ext: boilerplate code.

Renamed ext4 to ext since we are targeting ext(2/3/4).
Removed fs.go since we are targeting VFS2.
Added ext.go with filesystem struct.

PiperOrigin-RevId: 257689775

22 files changed, 2180 insertions, 0 deletions

diff --git a/pkg/sentry/fs/ext/BUILD b/pkg/sentry/fs/ext/BUILD
new file mode 100644
index 000000000..3c2a02eac
--- /dev/null
+++ b/pkg/sentry/fs/ext/BUILD
@@ -0,0 +1,11 @@
+package(licenses = ["notice"])
+
+load("//tools/go_stateify:defs.bzl", "go_library")
+
+go_library(
+    name = "ext",
+    srcs = ["ext.go"],
+    importpath = "gvisor.dev/gvisor/pkg/sentry/fs/ext",
+    visibility = ["//pkg/sentry:internal"],
+    deps = ["//pkg/sentry/fs/ext/disklayout"],
+)
diff --git a/pkg/sentry/fs/ext/disklayout/BUILD b/pkg/sentry/fs/ext/disklayout/BUILD
new file mode 100644
index 000000000..e4cb26645
--- /dev/null
+++ b/pkg/sentry/fs/ext/disklayout/BUILD
@@ -0,0 +1,46 @@
+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",
+        "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/fs/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",
+        "inode_test.go",
+        "superblock_test.go",
+    ],
+    embed = [":disklayout"],
+    deps = ["//pkg/sentry/kernel/time"],
+)
diff --git a/pkg/sentry/fs/ext/disklayout/block_group.go b/pkg/sentry/fs/ext/disklayout/block_group.go
new file mode 100644
index 000000000..32ea3d97d
--- /dev/null
+++ b/pkg/sentry/fs/ext/disklayout/block_group.go
@@ -0,0 +1,127 @@
+// 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.
+//
+// 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/fs/ext/disklayout/block_group_32.go b/pkg/sentry/fs/ext/disklayout/block_group_32.go
new file mode 100644
index 000000000..3e16c76db
--- /dev/null
+++ b/pkg/sentry/fs/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/fs/ext/disklayout/block_group_64.go b/pkg/sentry/fs/ext/disklayout/block_group_64.go
new file mode 100644
index 000000000..9a809197a
--- /dev/null
+++ b/pkg/sentry/fs/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/fs/ext/disklayout/block_group_test.go b/pkg/sentry/fs/ext/disklayout/block_group_test.go
new file mode 100644
index 000000000..0ef4294c0
--- /dev/null
+++ b/pkg/sentry/fs/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/fs/ext/disklayout/dirent.go b/pkg/sentry/fs/ext/disklayout/dirent.go
new file mode 100644
index 000000000..685bf57b8
--- /dev/null
+++ b/pkg/sentry/fs/ext/disklayout/dirent.go
@@ -0,0 +1,69 @@
+// 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
+)
+
+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/fs/ext/disklayout/dirent_new.go b/pkg/sentry/fs/ext/disklayout/dirent_new.go
new file mode 100644
index 000000000..29ae4a5c2
--- /dev/null
+++ b/pkg/sentry/fs/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/fs/ext/disklayout/dirent_old.go b/pkg/sentry/fs/ext/disklayout/dirent_old.go
new file mode 100644
index 000000000..2e0f9c812
--- /dev/null
+++ b/pkg/sentry/fs/ext/disklayout/dirent_old.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
+
+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. This is used in
+// ext2, ext3 and sometimes in ext4.
+//
+// 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/fs/ext/disklayout/dirent_test.go b/pkg/sentry/fs/ext/disklayout/dirent_test.go
new file mode 100644
index 000000000..cc6dff2c9
--- /dev/null
+++ b/pkg/sentry/fs/ext/disklayout/dirent_test.go
@@ -0,0 +1,28 @@
+// 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) {
+	want := uintptr(263)
+
+	assertSize(t, DirentOld{}, want)
+	assertSize(t, DirentNew{}, want)
+}
diff --git a/pkg/sentry/fs/ext/disklayout/disklayout.go b/pkg/sentry/fs/ext/disklayout/disklayout.go
new file mode 100644
index 000000000..bdf4e2132
--- /dev/null
+++ b/pkg/sentry/fs/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/fs/ext/disklayout/inode.go b/pkg/sentry/fs/ext/disklayout/inode.go
new file mode 100644
index 000000000..b48001910
--- /dev/null
+++ b/pkg/sentry/fs/ext/disklayout/inode.go
@@ -0,0 +1,267 @@
+// 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"
+)
+
+// 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
+
+	// Blocks returns the underlying inode.i_block array. This field is special
+	// and is used to store various kinds of things depending on the filesystem
+	// version and inode type.
+	//   - In ext2/ext3, it contains the block map.
+	//   - In ext4, it contains the extent tree.
