompatFeatures.InlineData {
+		log.Warningf("ext fs: inline files not supported")
+		return false
+	}
+	return true
+}
+
+// NewFilesystem implements vfs.FilesystemType.NewFilesystem.
+func (FilesystemType) NewFilesystem(ctx context.Context, creds *auth.Credentials, source string, opts vfs.NewFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
+	// TODO(b/134676337): Ensure that the user is mounting readonly. If not,
+	// EACCESS should be returned according to mount(2). Filesystem independent
+	// flags (like readonly) are currently not available in pkg/sentry/vfs.
+
+	dev, err := getDeviceFd(source, opts)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	fs := filesystem{dev: dev, inodeCache: make(map[uint32]*inode)}
+	fs.vfsfs.Init(&fs)
+	fs.sb, err = readSuperBlock(dev)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	if fs.sb.Magic() != linux.EXT_SUPER_MAGIC {
+		// mount(2) specifies that EINVAL should be returned if the superblock is
+		// invalid.
+		return nil, nil, syserror.EINVAL
+	}
+
+	// Refuse to mount if the filesystem is incompatible.
+	if !isCompatible(fs.sb) {
+		return nil, nil, syserror.EINVAL
+	}
+
+	fs.bgs, err = readBlockGroups(dev, fs.sb)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	rootInode, err := fs.getOrCreateInodeLocked(disklayout.RootDirInode)
+	if err != nil {
+		return nil, nil, err
+	}
+	rootInode.incRef()
+
+	return &fs.vfsfs, &newDentry(rootInode).vfsd, nil
+}
diff --git a/pkg/sentry/fsimpl/ext/ext_test.go b/pkg/sentry/fsimpl/ext/ext_test.go
new file mode 100644
index 000000000..49b57a2d6
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/ext_test.go
@@ -0,0 +1,917 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ext
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"path"
+	"sort"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/context/contexttest"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/ext/disklayout"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/usermem"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+
+	"gvisor.dev/gvisor/runsc/test/testutil"
+)
+
+const (
+	assetsDir = "pkg/sentry/fsimpl/ext/assets"
+)
+
+var (
+	ext2ImagePath = path.Join(assetsDir, "tiny.ext2")
+	ext3ImagePath = path.Join(assetsDir, "tiny.ext3")
+	ext4ImagePath = path.Join(assetsDir, "tiny.ext4")
+)
+
+// setUp opens imagePath as an ext Filesystem and returns all necessary
+// elements required to run tests. If error is non-nil, it also returns a tear
+// down function which must be called after the test is run for clean up.
+func setUp(t *testing.T, imagePath string) (context.Context, *vfs.VirtualFilesystem, *vfs.VirtualDentry, func(), error) {
+	localImagePath, err := testutil.FindFile(imagePath)
+	if err != nil {
+		return nil, nil, nil, nil, fmt.Errorf("failed to open local image at path %s: %v", imagePath, err)
+	}
+
+	f, err := os.Open(localImagePath)
+	if err != nil {
+		return nil, nil, nil, nil, err
+	}
+
+	ctx := contexttest.Context(t)
+	creds := auth.CredentialsFromContext(ctx)
+
+	// Create VFS.
+	vfsObj := vfs.New()
+	vfsObj.MustRegisterFilesystemType("extfs", FilesystemType{})
+	mntns, err := vfsObj.NewMountNamespace(ctx, creds, localImagePath, "extfs", &vfs.NewFilesystemOptions{InternalData: int(f.Fd())})
+	if err != nil {
+		f.Close()
+		return nil, nil, nil, nil, err
+	}
+
+	root := mntns.Root()
+
+	tearDown := func() {
+		root.DecRef()
+
+		if err := f.Close(); err != nil {
+			t.Fatalf("tearDown failed: %v", err)
+		}
+	}
+	return ctx, vfsObj, &root, tearDown, nil
+}
+
+// TODO(b/134676337): Test vfs.FilesystemImpl.ReadlinkAt and
+// vfs.FilesystemImpl.StatFSAt which are not implemented in
+// vfs.VirtualFilesystem yet.
+
+// TestSeek tests vfs.FileDescriptionImpl.Seek functionality.
+func TestSeek(t *testing.T) {
+	type seekTest struct {
+		name  string
+		image string
+		path  string
+	}
+
+	tests := []seekTest{
+		{
+			name:  "ext4 root dir seek",
+			image: ext4ImagePath,
+			path:  "/",
+		},
+		{
+			name:  "ext3 root dir seek",
+			image: ext3ImagePath,
+			path:  "/",
+		},
+		{
+			name:  "ext2 root dir seek",
+			image: ext2ImagePath,
+			path:  "/",
+		},
+		{
+			name:  "ext4 reg file seek",
+			image: ext4ImagePath,
+			path:  "/file.txt",
+		},
+		{
+			name:  "ext3 reg file seek",
+			image: ext3ImagePath,
+			path:  "/file.txt",
+		},
+		{
+			name:  "ext2 reg file seek",
+			image: ext2ImagePath,
+			path:  "/file.txt",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			ctx, vfsfs, root, tearDown, err := setUp(t, test.image)
+			if err != nil {
+				t.Fatalf("setUp failed: %v", err)
+			}
+			defer tearDown()
+
+			fd, err := vfsfs.OpenAt(
+				ctx,
+				auth.CredentialsFromContext(ctx),
+				&vfs.PathOperation{Root: *root, Start: *root, Pathname: test.path},
+				&vfs.OpenOptions{},
+			)
+			if err != nil {
+				t.Fatalf("vfsfs.OpenAt failed: %v", err)
+			}
+
+			if n, err := fd.Impl().Seek(ctx, 0, linux.SEEK_SET); n != 0 || err != nil {
+				t.Errorf("expected seek position 0, got %d and error %v", n, err)
+			}
+
+			stat, err := fd.Impl().Stat(ctx, vfs.StatOptions{})
+			if err != nil {
+				t.Errorf("fd.stat failed for file %s in image %s: %v", test.path, test.image, err)
+			}
+
+			// We should be able to seek beyond the end of file.
+			size := int64(stat.Size)
+			if n, err := fd.Impl().Seek(ctx, size, linux.SEEK_SET); n != size || err != nil {
+				t.Errorf("expected seek position %d, got %d and error %v", size, n, err)
+			}
+
+			// EINVAL should be returned if the resulting offset is negative.
+			if _, err := fd.Impl().Seek(ctx, -1, linux.SEEK_SET); err != syserror.EINVAL {
+				t.Errorf("expected error EINVAL but got %v", err)
+			}
+
+			if n, err := fd.Impl().Seek(ctx, 3, linux.SEEK_CUR); n != size+3 || err != nil {
+				t.Errorf("expected seek position %d, got %d and error %v", size+3, n, err)
+			}
+
+			// Make sure negative offsets work with SEEK_CUR.
+			if n, err := fd.Impl().Seek(ctx, -2, linux.SEEK_CUR); n != size+1 || err != nil {
+				t.Errorf("expected seek position %d, got %d and error %v", size+1, n, err)
+			}
+
+			// EINVAL should be returned if the resulting offset is negative.
+			if _, err := fd.Impl().Seek(ctx, -(size + 2), linux.SEEK_CUR); err != syserror.EINVAL {
+				t.Errorf("expected error EINVAL but got %v", err)
+			}
+
+			// Make sure SEEK_END works with regular files.
+			switch fd.Impl().(type) {
+			case *regularFileFD:
+				// Seek back to 0.
+				if n, err := fd.Impl().Seek(ctx, -size, linux.SEEK_END); n != 0 || err != nil {
+					t.Errorf("expected seek position %d, got %d and error %v", 0, n, err)
+				}
+
+				// Seek forward beyond EOF.
+				if n, err := fd.Impl().Seek(ctx, 1, linux.SEEK_END); n != size+1 || err != nil {
+					t.Errorf("expected seek position %d, got %d and error %v", size+1, n, err)
+				}
+
+				// EINVAL should be returned if the resulting offset is negative.
+				if _, err := fd.Impl().Seek(ctx, -(size + 1), linux.SEEK_END); err != syserror.EINVAL {
+					t.Errorf("expected error EINVAL but got %v", err)
+				}
+			}
+		})
+	}
+}
+
+// TestStatAt tests filesystem.StatAt functionality.
+func TestStatAt(t *testing.T) {
+	type statAtTest struct {
+		name  string
+		image string
+		path  string
+		want  linux.Statx
+	}
+
+	tests := []statAtTest{
+		{
+			name:  "ext4 statx small file",
+			image: ext4ImagePath,
+			path:  "/file.txt",
+			want: linux.Statx{
+				Blksize: 0x400,
+				Nlink:   1,
+				UID:     0,
+				GID:     0,
+				Mode:    0644 | linux.ModeRegular,
+				Size:    13,
+			},
+		},
+		{
+			name:  "ext3 statx small file",
+			image: ext3ImagePath,
+			path:  "/file.txt",
+			want: linux.Statx{
+				Blksize: 0x400,
+				Nlink:   1,
+				UID:     0,
+				GID:     0,
+				Mode:    0644 | linux.ModeRegular,
+				Size:    13,
+			},
+		},
+		{
+			name:  "ext2 statx small file",
+			image: ext2ImagePath,
+			path:  "/file.txt",
+			want: linux.Statx{
+				Blksize: 0x400,
+				Nlink:   1,
+				UID:     0,
+				GID:     0,
+				Mode:    0644 | linux.ModeRegular,
+				Size:    13,
+			},
+		},
+		{
+			name:  "ext4 statx big file",
+			image: ext4ImagePath,
+			path:  "/bigfile.txt",
+			want: linux.Statx{
+				Blksize: 0x400,
+				Nlink:   1,
+				UID:     0,
+				GID:     0,
+				Mode:    0644 | linux.ModeRegular,
+				Size:    13042,
+			},
+		},
+		{
+			name:  "ext3 statx big file",
+			image: ext3ImagePath,
+			path:  "/bigfile.txt",
+			want: linux.Statx{
+				Blksize: 0x400,
+				Nlink:   1,
+				UID:     0,
+				GID:     0,
+				Mode:    0644 | linux.ModeRegular,
+				Size:    13042,
+			},
+		},
+		{
+			name:  "ext2 statx big file",
+			image: ext2ImagePath,
+			path:  "/bigfile.txt",
+			want: linux.Statx{
+				Blksize: 0x400,
+				Nlink:   1,
+				UID:     0,
+				GID:     0,
+				Mode:    0644 | linux.ModeRegular,
+				Size:    13042,
+			},
+		},
+		{
+			name:  "ext4 statx symlink file",
+			image: ext4ImagePath,
+			path:  "/symlink.txt",
+			want: linux.Statx{
+				Blksize: 0x400,
+				Nlink:   1,
+				UID:     0,
+				GID:     0,
+				Mode:    0777 | linux.ModeSymlink,
+				Size:    8,
+			},
+		},
+		{
+			name:  "ext3 statx symlink file",
+			image: ext3ImagePath,
+			path:  "/symlink.txt",
+			want: linux.Statx{
+				Blksize: 0x400,
+				Nlink:   1,
+				UID:     0,
+				GID:     0,
+				Mode:    0777 | linux.ModeSymlink,
+				Size:    8,
+			},
+		},
+		{
+			name:  "ext2 statx symlink file",
+			image: ext2ImagePath,
+			path:  "/symlink.txt",
+			want: linux.Statx{
+				Blksize: 0x400,
+				Nlink:   1,
+				UID:     0,
+				GID:     0,
+				Mode:    0777 | linux.ModeSymlink,
+				Size:    8,
+			},
+		},
+	}
+
+	// Ignore the fields that are not supported by filesystem.StatAt yet and
+	// those which are likely to change as the image does.
