1. 13 2月, 2014 31 次提交
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      Btrfs: fix data corruption when reading/updating compressed extents · a2aa75e1
      Filipe David Borba Manana 提交于
      When using a mix of compressed file extents and prealloc extents, it
      is possible to fill a page of a file with random, garbage data from
      some unrelated previous use of the page, instead of a sequence of zeroes.
      
      A simple sequence of steps to get into such case, taken from the test
      case I made for xfstests, is:
      
         _scratch_mkfs
         _scratch_mount "-o compress-force=lzo"
         $XFS_IO_PROG -f -c "pwrite -S 0x06 -b 18670 266978 18670" $SCRATCH_MNT/foobar
         $XFS_IO_PROG -c "falloc 26450 665194" $SCRATCH_MNT/foobar
         $XFS_IO_PROG -c "truncate 542872" $SCRATCH_MNT/foobar
         $XFS_IO_PROG -c "fsync" $SCRATCH_MNT/foobar
      
      This results in the following file items in the fs tree:
      
         item 4 key (257 INODE_ITEM 0) itemoff 15879 itemsize 160
             inode generation 6 transid 6 size 542872 block group 0 mode 100600
         item 5 key (257 INODE_REF 256) itemoff 15863 itemsize 16
             inode ref index 2 namelen 6 name: foobar
         item 6 key (257 EXTENT_DATA 0) itemoff 15810 itemsize 53
             extent data disk byte 0 nr 0 gen 6
             extent data offset 0 nr 24576 ram 266240
             extent compression 0
         item 7 key (257 EXTENT_DATA 24576) itemoff 15757 itemsize 53
             prealloc data disk byte 12849152 nr 241664 gen 6
             prealloc data offset 0 nr 241664
         item 8 key (257 EXTENT_DATA 266240) itemoff 15704 itemsize 53
             extent data disk byte 12845056 nr 4096 gen 6
             extent data offset 0 nr 20480 ram 20480
             extent compression 2
         item 9 key (257 EXTENT_DATA 286720) itemoff 15651 itemsize 53
             prealloc data disk byte 13090816 nr 405504 gen 6
             prealloc data offset 0 nr 258048
      
      The on disk extent at offset 266240 (which corresponds to 1 single disk block),
      contains 5 compressed chunks of file data. Each of the first 4 compress 4096
      bytes of file data, while the last one only compresses 3024 bytes of file data.
      Therefore a read into the file region [285648 ; 286720[ (length = 4096 - 3024 =
      1072 bytes) should always return zeroes (our next extent is a prealloc one).
      
      The solution here is the compression code path to zero the remaining (untouched)
      bytes of the last page it uncompressed data into, as the information about how
      much space the file data consumes in the last page is not known in the upper layer
      fs/btrfs/extent_io.c:__do_readpage(). In __do_readpage we were correctly zeroing
      the remainder of the page but only if it corresponds to the last page of the inode
      and if the inode's size is not a multiple of the page size.
      
      This would cause not only returning random data on reads, but also permanently
      storing random data when updating parts of the region that should be zeroed.
      For the example above, it means updating a single byte in the region [285648 ; 286720[
      would store that byte correctly but also store random data on disk.
      
      A test case for xfstests follows soon.
      Signed-off-by: NFilipe David Borba Manana <fdmanana@gmail.com>
      Signed-off-by: NChris Mason <clm@fb.com>
      a2aa75e1