+	//   - 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.
+	Blocks() [60]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/fs/ext/disklayout/inode_new.go b/pkg/sentry/fs/ext/disklayout/inode_new.go
new file mode 100644
index 000000000..4f5348372
--- /dev/null
+++ b/pkg/sentry/fs/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/fs/ext/disklayout/inode_old.go b/pkg/sentry/fs/ext/disklayout/inode_old.go
new file mode 100644
index 000000000..dc4c9d8e4
--- /dev/null
+++ b/pkg/sentry/fs/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.
+	BlocksRaw     [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) }
+
+// Blocks implements Inode.Blocks.
+func (in *InodeOld) Blocks() [60]byte { return in.BlocksRaw }
diff --git a/pkg/sentry/fs/ext/disklayout/inode_test.go b/pkg/sentry/fs/ext/disklayout/inode_test.go
new file mode 100644
index 000000000..9cae9e4f0
--- /dev/null
+++ b/pkg/sentry/fs/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/fs/ext/disklayout/superblock.go b/pkg/sentry/fs/ext/disklayout/superblock.go
new file mode 100644
index 000000000..e4b8f46fb
--- /dev/null
+++ b/pkg/sentry/fs/ext/disklayout/superblock.go
@@ -0,0 +1,468 @@
+// 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
+
+// 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.
+	//
+	// The first block group descriptor is in (FirstDataBlock() + 1)th block.
+	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 int
+
+// 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 ext4_dir_entry_2 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/fs/ext/disklayout/superblock_32.go b/pkg/sentry/fs/ext/disklayout/superblock_32.go
new file mode 100644
index 000000000..587e4afaa
--- /dev/null
+++ b/pkg/sentry/fs/ext/disklayout/superblock_32.go
@@ -0,0 +1,75 @@
+// 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.
+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/fs/ext/disklayout/superblock_64.go b/pkg/sentry/fs/ext/disklayout/superblock_64.go
new file mode 100644
index 000000000..a2c2278fb
--- /dev/null
+++ b/pkg/sentry/fs/ext/disklayout/superblock_64.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 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.
+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/fs/ext/disklayout/superblock_old.go b/pkg/sentry/fs/ext/disklayout/superblock_old.go
new file mode 100644
index 000000000..c74953610
--- /dev/null
+++ b/pkg/sentry/fs/ext/disklayout/superblock_old.go
@@ -0,0 +1,102 @@
+// 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 in ext2 and ext3 systems.
+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
+}
+
+// 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/fs/ext/disklayout/superblock_test.go b/pkg/sentry/fs/ext/disklayout/superblock_test.go
new file mode 100644
index 000000000..463b5ba21
--- /dev/null
+++ b/pkg/sentry/fs/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/fs/ext/disklayout/test_utils.go b/pkg/sentry/fs/ext/disklayout/test_utils.go
new file mode 100644
index 000000000..9c63f04c0
--- /dev/null
+++ b/pkg/sentry/fs/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/fs/ext/ext.go b/pkg/sentry/fs/ext/ext.go
new file mode 100644
index 000000000..7602e2bf0
--- /dev/null
+++ b/pkg/sentry/fs/ext/ext.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 ext implements readonly ext(2/3/4) filesystems.
+package ext
+
+import (
+	"io"
+	"sync"
+
+	"gvisor.dev/gvisor/pkg/sentry/fs/ext/disklayout"
+)
+
+// Filesystem implements vfs.FilesystemImpl.
+type Filesystem struct {
+	// dev is the ReadSeeker for the underlying fs device and is protected by mu.
+	dev io.ReadSeeker
+
+	// mu synchronizes the usage of dev. The ext filesystems take locality into
+	// condsideration, i.e. data blocks of a file will tend to be placed close
+	// together. On a spinning disk, locality reduces the amount of movement of
+	// the head hence speeding up IO operations. On an SSD there are no moving
+	// parts but locality increases the size of each transer request. Hence,
+	// having mutual exclusion on the read seeker while reading a file *should*
+	// help in achieving the intended performance gains.
+	//
+	// Note: This synchronization was not coupled with the ReadSeeker itself
+	// because we want to synchronize across read/seek operations for the
+	// performance gains mentioned above. Helps enforcing one-file-at-a-time IO.
+	mu sync.Mutex
+
+	// 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
+}