+	ignoredFields := map[string]bool{
+		"Attributes":     true,
+		"AttributesMask": true,
+		"Atime":          true,
+		"Blocks":         true,
+		"Btime":          true,
+		"Ctime":          true,
+		"DevMajor":       true,
+		"DevMinor":       true,
+		"Ino":            true,
+		"Mask":           true,
+		"Mtime":          true,
+		"RdevMajor":      true,
+		"RdevMinor":      true,
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			ctx, vfsfs, root, tearDown, err := setUp(t, test.image)
+			if err != nil {
+				t.Fatalf("setUp failed: %v", err)
+			}
+			defer tearDown()
+
+			got, err := vfsfs.StatAt(ctx,
+				auth.CredentialsFromContext(ctx),
+				&vfs.PathOperation{Root: *root, Start: *root, Pathname: test.path},
+				&vfs.StatOptions{},
+			)
+			if err != nil {
+				t.Fatalf("vfsfs.StatAt failed for file %s in image %s: %v", test.path, test.image, err)
+			}
+
+			cmpIgnoreFields := cmp.FilterPath(func(p cmp.Path) bool {
+				_, ok := ignoredFields[p.String()]
+				return ok
+			}, cmp.Ignore())
+			if diff := cmp.Diff(got, test.want, cmpIgnoreFields, cmpopts.IgnoreUnexported(linux.Statx{})); diff != "" {
+				t.Errorf("stat mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
+// TestRead tests the read functionality for vfs file descriptions.
+func TestRead(t *testing.T) {
+	type readTest struct {
+		name    string
+		image   string
+		absPath string
+	}
+
+	tests := []readTest{
+		{
+			name:    "ext4 read small file",
+			image:   ext4ImagePath,
+			absPath: "/file.txt",
+		},
+		{
+			name:    "ext3 read small file",
+			image:   ext3ImagePath,
+			absPath: "/file.txt",
+		},
+		{
+			name:    "ext2 read small file",
+			image:   ext2ImagePath,
+			absPath: "/file.txt",
+		},
+		{
+			name:    "ext4 read big file",
+			image:   ext4ImagePath,
+			absPath: "/bigfile.txt",
+		},
+		{
+			name:    "ext3 read big file",
+			image:   ext3ImagePath,
+			absPath: "/bigfile.txt",
+		},
+		{
+			name:    "ext2 read big file",
+			image:   ext2ImagePath,
+			absPath: "/bigfile.txt",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			ctx, vfsfs, root, tearDown, err := setUp(t, test.image)
+			if err != nil {
+				t.Fatalf("setUp failed: %v", err)
+			}
+			defer tearDown()
+
+			fd, err := vfsfs.OpenAt(
+				ctx,
+				auth.CredentialsFromContext(ctx),
+				&vfs.PathOperation{Root: *root, Start: *root, Pathname: test.absPath},
+				&vfs.OpenOptions{},
+			)
+			if err != nil {
+				t.Fatalf("vfsfs.OpenAt failed: %v", err)
+			}
+
+			// Get a local file descriptor and compare its functionality with a vfs file
+			// description for the same file.
+			localFile, err := testutil.FindFile(path.Join(assetsDir, test.absPath))
+			if err != nil {
+				t.Fatalf("testutil.FindFile failed for %s: %v", test.absPath, err)
+			}
+
+			f, err := os.Open(localFile)
+			if err != nil {
+				t.Fatalf("os.Open failed for %s: %v", localFile, err)
+			}
+			defer f.Close()
+
+			// Read the entire file by reading one byte repeatedly. Doing this stress
+			// tests the underlying file reader implementation.
+			got := make([]byte, 1)
+			want := make([]byte, 1)
+			for {
+				n, err := f.Read(want)
+				fd.Impl().Read(ctx, usermem.BytesIOSequence(got), vfs.ReadOptions{})
+
+				if diff := cmp.Diff(got, want); diff != "" {
+					t.Errorf("file data mismatch (-want +got):\n%s", diff)
+				}
+
+				// Make sure there is no more file data left after getting EOF.
+				if n == 0 || err == io.EOF {
+					if n, _ := fd.Impl().Read(ctx, usermem.BytesIOSequence(got), vfs.ReadOptions{}); n != 0 {
+						t.Errorf("extra unexpected file data in file %s in image %s", test.absPath, test.image)
+					}
+
+					break
+				}
+
+				if err != nil {
+					t.Fatalf("read failed: %v", err)
+				}
+			}
+		})
+	}
+}
+
+// iterDirentsCb is a simple callback which just keeps adding the dirents to an
+// internal list. Implements vfs.IterDirentsCallback.
+type iterDirentsCb struct {
+	dirents []vfs.Dirent
+}
+
+// Compiles only if iterDirentCb implements vfs.IterDirentsCallback.
+var _ vfs.IterDirentsCallback = (*iterDirentsCb)(nil)
+
+// newIterDirentsCb is the iterDirent
+func newIterDirentCb() *iterDirentsCb {
+	return &iterDirentsCb{dirents: make([]vfs.Dirent, 0)}
+}
+
+// Handle implements vfs.IterDirentsCallback.Handle.
+func (cb *iterDirentsCb) Handle(dirent vfs.Dirent) bool {
+	cb.dirents = append(cb.dirents, dirent)
+	return true
+}
+
+// TestIterDirents tests the FileDescriptionImpl.IterDirents functionality.
+func TestIterDirents(t *testing.T) {
+	type iterDirentTest struct {
+		name  string
+		image string
+		path  string
+		want  []vfs.Dirent
+	}
+
+	wantDirents := []vfs.Dirent{
+		vfs.Dirent{
+			Name: ".",
+			Type: linux.DT_DIR,
+		},
+		vfs.Dirent{
+			Name: "..",
+			Type: linux.DT_DIR,
+		},
+		vfs.Dirent{
+			Name: "lost+found",
+			Type: linux.DT_DIR,
+		},
+		vfs.Dirent{
+			Name: "file.txt",
+			Type: linux.DT_REG,
+		},
+		vfs.Dirent{
+			Name: "bigfile.txt",
+			Type: linux.DT_REG,
+		},
+		vfs.Dirent{
+			Name: "symlink.txt",
+			Type: linux.DT_LNK,
+		},
+	}
+	tests := []iterDirentTest{
+		{
+			name:  "ext4 root dir iteration",
+			image: ext4ImagePath,
+			path:  "/",
+			want:  wantDirents,
+		},
+		{
+			name:  "ext3 root dir iteration",
+			image: ext3ImagePath,
+			path:  "/",
+			want:  wantDirents,
+		},
+		{
+			name:  "ext2 root dir iteration",
+			image: ext2ImagePath,
+			path:  "/",
+			want:  wantDirents,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			ctx, vfsfs, root, tearDown, err := setUp(t, test.image)
+			if err != nil {
+				t.Fatalf("setUp failed: %v", err)
+			}
+			defer tearDown()
+
+			fd, err := vfsfs.OpenAt(
+				ctx,
+				auth.CredentialsFromContext(ctx),
+				&vfs.PathOperation{Root: *root, Start: *root, Pathname: test.path},
+				&vfs.OpenOptions{},
+			)
+			if err != nil {
+				t.Fatalf("vfsfs.OpenAt failed: %v", err)
+			}
+
+			cb := &iterDirentsCb{}
+			if err = fd.Impl().IterDirents(ctx, cb); err != nil {
+				t.Fatalf("dir fd.IterDirents() failed: %v", err)
+			}
+
+			sort.Slice(cb.dirents, func(i int, j int) bool { return cb.dirents[i].Name < cb.dirents[j].Name })
+			sort.Slice(test.want, func(i int, j int) bool { return test.want[i].Name < test.want[j].Name })
+
+			// Ignore the inode number and offset of dirents because those are likely to
+			// change as the underlying image changes.
+			cmpIgnoreFields := cmp.FilterPath(func(p cmp.Path) bool {
+				return p.String() == "Ino" || p.String() == "Off"
+			}, cmp.Ignore())
+			if diff := cmp.Diff(cb.dirents, test.want, cmpIgnoreFields); diff != "" {
+				t.Errorf("dirents mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
+// TestRootDir tests that the root directory inode is correctly initialized and
+// returned from setUp.
+func TestRootDir(t *testing.T) {
+	type inodeProps struct {
+		Mode      linux.FileMode
+		UID       auth.KUID
+		GID       auth.KGID
+		Size      uint64
+		InodeSize uint16
+		Links     uint16
+		Flags     disklayout.InodeFlags
+	}
+
+	type rootDirTest struct {
+		name      string
+		image     string
+		wantInode inodeProps
+	}
+
+	tests := []rootDirTest{
+		{
+			name:  "ext4 root dir",
+			image: ext4ImagePath,
+			wantInode: inodeProps{
+				Mode:      linux.ModeDirectory | 0755,
+				Size:      0x400,
+				InodeSize: 0x80,
+				Links:     3,
+				Flags:     disklayout.InodeFlags{Extents: true},
+			},
+		},
+		{
+			name:  "ext3 root dir",
+			image: ext3ImagePath,
+			wantInode: inodeProps{
+				Mode:      linux.ModeDirectory | 0755,
+				Size:      0x400,
+				InodeSize: 0x80,
+				Links:     3,
+			},
+		},
+		{
+			name:  "ext2 root dir",
+			image: ext2ImagePath,
+			wantInode: inodeProps{
+				Mode:      linux.ModeDirectory | 0755,
+				Size:      0x400,
+				InodeSize: 0x80,
+				Links:     3,
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			_, _, vd, tearDown, err := setUp(t, test.image)
+			if err != nil {
+				t.Fatalf("setUp failed: %v", err)
+			}
+			defer tearDown()
+
+			d, ok := vd.Dentry().Impl().(*dentry)
+			if !ok {
+				t.Fatalf("ext dentry of incorrect type: %T", vd.Dentry().Impl())
+			}
+
+			// Offload inode contents into local structs for comparison.
