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// 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.
//
// Precondition: Must hold the mutex of the filesystem containing dev.
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
}
// readFull is a wrapper around io.ReadFull which enforces the absolute offset
// parameter so that we can ensure that we do not perform incorrect reads from
// stale previously used offsets.
//
// Precondition: Must hold the mutex of the filesystem containing dev.
func readFull(dev io.ReadSeeker, abOff int64, dst []byte) (int, error) {
if _, err := dev.Seek(abOff, io.SeekStart); err != nil {
return 0, syserror.EIO
}
n, err := io.ReadFull(dev, dst)
if err != nil {
err = syserror.EIO
}
return n, err
}
// 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.
//
// Precondition: Must hold the mutex of the filesystem containing dev.
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.
return (blocksCount + blocksPerGroup - 1) / blocksPerGroup
}
// readBlockGroups reads the block group descriptor table from block group 0 in
// the underlying device.
//
// Precondition: Must hold the mutex of the filesystem containing dev.
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
}
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