github.com/SagerNet/gvisor@v0.0.0-20210707092255-7731c139d75c/pkg/sentry/fsimpl/ext/utils.go (about)

     1  // Copyright 2019 The gVisor Authors.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  package ext
    16  
    17  import (
    18  	"io"
    19  
    20  	"github.com/SagerNet/gvisor/pkg/marshal"
    21  	"github.com/SagerNet/gvisor/pkg/sentry/fsimpl/ext/disklayout"
    22  	"github.com/SagerNet/gvisor/pkg/syserror"
    23  )
    24  
    25  // readFromDisk performs a binary read from disk into the given struct from
    26  // the absolute offset provided.
    27  func readFromDisk(dev io.ReaderAt, abOff int64, v marshal.Marshallable) error {
    28  	n := v.SizeBytes()
    29  	buf := make([]byte, n)
    30  	if read, _ := dev.ReadAt(buf, abOff); read < int(n) {
    31  		return syserror.EIO
    32  	}
    33  
    34  	v.UnmarshalBytes(buf)
    35  	return nil
    36  }
    37  
    38  // readSuperBlock reads the SuperBlock from block group 0 in the underlying
    39  // device. There are three versions of the superblock. This function identifies
    40  // and returns the correct version.
    41  func readSuperBlock(dev io.ReaderAt) (disklayout.SuperBlock, error) {
    42  	var sb disklayout.SuperBlock = &disklayout.SuperBlockOld{}
    43  	if err := readFromDisk(dev, disklayout.SbOffset, sb); err != nil {
    44  		return nil, err
    45  	}
    46  	if sb.Revision() == disklayout.OldRev {
    47  		return sb, nil
    48  	}
    49  
    50  	sb = &disklayout.SuperBlock32Bit{}
    51  	if err := readFromDisk(dev, disklayout.SbOffset, sb); err != nil {
    52  		return nil, err
    53  	}
    54  	if !sb.IncompatibleFeatures().Is64Bit {
    55  		return sb, nil
    56  	}
    57  
    58  	sb = &disklayout.SuperBlock64Bit{}
    59  	if err := readFromDisk(dev, disklayout.SbOffset, sb); err != nil {
    60  		return nil, err
    61  	}
    62  	return sb, nil
    63  }
    64  
    65  // blockGroupsCount returns the number of block groups in the ext fs.
    66  func blockGroupsCount(sb disklayout.SuperBlock) uint64 {
    67  	blocksCount := sb.BlocksCount()
    68  	blocksPerGroup := uint64(sb.BlocksPerGroup())
    69  
    70  	// Round up the result. float64 can compromise precision so do it manually.
    71  	return (blocksCount + blocksPerGroup - 1) / blocksPerGroup
    72  }
    73  
    74  // readBlockGroups reads the block group descriptor table from block group 0 in
    75  // the underlying device.
    76  func readBlockGroups(dev io.ReaderAt, sb disklayout.SuperBlock) ([]disklayout.BlockGroup, error) {
    77  	bgCount := blockGroupsCount(sb)
    78  	bgdSize := uint64(sb.BgDescSize())
    79  	is64Bit := sb.IncompatibleFeatures().Is64Bit
    80  	bgds := make([]disklayout.BlockGroup, bgCount)
    81  
    82  	for i, off := uint64(0), uint64(sb.FirstDataBlock()+1)*sb.BlockSize(); i < bgCount; i, off = i+1, off+bgdSize {
    83  		if is64Bit {
    84  			bgds[i] = &disklayout.BlockGroup64Bit{}
    85  		} else {
    86  			bgds[i] = &disklayout.BlockGroup32Bit{}
    87  		}
    88  
    89  		if err := readFromDisk(dev, int64(off), bgds[i]); err != nil {
    90  			return nil, err
    91  		}
    92  	}
    93  	return bgds, nil
    94  }