+			gotInode := inodeProps{
+				Mode:      d.inode.diskInode.Mode(),
+				UID:       d.inode.diskInode.UID(),
+				GID:       d.inode.diskInode.GID(),
+				Size:      d.inode.diskInode.Size(),
+				InodeSize: d.inode.diskInode.InodeSize(),
+				Links:     d.inode.diskInode.LinksCount(),
+				Flags:     d.inode.diskInode.Flags(),
+			}
+
+			if diff := cmp.Diff(gotInode, test.wantInode); diff != "" {
+				t.Errorf("inode mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
+// TestFilesystemInit tests that the filesystem superblock and block group
+// descriptors are correctly read in and initialized.
+func TestFilesystemInit(t *testing.T) {
+	// sb only contains the immutable properties of the superblock.
+	type sb struct {
+		InodesCount      uint32
+		BlocksCount      uint64
+		MaxMountCount    uint16
+		FirstDataBlock   uint32
+		BlockSize        uint64
+		BlocksPerGroup   uint32
+		ClusterSize      uint64
+		ClustersPerGroup uint32
+		InodeSize        uint16
+		InodesPerGroup   uint32
+		BgDescSize       uint16
+		Magic            uint16
+		Revision         disklayout.SbRevision
+		CompatFeatures   disklayout.CompatFeatures
+		IncompatFeatures disklayout.IncompatFeatures
+		RoCompatFeatures disklayout.RoCompatFeatures
+	}
+
+	// bg only contains the immutable properties of the block group descriptor.
+	type bg struct {
+		InodeTable      uint64
+		BlockBitmap     uint64
+		InodeBitmap     uint64
+		ExclusionBitmap uint64
+		Flags           disklayout.BGFlags
+	}
+
+	type fsInitTest struct {
+		name    string
+		image   string
+		wantSb  sb
+		wantBgs []bg
+	}
+
+	tests := []fsInitTest{
+		{
+			name:  "ext4 filesystem init",
+			image: ext4ImagePath,
+			wantSb: sb{
+				InodesCount:      0x10,
+				BlocksCount:      0x40,
+				MaxMountCount:    0xffff,
+				FirstDataBlock:   0x1,
+				BlockSize:        0x400,
+				BlocksPerGroup:   0x2000,
+				ClusterSize:      0x400,
+				ClustersPerGroup: 0x2000,
+				InodeSize:        0x80,
+				InodesPerGroup:   0x10,
+				BgDescSize:       0x40,
+				Magic:            linux.EXT_SUPER_MAGIC,
+				Revision:         disklayout.DynamicRev,
+				CompatFeatures: disklayout.CompatFeatures{
+					ExtAttr:     true,
+					ResizeInode: true,
+					DirIndex:    true,
+				},
+				IncompatFeatures: disklayout.IncompatFeatures{
+					DirentFileType: true,
+					Extents:        true,
+					Is64Bit:        true,
+					FlexBg:         true,
+				},
+				RoCompatFeatures: disklayout.RoCompatFeatures{
+					Sparse:       true,
+					LargeFile:    true,
+					HugeFile:     true,
+					DirNlink:     true,
+					ExtraIsize:   true,
+					MetadataCsum: true,
+				},
+			},
+			wantBgs: []bg{
+				{
+					InodeTable:  0x23,
+					BlockBitmap: 0x3,
+					InodeBitmap: 0x13,
+					Flags: disklayout.BGFlags{
+						InodeZeroed: true,
+					},
+				},
+			},
+		},
+		{
+			name:  "ext3 filesystem init",
+			image: ext3ImagePath,
+			wantSb: sb{
+				InodesCount:      0x10,
+				BlocksCount:      0x40,
+				MaxMountCount:    0xffff,
+				FirstDataBlock:   0x1,
+				BlockSize:        0x400,
+				BlocksPerGroup:   0x2000,
+				ClusterSize:      0x400,
+				ClustersPerGroup: 0x2000,
+				InodeSize:        0x80,
+				InodesPerGroup:   0x10,
+				BgDescSize:       0x20,
+				Magic:            linux.EXT_SUPER_MAGIC,
+				Revision:         disklayout.DynamicRev,
+				CompatFeatures: disklayout.CompatFeatures{
+					ExtAttr:     true,
+					ResizeInode: true,
+					DirIndex:    true,
+				},
+				IncompatFeatures: disklayout.IncompatFeatures{
+					DirentFileType: true,
+				},
+				RoCompatFeatures: disklayout.RoCompatFeatures{
+					Sparse:    true,
+					LargeFile: true,
+				},
+			},
+			wantBgs: []bg{
+				{
+					InodeTable:  0x5,
+					BlockBitmap: 0x3,
+					InodeBitmap: 0x4,
+					Flags: disklayout.BGFlags{
+						InodeZeroed: true,
+					},
+				},
+			},
+		},
+		{
+			name:  "ext2 filesystem init",
+			image: ext2ImagePath,
+			wantSb: sb{
+				InodesCount:      0x10,
+				BlocksCount:      0x40,
+				MaxMountCount:    0xffff,
+				FirstDataBlock:   0x1,
+				BlockSize:        0x400,
+				BlocksPerGroup:   0x2000,
+				ClusterSize:      0x400,
+				ClustersPerGroup: 0x2000,
+				InodeSize:        0x80,
+				InodesPerGroup:   0x10,
+				BgDescSize:       0x20,
+				Magic:            linux.EXT_SUPER_MAGIC,
+				Revision:         disklayout.DynamicRev,
+				CompatFeatures: disklayout.CompatFeatures{
+					ExtAttr:     true,
+					ResizeInode: true,
+					DirIndex:    true,
+				},
+				IncompatFeatures: disklayout.IncompatFeatures{
+					DirentFileType: true,
+				},
+				RoCompatFeatures: disklayout.RoCompatFeatures{
+					Sparse:    true,
+					LargeFile: true,
+				},
+			},
+			wantBgs: []bg{
+				{
+					InodeTable:  0x5,
+					BlockBitmap: 0x3,
+					InodeBitmap: 0x4,
+					Flags: disklayout.BGFlags{
+						InodeZeroed: true,
+					},
+				},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			_, _, vd, tearDown, err := setUp(t, test.image)
+			if err != nil {
+				t.Fatalf("setUp failed: %v", err)
+			}
+			defer tearDown()
+
+			fs, ok := vd.Mount().Filesystem().Impl().(*filesystem)
+			if !ok {
+				t.Fatalf("ext filesystem of incorrect type: %T", vd.Mount().Filesystem().Impl())
+			}
+
+			// Offload superblock and block group descriptors contents into
+			// local structs for comparison.
+			totalFreeInodes := uint32(0)
+			totalFreeBlocks := uint64(0)
+			gotSb := sb{
+				InodesCount:      fs.sb.InodesCount(),
+				BlocksCount:      fs.sb.BlocksCount(),
+				MaxMountCount:    fs.sb.MaxMountCount(),
+				FirstDataBlock:   fs.sb.FirstDataBlock(),
+				BlockSize:        fs.sb.BlockSize(),
+				BlocksPerGroup:   fs.sb.BlocksPerGroup(),
+				ClusterSize:      fs.sb.ClusterSize(),
+				ClustersPerGroup: fs.sb.ClustersPerGroup(),
+				InodeSize:        fs.sb.InodeSize(),
+				InodesPerGroup:   fs.sb.InodesPerGroup(),
+				BgDescSize:       fs.sb.BgDescSize(),
+				Magic:            fs.sb.Magic(),
+				Revision:         fs.sb.Revision(),
+				CompatFeatures:   fs.sb.CompatibleFeatures(),
+				IncompatFeatures: fs.sb.IncompatibleFeatures(),
+				RoCompatFeatures: fs.sb.ReadOnlyCompatibleFeatures(),
+			}
+			gotNumBgs := len(fs.bgs)
+			gotBgs := make([]bg, gotNumBgs)
+			for i := 0; i < gotNumBgs; i++ {
+				gotBgs[i].InodeTable = fs.bgs[i].InodeTable()
+				gotBgs[i].BlockBitmap = fs.bgs[i].BlockBitmap()
+				gotBgs[i].InodeBitmap = fs.bgs[i].InodeBitmap()
+				gotBgs[i].ExclusionBitmap = fs.bgs[i].ExclusionBitmap()
+				gotBgs[i].Flags = fs.bgs[i].Flags()
+
+				totalFreeInodes += fs.bgs[i].FreeInodesCount()
+				totalFreeBlocks += uint64(fs.bgs[i].FreeBlocksCount())
+			}
+
+			if diff := cmp.Diff(gotSb, test.wantSb); diff != "" {
+				t.Errorf("superblock mismatch (-want +got):\n%s", diff)
+			}
+
+			if diff := cmp.Diff(gotBgs, test.wantBgs); diff != "" {
+				t.Errorf("block group descriptors mismatch (-want +got):\n%s", diff)
+			}
+
+			if diff := cmp.Diff(totalFreeInodes, fs.sb.FreeInodesCount()); diff != "" {
+				t.Errorf("total free inodes mismatch (-want +got):\n%s", diff)
+			}
+
+			if diff := cmp.Diff(totalFreeBlocks, fs.sb.FreeBlocksCount()); diff != "" {
+				t.Errorf("total free blocks mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
diff --git a/pkg/sentry/fsimpl/ext/extent_file.go b/pkg/sentry/fsimpl/ext/extent_file.go
new file mode 100644
index 000000000..38b68a2d3
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/extent_file.go
@@ -0,0 +1,237 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ext
+
+import (
+	"io"
+	"sort"
+
+	"gvisor.dev/gvisor/pkg/binary"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/ext/disklayout"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// extentFile is a type of regular file which uses extents to store file data.
+type extentFile struct {
+	regFile regularFile
+
+	// root is the root extent node. This lives in the 60 byte diskInode.Data().
+	// Immutable.
+	root disklayout.ExtentNode
+}
+
+// Compiles only if extentFile implements io.ReaderAt.
+var _ io.ReaderAt = (*extentFile)(nil)
+
+// newExtentFile is the extent file constructor. It reads the entire extent
+// tree into memory.
+// TODO(b/134676337): Build extent tree on demand to reduce memory usage.
+func newExtentFile(regFile regularFile) (*extentFile, error) {
+	file := &extentFile{regFile: regFile}
+	file.regFile.impl = file
+	err := file.buildExtTree()
+	if err != nil {
+		return nil, err
+	}
+	return file, nil
+}
+
+// buildExtTree builds the extent tree by reading it from disk by doing
+// running a simple DFS. It first reads the root node from the inode struct in
+// memory. Then it recursively builds the rest of the tree by reading it off
+// disk.
+//
+// Precondition: inode flag InExtents must be set.
+func (f *extentFile) buildExtTree() error {
+	rootNodeData := f.regFile.inode.diskInode.Data()
+
+	binary.Unmarshal(rootNodeData[:disklayout.ExtentStructsSize], binary.LittleEndian, &f.root.Header)
+
+	// Root node can not have more than 4 entries: 60 bytes = 1 header + 4 entries.
