ext: Filesystem init implementation.

PiperOrigin-RevId: 258645957

1 files changed, 103 insertions, 0 deletions

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
+}