Description of problem:
There is a checksum mismatch on the last two blocks of a vhd/vhdx if the disk is 1 GB (if it is >2GB there is no error).
Tested this by creating a data in 30 blocks (in case of a disk of 1 GB) and check it afterwards if it’s not corrupted.


Version-Release number of selected component (if applicable):
v4.4.1

How reproducible: 100%


Steps to Reproduce:
1.  Attach a 1 GB VHD to a SCSI controller
2.  Format it, create a btrfs filesystem and mount it
3.  We perform a data integrity check by creating a file that will fill the mounted device. This will have 30 blocks in case of a 1 GB large VHD/VHDX
For example:
    targetDevice="/dev/sdb1/"
    testFile="/dev/shm/testsource"
    blockSize=$((32*1024*1024))
    _gb=$((1*1024*1024*1024))
    targetSize=$(blockdev --getsize64 $targetDevice)
    let "blocks=$targetSize / $blockSize"

    if [ "$targetSize" -gt "$_gb" ] ; then
      targetSize=$_gb
      let "blocks=$targetSize / $blockSize"
    fi

    blocks=$((blocks-1))
    mount $targetDevice /mnt/
    targetDevice="/mnt/1"

    dd if=/dev/urandom of=$testFile bs=$blockSize count=1 status=noxfer 2> /dev/null

4.  Calculate its checksum then check the checksum for all the blocks individually.
checksum=$(sha1sum $testFile | cut -d " " -f 1)

for ((y=0 ; y<$blocks ; y++)) ; do
      dd if=$testFile of=$targetDevice bs=$blockSize count=1 seek=$y status=noxfer 2> /dev/null
      echo -n "Checking block $y ..."
      testChecksum=$(dd if=$targetDevice bs=$blockSize count=1 skip=$y status=noxfer 2> /dev/null | sha1sum | cut -d " " -f 1)
      if [ "$checksum" == "$testChecksum" ] ; then
        echo "Checksum matched for block $y"
      else
        echo "Checksum mismatch at block $y"
        exit 80
      fi
    done

5.  You can see that the last two blocks have different checksum


Actual results:
[...]
Checking block 24 ...checksum: facb2ef7b500779e3857c7787a237b693714c2c9, testChecksum: facb2ef7b500779e3857c7787a237b693714c2c9
Checksum matched for block 24

Checking block 25 ...checksum: facb2ef7b500779e3857c7787a237b693714c2c9, testChecksum: facb2ef7b500779e3857c7787a237b693714c2c9
Checksum matched for block 25

Checking block 26 ...checksum: facb2ef7b500779e3857c7787a237b693714c2c9, testChecksum: facb2ef7b500779e3857c7787a237b693714c2c9
Checksum matched for block 26

Checking block 27 ...checksum: facb2ef7b500779e3857c7787a237b693714c2c9, testChecksum: facb2ef7b500779e3857c7787a237b693714c2c9
Checksum matched for block 27

Checking block 28 ...checksum: facb2ef7b500779e3857c7787a237b693714c2c9, testChecksum: 8ef29351cce142235e7eeba8ffe913341d714ccb
Checksum mismatch at block 28

Checking block 29 ...checksum: facb2ef7b500779e3857c7787a237b693714c2c9, testChecksum: da39a3ee5e6b4b0d3255bfef95601890afd80709
Checksum mismatch at block 29

Expected results:
Checksum should match on all blocks.

Additional info:
- With all other filesystems tested (ext3, ext4, xfs) this could not be reproduced.
- This issue didn’t persist on disks larger than 1 GB (tested with 2 GB, 3 GB and 2 TB)
- Could not reproduce on RHEL 7.2 with btrfs-progs v3.19.1