+	if f.root.Header.NumEntries > 4 {
+		// read(2) specifies that EINVAL should be returned if the file is unsuitable
+		// for reading.
+		return syserror.EINVAL
+	}
+
+	f.root.Entries = make([]disklayout.ExtentEntryPair, f.root.Header.NumEntries)
+	for i, off := uint16(0), disklayout.ExtentStructsSize; i < f.root.Header.NumEntries; i, off = i+1, off+disklayout.ExtentStructsSize {
+		var curEntry disklayout.ExtentEntry
+		if f.root.Header.Height == 0 {
+			// Leaf node.
+			curEntry = &disklayout.Extent{}
+		} else {
+			// Internal node.
+			curEntry = &disklayout.ExtentIdx{}
+		}
+		binary.Unmarshal(rootNodeData[off:off+disklayout.ExtentStructsSize], binary.LittleEndian, curEntry)
+		f.root.Entries[i].Entry = curEntry
+	}
+
+	// If this node is internal, perform DFS.
+	if f.root.Header.Height > 0 {
+		for i := uint16(0); i < f.root.Header.NumEntries; i++ {
+			var err error
+			if f.root.Entries[i].Node, err = f.buildExtTreeFromDisk(f.root.Entries[i].Entry); err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
+}
+
+// buildExtTreeFromDisk reads the extent tree nodes from disk and recursively
+// builds the tree. Performs a simple DFS. It returns the ExtentNode pointed to
+// by the ExtentEntry.
+func (f *extentFile) buildExtTreeFromDisk(entry disklayout.ExtentEntry) (*disklayout.ExtentNode, error) {
+	var header disklayout.ExtentHeader
+	off := entry.PhysicalBlock() * f.regFile.inode.blkSize
+	err := readFromDisk(f.regFile.inode.dev, int64(off), &header)
+	if err != nil {
+		return nil, err
+	}
+
+	entries := make([]disklayout.ExtentEntryPair, header.NumEntries)
+	for i, off := uint16(0), off+disklayout.ExtentStructsSize; i < header.NumEntries; i, off = i+1, off+disklayout.ExtentStructsSize {
+		var curEntry disklayout.ExtentEntry
+		if header.Height == 0 {
+			// Leaf node.
+			curEntry = &disklayout.Extent{}
+		} else {
+			// Internal node.
+			curEntry = &disklayout.ExtentIdx{}
+		}
+
+		err := readFromDisk(f.regFile.inode.dev, int64(off), curEntry)
+		if err != nil {
+			return nil, err
+		}
+		entries[i].Entry = curEntry
+	}
+
+	// If this node is internal, perform DFS.
+	if header.Height > 0 {
+		for i := uint16(0); i < header.NumEntries; i++ {
+			var err error
+			entries[i].Node, err = f.buildExtTreeFromDisk(entries[i].Entry)
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+
+	return &disklayout.ExtentNode{header, entries}, nil
+}
+
+// ReadAt implements io.ReaderAt.ReadAt.
+func (f *extentFile) ReadAt(dst []byte, off int64) (int, error) {
+	if len(dst) == 0 {
+		return 0, nil
+	}
+
+	if off < 0 {
+		return 0, syserror.EINVAL
+	}
+
+	if uint64(off) >= f.regFile.inode.diskInode.Size() {
+		return 0, io.EOF
+	}
+
+	n, err := f.read(&f.root, uint64(off), dst)
+	if n < len(dst) && err == nil {
+		err = io.EOF
+	}
+	return n, err
+}
+
+// read is the recursive step of extentFile.ReadAt which traverses the extent
+// tree from the node passed and reads file data.
+func (f *extentFile) read(node *disklayout.ExtentNode, off uint64, dst []byte) (int, error) {
+	// Perform a binary search for the node covering bytes starting at r.fileOff.
+	// A highly fragmented filesystem can have upto 340 entries and so linear
+	// search should be avoided. Finds the first entry which does not cover the
+	// file block we want and subtracts 1 to get the desired index.
+	fileBlk := uint32(off / f.regFile.inode.blkSize)
+	n := len(node.Entries)
+	found := sort.Search(n, func(i int) bool {
+		return node.Entries[i].Entry.FileBlock() > fileBlk
+	}) - 1
+
+	// We should be in this recursive step only if the data we want exists under
+	// the current node.
+	if found < 0 {
+		panic("searching for a file block in an extent entry which does not cover it")
+	}
+
+	read := 0
+	toRead := len(dst)
+	var curR int
+	var err error
+	for i := found; i < n && read < toRead; i++ {
+		if node.Header.Height == 0 {
+			curR, err = f.readFromExtent(node.Entries[i].Entry.(*disklayout.Extent), off, dst[read:])
+		} else {
+			curR, err = f.read(node.Entries[i].Node, off, dst[read:])
+		}
+
+		read += curR
+		off += uint64(curR)
+		if err != nil {
+			return read, err
+		}
+	}
+
+	return read, nil
+}
+
+// readFromExtent reads file data from the extent. It takes advantage of the
+// sequential nature of extents and reads file data from multiple blocks in one
+// call.
+//
+// A non-nil error indicates that this is a partial read and there is probably
+// more to read from this extent. The caller should propagate the error upward
+// and not move to the next extent in the tree.
+//
+// A subsequent call to extentReader.Read should continue reading from where we
+// left off as expected.
+func (f *extentFile) readFromExtent(ex *disklayout.Extent, off uint64, dst []byte) (int, error) {
+	curFileBlk := uint32(off / f.regFile.inode.blkSize)
+	exFirstFileBlk := ex.FileBlock()
+	exLastFileBlk := exFirstFileBlk + uint32(ex.Length) // This is exclusive.
+
+	// We should be in this recursive step only if the data we want exists under
+	// the current extent.
+	if curFileBlk < exFirstFileBlk || exLastFileBlk <= curFileBlk {
+		panic("searching for a file block in an extent which does not cover it")
+	}
+
+	curPhyBlk := uint64(curFileBlk-exFirstFileBlk) + ex.PhysicalBlock()
+	readStart := curPhyBlk*f.regFile.inode.blkSize + (off % f.regFile.inode.blkSize)
+
+	endPhyBlk := ex.PhysicalBlock() + uint64(ex.Length)
+	extentEnd := endPhyBlk * f.regFile.inode.blkSize // This is exclusive.
+
+	toRead := int(extentEnd - readStart)
+	if len(dst) < toRead {
+		toRead = len(dst)
+	}
+
+	n, _ := f.regFile.inode.dev.ReadAt(dst[:toRead], int64(readStart))
+	if n < toRead {
+		return n, syserror.EIO
+	}
+	return n, nil
+}
diff --git a/pkg/sentry/fsimpl/ext/extent_test.go b/pkg/sentry/fsimpl/ext/extent_test.go
new file mode 100644
index 000000000..42d0a484b
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/extent_test.go
@@ -0,0 +1,265 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ext
+
+import (
+	"bytes"
+	"math/rand"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+	"gvisor.dev/gvisor/pkg/binary"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/ext/disklayout"
+)
+
+const (
+	// mockExtentBlkSize is the mock block size used for testing.
+	// No block has more than 1 header + 4 entries.
+	mockExtentBlkSize = uint64(64)
+)
+
+// The tree described below looks like:
+//
+//            0.{Head}[Idx][Idx]
+//                     /     \
+//                    /       \
+//      1.{Head}[Ext][Ext]  2.{Head}[Idx]
+//               /    |               \
+//           [Phy]  [Phy, Phy]    3.{Head}[Ext]
+//                                          |
+//                                    [Phy, Phy, Phy]
+//
+// Legend:
+//   - Head = ExtentHeader
+//   - Idx  = ExtentIdx
+//   - Ext  = Extent
+//   - Phy  = Physical Block
+//
+// Please note that ext4 might not construct extent trees looking like this.
+// This is purely for testing the tree traversal logic.
+var (
+	node3 = &disklayout.ExtentNode{
+		Header: disklayout.ExtentHeader{
+			Magic:      disklayout.ExtentMagic,
+			NumEntries: 1,
+			MaxEntries: 4,
+			Height:     0,
+		},
+		Entries: []disklayout.ExtentEntryPair{
+			{
+				Entry: &disklayout.Extent{
+					FirstFileBlock: 3,
+					Length:         3,
+					StartBlockLo:   6,
+				},
+				Node: nil,
+			},
+		},
+	}
+
+	node2 = &disklayout.ExtentNode{
+		Header: disklayout.ExtentHeader{
+			Magic:      disklayout.ExtentMagic,
+			NumEntries: 1,
+			MaxEntries: 4,
+			Height:     1,
+		},
+		Entries: []disklayout.ExtentEntryPair{
+			{
+				Entry: &disklayout.ExtentIdx{
+					FirstFileBlock: 3,
+					ChildBlockLo:   2,
+				},
+				Node: node3,
+			},
+		},
+	}
+
+	node1 = &disklayout.ExtentNode{
+		Header: disklayout.ExtentHeader{
+			Magic:      disklayout.ExtentMagic,
+			NumEntries: 2,
+			MaxEntries: 4,
+			Height:     0,
+		},
+		Entries: []disklayout.ExtentEntryPair{
+			{
+				Entry: &disklayout.Extent{
+					FirstFileBlock: 0,
+					Length:         1,
+					StartBlockLo:   3,
+				},
+				Node: nil,
+			},
+			{
+				Entry: &disklayout.Extent{
+					FirstFileBlock: 1,
+					Length:         2,
+					StartBlockLo:   4,
+				},
+				Node: nil,
+			},
+		},
+	}
+
+	node0 = &disklayout.ExtentNode{
+		Header: disklayout.ExtentHeader{
+			Magic:      disklayout.ExtentMagic,
+			NumEntries: 2,
+			MaxEntries: 4,
+			Height:     2,
+		},
+		Entries: []disklayout.ExtentEntryPair{
+			{
+				Entry: &disklayout.ExtentIdx{
+					FirstFileBlock: 0,
+					ChildBlockLo:   0,
+				},
+				Node: node1,
+			},
+			{
+				Entry: &disklayout.ExtentIdx{
+					FirstFileBlock: 3,
+					ChildBlockLo:   1,
+				},
+				Node: node2,
+			},
+		},
+	}
+)
+
+// TestExtentReader stress tests extentReader functionality. It performs random
+// length reads from all possible positions in the extent tree.
+func TestExtentReader(t *testing.T) {
+	mockExtentFile, want := extentTreeSetUp(t, node0)
+	n := len(want)
+
+	for from := 0; from < n; from++ {
+		got := make([]byte, n-from)
+
+		if read, err := mockExtentFile.ReadAt(got, int64(from)); err != nil {
+			t.Fatalf("file read operation from offset %d to %d only read %d bytes: %v", from, n, read, err)
+		}
+
+		if diff := cmp.Diff(got, want[from:]); diff != "" {
+			t.Fatalf("file data from offset %d to %d mismatched (-want +got):\n%s", from, n, diff)
+		}
+	}
+}
+
+// TestBuildExtentTree tests the extent tree building logic.
