6 files changed, 157 insertions, 5 deletions
diff --git a/pkg/sentry/fs/ext/BUILD b/pkg/sentry/fs/ext/BUILD
index 3c2a02eac..b386f31c8 100644
--- a/pkg/sentry/fs/ext/BUILD
+++ b/pkg/sentry/fs/ext/BUILD
@@ -4,8 +4,15 @@ load("//tools/go_stateify:defs.bzl", "go_library")
 
 go_library(
     name = "ext",
-    srcs = ["ext.go"],
+    srcs = [
+        "ext.go",
+        "utils.go",
+    ],
     importpath = "gvisor.dev/gvisor/pkg/sentry/fs/ext",
     visibility = ["//pkg/sentry:internal"],
-    deps = ["//pkg/sentry/fs/ext/disklayout"],
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/sentry/fs/ext/disklayout",
+        "//pkg/syserror",
+    ],
 )
diff --git a/pkg/sentry/fs/ext/disklayout/block_group.go b/pkg/sentry/fs/ext/disklayout/block_group.go
index 32ea3d97d..ad6f4fef8 100644
--- a/pkg/sentry/fs/ext/disklayout/block_group.go
+++ b/pkg/sentry/fs/ext/disklayout/block_group.go
@@ -18,6 +18,16 @@ package disklayout
 // 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
diff --git a/pkg/sentry/fs/ext/disklayout/superblock.go b/pkg/sentry/fs/ext/disklayout/superblock.go
index e4b8f46fb..7a337a5e0 100644
--- a/pkg/sentry/fs/ext/disklayout/superblock.go
+++ b/pkg/sentry/fs/ext/disklayout/superblock.go
@@ -14,6 +14,11 @@
 
 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
@@ -57,8 +62,6 @@ type SuperBlock interface {
 	//
 	// 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.
@@ -128,7 +131,7 @@ type SuperBlock interface {
 }
 
 // SbRevision is the type for superblock revisions.
-type SbRevision int
+type SbRevision uint32
 
 // Super block revisions.
 const (
diff --git a/pkg/sentry/fs/ext/disklayout/superblock_old.go b/pkg/sentry/fs/ext/disklayout/superblock_old.go
index c74953610..aada8b550 100644
--- a/pkg/sentry/fs/ext/disklayout/superblock_old.go
+++ b/pkg/sentry/fs/ext/disklayout/superblock_old.go
@@ -44,6 +44,9 @@ type SuperBlockOld struct {
 	DefResGID           uint16
 }
 
+// Compiles only if SuperBlockOld implements SuperBlock.
+var _ SuperBlock = (*SuperBlockOld)(nil)
+
 // InodesCount implements SuperBlock.InodesCount.
 func (sb *SuperBlockOld) InodesCount() uint32 { return sb.InodesCountRaw }
 
diff --git a/pkg/sentry/fs/ext/ext.go b/pkg/sentry/fs/ext/ext.go
index 7602e2bf0..9d33be08f 100644
--- a/pkg/sentry/fs/ext/ext.go
+++ b/pkg/sentry/fs/ext/ext.go
@@ -19,7 +19,9 @@ import (
 	"io"
 	"sync"
 
+	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/sentry/fs/ext/disklayout"
+	"gvisor.dev/gvisor/pkg/syserror"
 )
 
 // Filesystem implements vfs.FilesystemImpl.
@@ -47,3 +49,27 @@ type Filesystem struct {
 	// Immutable after initialization.
 	bgs []disklayout.BlockGroup
 }
+
+// newFilesystem is the Filesystem constructor.
+func newFilesystem(dev io.ReadSeeker) (*Filesystem, error) {
+	fs := Filesystem{dev: dev}
+	var err error
+
+	fs.sb, err = readSuperBlock(dev)
+	if err != nil {
+		return 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, syserror.EINVAL
+	}
+
+	fs.bgs, err = readBlockGroups(dev, fs.sb)
+	if err != nil {
+		return nil, err
+	}
+
+	return &fs, nil
+}
diff --git a/pkg/sentry/fs/ext/utils.go b/pkg/sentry/fs/ext/utils.go
new file mode 100644
index 000000000..861ef9a73
--- /dev/null
+++ b/pkg/sentry/fs/ext/utils.go
@@ -0,0 +1,103 @@
+// 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 (
+	"encoding/binary"
+	"io"
+
+	"gvisor.dev/gvisor/pkg/sentry/fs/ext/disklayout"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// readFromDisk performs a binary read from disk into the given struct from
+// the absolute offset provided.
+//
+// All disk reads should use this helper so we avoid reading from stale
+// previously used offsets. This function forces the offset parameter.
+func readFromDisk(dev io.ReadSeeker, abOff int64, v interface{}) error {
+	if _, err := dev.Seek(abOff, io.SeekStart); err != nil {
+		return syserror.EIO
+	}
+
+	if err := binary.Read(dev, binary.LittleEndian, v); err != nil {
+		return syserror.EIO
+	}
+
+	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.ReadSeeker) (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.
+	bgCount := blocksCount / blocksPerGroup
+	if blocksCount%blocksPerGroup != 0 {
+		bgCount++
+	}
+
+	return bgCount
+}
+
+// readBlockGroups reads the block group descriptor table from block group 0 in
+// the underlying device.
+func readBlockGroups(dev io.ReadSeeker, 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
+}