+func TestBuildExtentTree(t *testing.T) {
+	mockExtentFile, _ := extentTreeSetUp(t, node0)
+
+	opt := cmpopts.IgnoreUnexported(disklayout.ExtentIdx{}, disklayout.ExtentHeader{})
+	if diff := cmp.Diff(&mockExtentFile.root, node0, opt); diff != "" {
+		t.Errorf("extent tree mismatch (-want +got):\n%s", diff)
+	}
+}
+
+// extentTreeSetUp writes the passed extent tree to a mock disk as an extent
+// tree. It also constucts a mock extent file with the same tree built in it.
+// It also writes random data file data and returns it.
+func extentTreeSetUp(t *testing.T, root *disklayout.ExtentNode) (*extentFile, []byte) {
+	t.Helper()
+
+	mockDisk := make([]byte, mockExtentBlkSize*10)
+	mockExtentFile := &extentFile{
+		regFile: regularFile{
+			inode: inode{
+				diskInode: &disklayout.InodeNew{
+					InodeOld: disklayout.InodeOld{
+						SizeLo: uint32(mockExtentBlkSize) * getNumPhyBlks(root),
+					},
+				},
+				blkSize: mockExtentBlkSize,
+				dev:     bytes.NewReader(mockDisk),
+			},
+		},
+	}
+
+	fileData := writeTree(&mockExtentFile.regFile.inode, mockDisk, node0, mockExtentBlkSize)
+
+	if err := mockExtentFile.buildExtTree(); err != nil {
+		t.Fatalf("inode.buildExtTree failed: %v", err)
+	}
+	return mockExtentFile, fileData
+}
+
+// writeTree writes the tree represented by `root` to the inode and disk. It
+// also writes random file data on disk.
+func writeTree(in *inode, disk []byte, root *disklayout.ExtentNode, mockExtentBlkSize uint64) []byte {
+	rootData := binary.Marshal(nil, binary.LittleEndian, root.Header)
+	for _, ep := range root.Entries {
+		rootData = binary.Marshal(rootData, binary.LittleEndian, ep.Entry)
+	}
+
+	copy(in.diskInode.Data(), rootData)
+
+	var fileData []byte
+	for _, ep := range root.Entries {
+		if root.Header.Height == 0 {
+			fileData = append(fileData, writeFileDataToExtent(disk, ep.Entry.(*disklayout.Extent))...)
+		} else {
+			fileData = append(fileData, writeTreeToDisk(disk, ep)...)
+		}
+	}
+	return fileData
+}
+
+// writeTreeToDisk is the recursive step for writeTree which writes the tree
+// on the disk only. Also writes random file data on disk.
+func writeTreeToDisk(disk []byte, curNode disklayout.ExtentEntryPair) []byte {
+	nodeData := binary.Marshal(nil, binary.LittleEndian, curNode.Node.Header)
+	for _, ep := range curNode.Node.Entries {
+		nodeData = binary.Marshal(nodeData, binary.LittleEndian, ep.Entry)
+	}
+
+	copy(disk[curNode.Entry.PhysicalBlock()*mockExtentBlkSize:], nodeData)
+
+	var fileData []byte
+	for _, ep := range curNode.Node.Entries {
+		if curNode.Node.Header.Height == 0 {
+			fileData = append(fileData, writeFileDataToExtent(disk, ep.Entry.(*disklayout.Extent))...)
+		} else {
+			fileData = append(fileData, writeTreeToDisk(disk, ep)...)
+		}
+	}
+	return fileData
+}
+
+// writeFileDataToExtent writes random bytes to the blocks on disk that the
+// passed extent points to.
+func writeFileDataToExtent(disk []byte, ex *disklayout.Extent) []byte {
+	phyExStartBlk := ex.PhysicalBlock()
+	phyExStartOff := phyExStartBlk * mockExtentBlkSize
+	phyExEndOff := phyExStartOff + uint64(ex.Length)*mockExtentBlkSize
+	rand.Read(disk[phyExStartOff:phyExEndOff])
+	return disk[phyExStartOff:phyExEndOff]
+}
+
+// getNumPhyBlks returns the number of physical blocks covered under the node.
+func getNumPhyBlks(node *disklayout.ExtentNode) uint32 {
+	var res uint32
+	for _, ep := range node.Entries {
+		if node.Header.Height == 0 {
+			res += uint32(ep.Entry.(*disklayout.Extent).Length)
+		} else {
+			res += getNumPhyBlks(ep.Node)
+		}
+	}
+	return res
+}
diff --git a/pkg/sentry/fsimpl/ext/file_description.go b/pkg/sentry/fsimpl/ext/file_description.go
new file mode 100644
index 000000000..d244cf1e7
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/file_description.go
@@ -0,0 +1,110 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ext
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/arch"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/usermem"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/waiter"
+)
+
+// fileDescription is embedded by ext implementations of
+// vfs.FileDescriptionImpl.
+type fileDescription struct {
+	vfsfd vfs.FileDescription
+
+	// flags is the same as vfs.OpenOptions.Flags which are passed to
+	// vfs.FilesystemImpl.OpenAt.
+	// TODO(b/134676337): syscalls like read(2), write(2), fchmod(2), fchown(2),
+	// fgetxattr(2), ioctl(2), mmap(2) should fail with EBADF if O_PATH is set.
+	// Only close(2), fstat(2), fstatfs(2) should work.
+	flags uint32
+}
+
+func (fd *fileDescription) filesystem() *filesystem {
+	return fd.vfsfd.VirtualDentry().Mount().Filesystem().Impl().(*filesystem)
+}
+
+func (fd *fileDescription) inode() *inode {
+	return fd.vfsfd.VirtualDentry().Dentry().Impl().(*dentry).inode
+}
+
+// OnClose implements vfs.FileDescriptionImpl.OnClose.
+func (fd *fileDescription) OnClose() error { return nil }
+
+// StatusFlags implements vfs.FileDescriptionImpl.StatusFlags.
+func (fd *fileDescription) StatusFlags(ctx context.Context) (uint32, error) {
+	return fd.flags, nil
+}
+
+// SetStatusFlags implements vfs.FileDescriptionImpl.SetStatusFlags.
+func (fd *fileDescription) SetStatusFlags(ctx context.Context, flags uint32) error {
+	// None of the flags settable by fcntl(F_SETFL) are supported, so this is a
+	// no-op.
+	return nil
+}
+
+// Stat implements vfs.FileDescriptionImpl.Stat.
+func (fd *fileDescription) Stat(ctx context.Context, opts vfs.StatOptions) (linux.Statx, error) {
+	var stat linux.Statx
+	fd.inode().statTo(&stat)
+	return stat, nil
+}
+
+// SetStat implements vfs.FileDescriptionImpl.SetStat.
+func (fd *fileDescription) SetStat(ctx context.Context, opts vfs.SetStatOptions) error {
+	if opts.Stat.Mask == 0 {
+		return nil
+	}
+	return syserror.EPERM
+}
+
+// SetStat implements vfs.FileDescriptionImpl.StatFS.
+func (fd *fileDescription) StatFS(ctx context.Context) (linux.Statfs, error) {
+	var stat linux.Statfs
+	fd.filesystem().statTo(&stat)
+	return stat, nil
+}
+
+// Readiness implements waiter.Waitable.Readiness analogously to
+// file_operations::poll == NULL in Linux.
+func (fd *fileDescription) Readiness(mask waiter.EventMask) waiter.EventMask {
+	// include/linux/poll.h:vfs_poll() => DEFAULT_POLLMASK
+	return waiter.EventIn | waiter.EventOut
+}
+
+// EventRegister implements waiter.Waitable.EventRegister analogously to
+// file_operations::poll == NULL in Linux.
+func (fd *fileDescription) EventRegister(e *waiter.Entry, mask waiter.EventMask) {}
+
+// EventUnregister implements waiter.Waitable.EventUnregister analogously to
+// file_operations::poll == NULL in Linux.
+func (fd *fileDescription) EventUnregister(e *waiter.Entry) {}
+
+// Sync implements vfs.FileDescriptionImpl.Sync.
+func (fd *fileDescription) Sync(ctx context.Context) error {
+	return nil
+}
+
+// Ioctl implements vfs.FileDescriptionImpl.Ioctl.
+func (fd *fileDescription) Ioctl(ctx context.Context, uio usermem.IO, args arch.SyscallArguments) (uintptr, error) {
+	// ioctl(2) specifies that ENOTTY must be returned if the file descriptor is
+	// not associated with a character special device (which is unimplemented).
+	return 0, syserror.ENOTTY
+}
diff --git a/pkg/sentry/fsimpl/ext/filesystem.go b/pkg/sentry/fsimpl/ext/filesystem.go
new file mode 100644
index 000000000..2d15e8aaf
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/filesystem.go
@@ -0,0 +1,443 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ext
+
+import (
+	"errors"
+	"io"
+	"sync"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/ext/disklayout"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+var (
+	// errResolveDirent indicates that the vfs.ResolvingPath.Component() does
+	// not exist on the dentry tree but does exist on disk. So it has to be read in
+	// using the in-memory dirent and added to the dentry tree. Usually indicates
+	// the need to lock filesystem.mu for writing.
+	errResolveDirent = errors.New("resolve path component using dirent")
+)
+
+// filesystem implements vfs.FilesystemImpl.
+type filesystem struct {
+	vfsfs vfs.Filesystem
+
+	// mu serializes changes to the Dentry tree.
+	mu sync.RWMutex
+
+	// dev represents the underlying fs device. It does not require protection
+	// because io.ReaderAt permits concurrent read calls to it. It translates to
+	// the pread syscall which passes on the read request directly to the device
+	// driver. Device drivers are intelligent in serving multiple concurrent read
+	// requests in the optimal order (taking locality into consideration).
+	dev io.ReaderAt
+
+	// inodeCache maps absolute inode numbers to the corresponding Inode struct.
+	// Inodes should be removed from this once their reference count hits 0.
+	//
+	// Protected by mu because most additions (see IterDirents) and all removals
+	// from this corresponds to a change in the dentry tree.
+	inodeCache map[uint32]*inode
+
+	// sb represents the filesystem superblock. Immutable after initialization.
+	sb disklayout.SuperBlock
+
+	// bgs represents all the block group descriptors for the filesystem.
+	// Immutable after initialization.
+	bgs []disklayout.BlockGroup
+}
+
+// Compiles only if filesystem implements vfs.FilesystemImpl.
+var _ vfs.FilesystemImpl = (*filesystem)(nil)
+
+// stepLocked resolves rp.Component() in parent directory vfsd. The write
+// parameter passed tells if the caller has acquired filesystem.mu for writing
+// or not. If set to true, an existing inode on disk can be added to the dentry
+// tree if not present already.
+//
+// stepLocked is loosely analogous to fs/namei.c:walk_component().
+//
+// Preconditions:
+//     - filesystem.mu must be locked (for writing if write param is true).
+//     - !rp.Done().
+//     - inode == vfsd.Impl().(*Dentry).inode.
+func stepLocked(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, inode *inode, write bool) (*vfs.Dentry, *inode, error) {
+	if !inode.isDir() {
+		return nil, nil, syserror.ENOTDIR
+	}
+	if err := inode.checkPermissions(rp.Credentials(), vfs.MayExec); err != nil {
+		return nil, nil, err
+	}
+
+	for {
+		nextVFSD, err := rp.ResolveComponent(vfsd)
+		if err != nil {
+			return nil, nil, err
+		}
+		if nextVFSD == nil {
+			// Since the Dentry tree is not the sole source of truth for extfs, if it's
+			// not in the Dentry tree, it might need to be pulled from disk.
+			childDirent, ok := inode.impl.(*directory).childMap[rp.Component()]
+			if !ok {
+				// The underlying inode does not exist on disk.
+				return nil, nil, syserror.ENOENT
+			}
+
+			if !write {
+				// filesystem.mu must be held for writing to add to the dentry tree.
+				return nil, nil, errResolveDirent
+			}
+
+			// Create and add the component's dirent to the dentry tree.
+			fs := rp.Mount().Filesystem().Impl().(*filesystem)
+			childInode, err := fs.getOrCreateInodeLocked(childDirent.diskDirent.Inode())
+			if err != nil {
+				return nil, nil, err
+			}
+			// incRef because this is being added to the dentry tree.
+			childInode.incRef()
+			child := newDentry(childInode)
+			vfsd.InsertChild(&child.vfsd, rp.Component())
+
+			// Continue as usual now that nextVFSD is not nil.
+			nextVFSD = &child.vfsd
+		}
+		nextInode := nextVFSD.Impl().(*dentry).inode
+		if nextInode.isSymlink() && rp.ShouldFollowSymlink() {
+			if err := rp.HandleSymlink(inode.impl.(*symlink).target); err != nil {
+				return nil, nil, err
+			}
+			continue
+		}
+		rp.Advance()
+		return nextVFSD, nextInode, nil
+	}
+}
+
+// walkLocked resolves rp to an existing file. The write parameter
+// passed tells if the caller has acquired filesystem.mu for writing or not.
+// If set to true, additions can be made to the dentry tree while walking.
+// If errResolveDirent is returned, the walk needs to be continued with an
+// upgraded filesystem.mu.
+//
+// walkLocked is loosely analogous to Linux's fs/namei.c:path_lookupat().
+//
+// Preconditions:
+//     - filesystem.mu must be locked (for writing if write param is true).
+func walkLocked(rp *vfs.ResolvingPath, write bool) (*vfs.Dentry, *inode, error) {
+	vfsd := rp.Start()
+	inode := vfsd.Impl().(*dentry).inode
+	for !rp.Done() {
+		var err error
+		vfsd, inode, err = stepLocked(rp, vfsd, inode, write)
+		if err != nil {
+			return nil, nil, err
+		}
+	}
+	if rp.MustBeDir() && !inode.isDir() {
+		return nil, nil, syserror.ENOTDIR
+	}
+	return vfsd, inode, nil
+}
+
+// walkParentLocked resolves all but the last path component of rp to an
+// existing directory. It does not check that the returned directory is
+// searchable by the provider of rp. The write parameter passed tells if the
+// caller has acquired filesystem.mu for writing or not. If set to true,
+// additions can be made to the dentry tree while walking.
+// If errResolveDirent is returned, the walk needs to be continued with an
+// upgraded filesystem.mu.
+//
+// walkParentLocked is loosely analogous to Linux's fs/namei.c:path_parentat().
+//
+// Preconditions:
+//     - filesystem.mu must be locked (for writing if write param is true).
+//     - !rp.Done().
+func walkParentLocked(rp *vfs.ResolvingPath, write bool) (*vfs.Dentry, *inode, error) {
+	vfsd := rp.Start()
+	inode := vfsd.Impl().(*dentry).inode
+	for !rp.Final() {
+		var err error
+		vfsd, inode, err = stepLocked(rp, vfsd, inode, write)
+		if err != nil {
+			return nil, nil, err
+		}
+	}
+	if !inode.isDir() {
+		return nil, nil, syserror.ENOTDIR
+	}
+	return vfsd, inode, nil
+}
+
+// walk resolves rp to an existing file. If parent is set to true, it resolves
+// the rp till the parent of the last component which should be an existing
+// directory. If parent is false then resolves rp entirely. Attemps to resolve
+// the path as far as it can with a read lock and upgrades the lock if needed.
+func (fs *filesystem) walk(rp *vfs.ResolvingPath, parent bool) (*vfs.Dentry, *inode, error) {
+	var (
+		vfsd  *vfs.Dentry
+		inode *inode
+		err   error
+	)
+
+	// Try walking with the hopes that all dentries have already been pulled out
+	// of disk. This reduces congestion (allows concurrent walks).
+	fs.mu.RLock()
+	if parent {
+		vfsd, inode, err = walkParentLocked(rp, false)
+	} else {
+		vfsd, inode, err = walkLocked(rp, false)
+	}
+	fs.mu.RUnlock()
+
+	if err == errResolveDirent {
+		// Upgrade lock and continue walking. Lock upgrading in the middle of the
+		// walk is fine as this is a read only filesystem.
+		fs.mu.Lock()
+		if parent {
+			vfsd, inode, err = walkParentLocked(rp, true)
+		} else {
+			vfsd, inode, err = walkLocked(rp, true)
+		}
+		fs.mu.Unlock()
+	}
+
+	return vfsd, inode, err
+}
+
+// getOrCreateInodeLocked gets the inode corresponding to the inode number passed in.
+// It creates a new one with the given inode number if one does not exist.
+// The caller must increment the ref count if adding this to the dentry tree.
+//
+// Precondition: must be holding fs.mu for writing.
+func (fs *filesystem) getOrCreateInodeLocked(inodeNum uint32) (*inode, error) {
+	if in, ok := fs.inodeCache[inodeNum]; ok {
+		return in, nil
+	}
+
+	in, err := newInode(fs, inodeNum)
+	if err != nil {
+		return nil, err
+	}
+
+	fs.inodeCache[inodeNum] = in
+	return in, nil
+}
+
+// statTo writes the statfs fields to the output parameter.
+func (fs *filesystem) statTo(stat *linux.Statfs) {
+	stat.Type = uint64(fs.sb.Magic())
+	stat.BlockSize = int64(fs.sb.BlockSize())
+	stat.Blocks = fs.sb.BlocksCount()
+	stat.BlocksFree = fs.sb.FreeBlocksCount()
+	stat.BlocksAvailable = fs.sb.FreeBlocksCount()
+	stat.Files = uint64(fs.sb.InodesCount())
+	stat.FilesFree = uint64(fs.sb.FreeInodesCount())
+	stat.NameLength = disklayout.MaxFileName
+	stat.FragmentSize = int64(fs.sb.BlockSize())
+	// TODO(b/134676337): Set Statfs.Flags and Statfs.FSID.
+}
+
+// GetDentryAt implements vfs.FilesystemImpl.GetDentryAt.
+func (fs *filesystem) GetDentryAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.GetDentryOptions) (*vfs.Dentry, error) {
+	vfsd, inode, err := fs.walk(rp, false)
+	if err != nil {
+		return nil, err
+	}
+
+	if opts.CheckSearchable {
+		if !inode.isDir() {
+			return nil, syserror.ENOTDIR
+		}
+		if err := inode.checkPermissions(rp.Credentials(), vfs.MayExec); err != nil {
+			return nil, err
+		}
+	}
+
+	inode.incRef()
+	return vfsd, nil
+}
+
+// OpenAt implements vfs.FilesystemImpl.OpenAt.
+func (fs *filesystem) OpenAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
+	vfsd, inode, err := fs.walk(rp, false)
+	if err != nil {
+		return nil, err
+	}
+
+	// EROFS is returned if write access is needed.
+	if vfs.MayWriteFileWithOpenFlags(opts.Flags) || opts.Flags&(linux.O_CREAT|linux.O_EXCL|linux.O_TMPFILE) != 0 {
+		return nil, syserror.EROFS
+	}
+	return inode.open(rp, vfsd, opts.Flags)
+}
+
+// ReadlinkAt implements vfs.FilesystemImpl.ReadlinkAt.
+func (fs *filesystem) ReadlinkAt(ctx context.Context, rp *vfs.ResolvingPath) (string, error) {
+	_, inode, err := fs.walk(rp, false)
+	if err != nil {
+		return "", err
+	}
+	symlink, ok := inode.impl.(*symlink)
+	if !ok {
+		return "", syserror.EINVAL
+	}
+	return symlink.target, nil
+}
+
+// StatAt implements vfs.FilesystemImpl.StatAt.
+func (fs *filesystem) StatAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.StatOptions) (linux.Statx, error) {
+	_, inode, err := fs.walk(rp, false)
+	if err != nil {
+		return linux.Statx{}, err
+	}
+	var stat linux.Statx
+	inode.statTo(&stat)
+	return stat, nil
+}
+
+// StatFSAt implements vfs.FilesystemImpl.StatFSAt.
+func (fs *filesystem) StatFSAt(ctx context.Context, rp *vfs.ResolvingPath) (linux.Statfs, error) {
+	if _, _, err := fs.walk(rp, false); err != nil {
+		return linux.Statfs{}, err
+	}
+
+	var stat linux.Statfs
+	fs.statTo(&stat)
+	return stat, nil
+}
+
+// Release implements vfs.FilesystemImpl.Release.
+func (fs *filesystem) Release() {}
+
+// Sync implements vfs.FilesystemImpl.Sync.
+func (fs *filesystem) Sync(ctx context.Context) error {
+	// This is a readonly filesystem for now.
+	return nil
+}
+
+// The vfs.FilesystemImpl functions below return EROFS because their respective
+// man pages say that EROFS must be returned if the path resolves to a file on
+// this read-only filesystem.
+
+// LinkAt implements vfs.FilesystemImpl.LinkAt.
+func (fs *filesystem) LinkAt(ctx context.Context, rp *vfs.ResolvingPath, vd vfs.VirtualDentry) error {
+	if rp.Done() {
+		return syserror.EEXIST
+	}
+
+	if _, _, err := fs.walk(rp, true); err != nil {
+		return err
+	}
+
+	return syserror.EROFS
+}
+
+// MkdirAt implements vfs.FilesystemImpl.MkdirAt.
+func (fs *filesystem) MkdirAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.MkdirOptions) error {
+	if rp.Done() {
+		return syserror.EEXIST
+	}
+
+	if _, _, err := fs.walk(rp, true); err != nil {
+		return err
+	}
+
+	return syserror.EROFS
+}
+
+// MknodAt implements vfs.FilesystemImpl.MknodAt.
+func (fs *filesystem) MknodAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.MknodOptions) error {
+	if rp.Done() {
+		return syserror.EEXIST
+	}
+
+	_, _, err := fs.walk(rp, true)
+	if err != nil {
+		return err
+	}
+
+	return syserror.EROFS
+}
+
+// RenameAt implements vfs.FilesystemImpl.RenameAt.
+func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, vd vfs.VirtualDentry, opts vfs.RenameOptions) error {
+	if rp.Done() {
+		return syserror.ENOENT
+	}
+
+	_, _, err := fs.walk(rp, false)
+	if err != nil {
+		return err
+	}
+
+	return syserror.EROFS
+}
+
+// RmdirAt implements vfs.FilesystemImpl.RmdirAt.
+func (fs *filesystem) RmdirAt(ctx context.Context, rp *vfs.ResolvingPath) error {
+	_, inode, err := fs.walk(rp, false)
+	if err != nil {
+		return err
+	}
+
+	if !inode.isDir() {
+		return syserror.ENOTDIR
+	}
+
+	return syserror.EROFS
+}
+
+// SetStatAt implements vfs.FilesystemImpl.SetStatAt.
+func (fs *filesystem) SetStatAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.SetStatOptions) error {
+	_, _, err := fs.walk(rp, false)
+	if err != nil {
+		return err
+	}
+
+	return syserror.EROFS
+}
+
+// SymlinkAt implements vfs.FilesystemImpl.SymlinkAt.
+func (fs *filesystem) SymlinkAt(ctx context.Context, rp *vfs.ResolvingPath, target string) error {
+	if rp.Done() {
+		return syserror.EEXIST
+	}
+
+	_, _, err := fs.walk(rp, true)
+	if err != nil {
+		return err
+	}
+
+	return syserror.EROFS
+}
+
+// UnlinkAt implements vfs.FilesystemImpl.UnlinkAt.
+func (fs *filesystem) UnlinkAt(ctx context.Context, rp *vfs.ResolvingPath) error {
+	_, inode, err := fs.walk(rp, false)
+	if err != nil {
+		return err
+	}
+
+	if inode.isDir() {
+		return syserror.EISDIR
+	}
+
+	return syserror.EROFS
+}
diff --git a/pkg/sentry/fsimpl/ext/inode.go b/pkg/sentry/fsimpl/ext/inode.go
new file mode 100644
index 000000000..e6c847a71
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/inode.go
@@ -0,0 +1,219 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ext
+
+import (
+	"fmt"
+	"io"
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/ext/disklayout"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// inode represents an ext inode.
+//
+// inode uses the same inheritance pattern that pkg/sentry/vfs structures use.
+// This has been done to increase memory locality.
+//
+// Implementations:
+//    inode --
+//           |-- dir
+//           |-- symlink
+//           |-- regular--
+//                       |-- extent file
+//                       |-- block map file
+type inode struct {
+	// refs is a reference count. refs is accessed using atomic memory operations.
+	refs int64
+
+	// inodeNum is the inode number of this inode on disk. This is used to
+	// identify inodes within the ext filesystem.
+	inodeNum uint32
+
+	// dev represents the underlying device. Same as filesystem.dev.
+	dev io.ReaderAt
+
+	// blkSize is the fs data block size. Same as filesystem.sb.BlockSize().
+	blkSize uint64
+
+	// diskInode gives us access to the inode struct on disk. Immutable.
+	diskInode disklayout.Inode
+
+	// This is immutable. The first field of the implementations must have inode
+	// as the first field to ensure temporality.
+	impl interface{}
+}
+
+// incRef increments the inode ref count.
+func (in *inode) incRef() {
+	atomic.AddInt64(&in.refs, 1)
+}
+
+// tryIncRef tries to increment the ref count. Returns true if successful.
+func (in *inode) tryIncRef() bool {
+	for {
+		refs := atomic.LoadInt64(&in.refs)
+		if refs == 0 {
+			return false
+		}
+		if atomic.CompareAndSwapInt64(&in.refs, refs, refs+1) {
+			return true
+		}
+	}
+}
+
+// decRef decrements the inode ref count and releases the inode resources if
+// the ref count hits 0.
+//
+// Precondition: Must have locked fs.mu.
+func (in *inode) decRef(fs *filesystem) {
+	if refs := atomic.AddInt64(&in.refs, -1); refs == 0 {
+		delete(fs.inodeCache, in.inodeNum)
+	} else if refs < 0 {
+		panic("ext.inode.decRef() called without holding a reference")
+	}
+}
+
+// newInode is the inode constructor. Reads the inode off disk. Identifies
+// inodes based on the absolute inode number on disk.
+func newInode(fs *filesystem, inodeNum uint32) (*inode, error) {
+	if inodeNum == 0 {
+		panic("inode number 0 on ext filesystems is not possible")
+	}
+
+	inodeRecordSize := fs.sb.InodeSize()
+	var diskInode disklayout.Inode
+	if inodeRecordSize == disklayout.OldInodeSize {
+		diskInode = &disklayout.InodeOld{}
+	} else {
+		diskInode = &disklayout.InodeNew{}
+	}
+
+	// Calculate where the inode is actually placed.
+	inodesPerGrp := fs.sb.InodesPerGroup()
+	blkSize := fs.sb.BlockSize()
+	inodeTableOff := fs.bgs[getBGNum(inodeNum, inodesPerGrp)].InodeTable() * blkSize
+	inodeOff := inodeTableOff + uint64(uint32(inodeRecordSize)*getBGOff(inodeNum, inodesPerGrp))
+
+	if err := readFromDisk(fs.dev, int64(inodeOff), diskInode); err != nil {
+		return nil, err
+	}
+
+	// Build the inode based on its type.
+	inode := inode{
+		inodeNum:  inodeNum,
+		dev:       fs.dev,
+		blkSize:   blkSize,
+		diskInode: diskInode,
+	}
+
+	switch diskInode.Mode().FileType() {
+	case linux.ModeSymlink:
+		f, err := newSymlink(inode)
+		if err != nil {
+			return nil, err
+		}
+		return &f.inode, nil
+	case linux.ModeRegular:
+		f, err := newRegularFile(inode)
+		if err != nil {
+			return nil, err
+		}
+		return &f.inode, nil
+	case linux.ModeDirectory:
+		f, err := newDirectroy(inode, fs.sb.IncompatibleFeatures().DirentFileType)
+		if err != nil {
+			return nil, err
+		}
+		return &f.inode, nil
+	default:
+		// TODO(b/134676337): Return appropriate errors for sockets, pipes and devices.
+		return nil, syserror.EINVAL
+	}
+}
+
+// open creates and returns a file description for the dentry passed in.
+func (in *inode) open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, flags uint32) (*vfs.FileDescription, error) {
+	ats := vfs.AccessTypesForOpenFlags(flags)
+	if err := in.checkPermissions(rp.Credentials(), ats); err != nil {
+		return nil, err
+	}
+	switch in.impl.(type) {
+	case *regularFile:
+		var fd regularFileFD
+		fd.flags = flags
+		fd.vfsfd.Init(&fd, rp.Mount(), vfsd)
+		return &fd.vfsfd, nil
+	case *directory:
+		// Can't open directories writably. This check is not necessary for a read
+		// only filesystem but will be required when write is implemented.
+		if ats&vfs.MayWrite != 0 {
+			return nil, syserror.EISDIR
+		}
+		var fd directoryFD
+		fd.vfsfd.Init(&fd, rp.Mount(), vfsd)
+		fd.flags = flags
+		return &fd.vfsfd, nil
+	case *symlink:
+		if flags&linux.O_PATH == 0 {
+			// Can't open symlinks without O_PATH.
+			return nil, syserror.ELOOP
+		}
+		var fd symlinkFD
+		fd.flags = flags
+		fd.vfsfd.Init(&fd, rp.Mount(), vfsd)
+		return &fd.vfsfd, nil
+	default:
+		panic(fmt.Sprintf("unknown inode type: %T", in.impl))
+	}
+}
+
+func (in *inode) checkPermissions(creds *auth.Credentials, ats vfs.AccessTypes) error {
+	return vfs.GenericCheckPermissions(creds, ats, in.isDir(), uint16(in.diskInode.Mode()), in.diskInode.UID(), in.diskInode.GID())
+}
+
+// statTo writes the statx fields to the output parameter.
+func (in *inode) statTo(stat *linux.Statx) {
+	stat.Mask = linux.STATX_TYPE | linux.STATX_MODE | linux.STATX_NLINK |
+		linux.STATX_UID | linux.STATX_GID | linux.STATX_INO | linux.STATX_SIZE |
+		linux.STATX_ATIME | linux.STATX_CTIME | linux.STATX_MTIME
+	stat.Blksize = uint32(in.blkSize)
+	stat.Mode = uint16(in.diskInode.Mode())
+	stat.Nlink = uint32(in.diskInode.LinksCount())
+	stat.UID = uint32(in.diskInode.UID())
+	stat.GID = uint32(in.diskInode.GID())
+	stat.Ino = uint64(in.inodeNum)
+	stat.Size = in.diskInode.Size()
+	stat.Atime = in.diskInode.AccessTime().StatxTimestamp()
+	stat.Ctime = in.diskInode.ChangeTime().StatxTimestamp()
+	stat.Mtime = in.diskInode.ModificationTime().StatxTimestamp()
+	// TODO(b/134676337): Set stat.Blocks which is the number of 512 byte blocks
+	// (including metadata blocks) required to represent this file.
+}
+
+// getBGNum returns the block group number that a given inode belongs to.
+func getBGNum(inodeNum uint32, inodesPerGrp uint32) uint32 {
+	return (inodeNum - 1) / inodesPerGrp
+}
+
+// getBGOff returns the offset at which the given inode lives in the block
+// group's inode table, i.e. the index of the inode in the inode table.
+func getBGOff(inodeNum uint32, inodesPerGrp uint32) uint32 {
+	return (inodeNum - 1) % inodesPerGrp
+}
diff --git a/pkg/sentry/fsimpl/ext/regular_file.go b/pkg/sentry/fsimpl/ext/regular_file.go
new file mode 100644
index 000000000..ffc76ba5b
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/regular_file.go
@@ -0,0 +1,159 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ext
+
+import (
+	"io"
+	"sync"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/memmap"
+	"gvisor.dev/gvisor/pkg/sentry/safemem"
+	"gvisor.dev/gvisor/pkg/sentry/usermem"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// regularFile represents a regular file's inode. This too follows the
+// inheritance pattern prevelant in the vfs layer described in
+// pkg/sentry/vfs/README.md.
+type regularFile struct {
+	inode inode
+
+	// This is immutable. The first field of fileReader implementations must be
+	// regularFile to ensure temporality.
+	// io.ReaderAt is more strict than io.Reader in the sense that a partial read
+	// is always accompanied by an error. If a read spans past the end of file, a
+	// partial read (within file range) is done and io.EOF is returned.
+	impl io.ReaderAt
+}
+
+// newRegularFile is the regularFile constructor. It figures out what kind of
+// file this is and initializes the fileReader.
+func newRegularFile(inode inode) (*regularFile, error) {
+	regFile := regularFile{
+		inode: inode,
+	}
+
+	inodeFlags := inode.diskInode.Flags()
+
+	if inodeFlags.Extents {
+		file, err := newExtentFile(regFile)
+		if err != nil {
+			return nil, err
+		}
+
+		file.regFile.inode.impl = &file.regFile
+		return &file.regFile, nil
+	}
+
+	file, err := newBlockMapFile(regFile)
+	if err != nil {
+		return nil, err
+	}
+	file.regFile.inode.impl = &file.regFile
+	return &file.regFile, nil
+}
+
+func (in *inode) isRegular() bool {
+	_, ok := in.impl.(*regularFile)
+	return ok
+}
+
+// directoryFD represents a directory file description. It implements
+// vfs.FileDescriptionImpl.
+type regularFileFD struct {
+	fileDescription
+
+	// off is the file offset. off is accessed using atomic memory operations.
+	off int64
+
+	// offMu serializes operations that may mutate off.
+	offMu sync.Mutex
+}
+
+// Release implements vfs.FileDescriptionImpl.Release.
+func (fd *regularFileFD) Release() {}
+
+// PRead implements vfs.FileDescriptionImpl.PRead.
+func (fd *regularFileFD) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
+	safeReader := safemem.FromIOReaderAt{
+		ReaderAt: fd.inode().impl.(*regularFile).impl,
+		Offset:   offset,
+	}
+
+	// Copies data from disk directly into usermem without any intermediate
+	// allocations (if dst is converted into BlockSeq such that it does not need
+	// safe copying).
+	return dst.CopyOutFrom(ctx, safeReader)
+}
+
+// Read implements vfs.FileDescriptionImpl.Read.
+func (fd *regularFileFD) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.ReadOptions) (int64, error) {
+	n, err := fd.PRead(ctx, dst, fd.off, opts)
+	fd.offMu.Lock()
+	fd.off += n
+	fd.offMu.Unlock()
+	return n, err
+}
+
+// PWrite implements vfs.FileDescriptionImpl.PWrite.
+func (fd *regularFileFD) PWrite(ctx context.Context, src usermem.IOSequence, offset int64, opts vfs.WriteOptions) (int64, error) {
+	// write(2) specifies that EBADF must be returned if the fd is not open for
+	// writing.
+	return 0, syserror.EBADF
+}
+
+// Write implements vfs.FileDescriptionImpl.Write.
+func (fd *regularFileFD) Write(ctx context.Context, src usermem.IOSequence, opts vfs.WriteOptions) (int64, error) {
+	n, err := fd.PWrite(ctx, src, fd.off, opts)
+	fd.offMu.Lock()
+	fd.off += n
+	fd.offMu.Unlock()
+	return n, err
+}
+
+// IterDirents implements vfs.FileDescriptionImpl.IterDirents.
+func (fd *regularFileFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallback) error {
+	return syserror.ENOTDIR
+}
+
+// Seek implements vfs.FileDescriptionImpl.Seek.
+func (fd *regularFileFD) Seek(ctx context.Context, offset int64, whence int32) (int64, error) {
+	fd.offMu.Lock()
+	defer fd.offMu.Unlock()
+	switch whence {
+	case linux.SEEK_SET:
+		// Use offset as specified.
+	case linux.SEEK_CUR:
+		offset += fd.off
+	case linux.SEEK_END:
+		offset += int64(fd.inode().diskInode.Size())
+	default:
+		return 0, syserror.EINVAL
+	}
+	if offset < 0 {
+		return 0, syserror.EINVAL
+	}
+	fd.off = offset
+	return offset, nil
+}
+
+// IterDirents implements vfs.FileDescriptionImpl.IterDirents.
+func (fd *regularFileFD) ConfigureMMap(ctx context.Context, opts memmap.MMapOpts) error {
+	// TODO(b/134676337): Implement mmap(2).
+	return syserror.ENODEV
+}
diff --git a/pkg/sentry/fsimpl/ext/symlink.go b/pkg/sentry/fsimpl/ext/symlink.go
new file mode 100644
index 000000000..e06548a98
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/symlink.go
@@ -0,0 +1,111 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ext
+
+import (
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/memmap"
+	"gvisor.dev/gvisor/pkg/sentry/usermem"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// symlink represents a symlink inode.
+type symlink struct {
+	inode  inode
+	target string // immutable
+}
+
+// newSymlink is the symlink constructor. It reads out the symlink target from
+// the inode (however it might have been stored).
+func newSymlink(inode inode) (*symlink, error) {
+	var file *symlink
+	var link []byte
+
+	// If the symlink target is lesser than 60 bytes, its stores in inode.Data().
+	// Otherwise either extents or block maps will be used to store the link.
+	size := inode.diskInode.Size()
+	if size < 60 {
+		link = inode.diskInode.Data()[:size]
+	} else {
+		// Create a regular file out of this inode and read out the target.
+		regFile, err := newRegularFile(inode)
+		if err != nil {
+			return nil, err
+		}
+
+		link = make([]byte, size)
+		if n, err := regFile.impl.ReadAt(link, 0); uint64(n) < size {
+			return nil, err
+		}
+	}
+
+	file = &symlink{inode: inode, target: string(link)}
+	file.inode.impl = file
+	return file, nil
+}
+
+func (in *inode) isSymlink() bool {
+	_, ok := in.impl.(*symlink)
+	return ok
+}
+
+// symlinkFD represents a symlink file description and implements implements
+// vfs.FileDescriptionImpl. which may only be used if open options contains
+// O_PATH. For this reason most of the functions return EBADF.
+type symlinkFD struct {
+	fileDescription
+}
+
+// Compiles only if symlinkFD implements vfs.FileDescriptionImpl.
+var _ vfs.FileDescriptionImpl = (*symlinkFD)(nil)
+
+// Release implements vfs.FileDescriptionImpl.Release.
+func (fd *symlinkFD) Release() {}
+
+// PRead implements vfs.FileDescriptionImpl.PRead.
+func (fd *symlinkFD) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
+	return 0, syserror.EBADF
+}
+
+// Read implements vfs.FileDescriptionImpl.Read.
+func (fd *symlinkFD) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.ReadOptions) (int64, error) {
+	return 0, syserror.EBADF
+}
+
+// PWrite implements vfs.FileDescriptionImpl.PWrite.
+func (fd *symlinkFD) PWrite(ctx context.Context, src usermem.IOSequence, offset int64, opts vfs.WriteOptions) (int64, error) {
+	return 0, syserror.EBADF
+}
+
+// Write implements vfs.FileDescriptionImpl.Write.
+func (fd *symlinkFD) Write(ctx context.Context, src usermem.IOSequence, opts vfs.WriteOptions) (int64, error) {
+	return 0, syserror.EBADF
+}
+
+// IterDirents implements vfs.FileDescriptionImpl.IterDirents.
+func (fd *symlinkFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallback) error {
+	return syserror.ENOTDIR
+}
+
+// Seek implements vfs.FileDescriptionImpl.Seek.
+func (fd *symlinkFD) Seek(ctx context.Context, offset int64, whence int32) (int64, error) {
+	return 0, syserror.EBADF
+}
+
+// IterDirents implements vfs.FileDescriptionImpl.IterDirents.
+func (fd *symlinkFD) ConfigureMMap(ctx context.Context, opts memmap.MMapOpts) error {
+	return syserror.EBADF
+}
diff --git a/pkg/sentry/fsimpl/ext/utils.go b/pkg/sentry/fsimpl/ext/utils.go
new file mode 100644
index 000000000..d8b728f8c
--- /dev/null
+++ b/pkg/sentry/fsimpl/ext/utils.go
@@ -0,0 +1,94 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ext
+
+import (
+	"io"
+
+	"gvisor.dev/gvisor/pkg/binary"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/ext/disklayout"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// readFromDisk performs a binary read from disk into the given struct from
+// the absolute offset provided.
+func readFromDisk(dev io.ReaderAt, abOff int64, v interface{}) error {
+	n := binary.Size(v)
+	buf := make([]byte, n)
+	if read, _ := dev.ReadAt(buf, abOff); read < int(n) {
+		return syserror.EIO
+	}
+
+	binary.Unmarshal(buf, binary.LittleEndian, v)
+	return nil
+}
+
+// readSuperBlock reads the SuperBlock from block group 0 in the underlying
+// device. There are three versions of the superblock. This function identifies
+// and returns the correct version.
+func readSuperBlock(dev io.ReaderAt) (disklayout.SuperBlock, error) {
+	var sb disklayout.SuperBlock = &disklayout.SuperBlockOld{}
+	if err := readFromDisk(dev, disklayout.SbOffset, sb); err != nil {
+		return nil, err
+	}
+	if sb.Revision() == disklayout.OldRev {
+		return sb, nil
+	}
+
+	sb = &disklayout.SuperBlock32Bit{}
+	if err := readFromDisk(dev, disklayout.SbOffset, sb); err != nil {
+		return nil, err
+	}
+	if !sb.IncompatibleFeatures().Is64Bit {
+		return sb, nil
+	}
+
+	sb = &disklayout.SuperBlock64Bit{}
+	if err := readFromDisk(dev, disklayout.SbOffset, sb); err != nil {
+		return nil, err
+	}
+	return sb, nil
+}
+
+// blockGroupsCount returns the number of block groups in the ext fs.
+func blockGroupsCount(sb disklayout.SuperBlock) uint64 {
+	blocksCount := sb.BlocksCount()
+	blocksPerGroup := uint64(sb.BlocksPerGroup())
+
+	// Round up the result. float64 can compromise precision so do it manually.
+	return (blocksCount + blocksPerGroup - 1) / blocksPerGroup
+}
+
+// readBlockGroups reads the block group descriptor table from block group 0 in
+// the underlying device.
+func readBlockGroups(dev io.ReaderAt, sb disklayout.SuperBlock) ([]disklayout.BlockGroup, error) {
+	bgCount := blockGroupsCount(sb)
+	bgdSize := uint64(sb.BgDescSize())
+	is64Bit := sb.IncompatibleFeatures().Is64Bit
+	bgds := make([]disklayout.BlockGroup, bgCount)
+
+	for i, off := uint64(0), uint64(sb.FirstDataBlock()+1)*sb.BlockSize(); i < bgCount; i, off = i+1, off+bgdSize {
+		if is64Bit {
+			bgds[i] = &disklayout.BlockGroup64Bit{}
+		} else {
+			bgds[i] = &disklayout.BlockGroup32Bit{}
+		}
+
+		if err := readFromDisk(dev, int64(off), bgds[i]); err != nil {
+			return nil, err
+		}
+	}
+	return bgds, nil
+}