[Bug 1850000] Re: 4.1.0 bogus QCOW2 corruption reported after compress

[Max Reitz]

There is definitely a bug in that patch, and that is that
QCOW2_COMPRESSED_SECTOR_MASK is an unsigned int instead of a uint64_t
(so the mask is too small).

It looks like the bug has existed in some places before that patch
(because they use ~511 as a mask), but not in others.

This would explain why the bug is visible only for some images, namely
for those with a compressed size of more than 4 GB, I presume.


And indeed, fixing QCOW2_COMPRESSED_SECTOR_MASK to be an unsigned long long 
fixes the bug.  I’ll send a patch (but I’ll have to write a more simple and 
quicker test case first).

Max

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https://bugs.launchpad.net/bugs/1850000

Title:
  4.1.0 bogus QCOW2 corruption reported after compress

Status in QEMU:
  New

Bug description:
  Creating a compressed image then running `qemu-img check <..>.qcow2'
  on said image seems to report bogus corruption in some (but not all)
  cases:

  Step 1.

  # qemu-img info win7-base.qcow2
  image: win7-base.qcow2
  file format: qcow2
  virtual size: 20 GiB (21474836480 bytes)
  disk size: 12.2 GiB
  cluster_size: 65536
  Format specific information:
      compat: 1.1
      lazy refcounts: true
      refcount bits: 16
      corrupt: false

  # qemu-img check win7-base.qcow2
  No errors were found on the image.
  327680/327680 = 100.00% allocated, 0.00% fragmented, 0.00% compressed clusters
  Image end offset: 21478375424

  Step 2.

  # qemu-img convert -f qcow2 -O qcow2 -c win7-base.qcow2 test1-z.qcow2

  Step 3.

  # qemu-img info test1-z.qcow2
  image: test1-z.qcow2
  file format: qcow2
  virtual size: 20 GiB (21474836480 bytes)
  disk size: 5.78 GiB
  cluster_size: 65536
  Format specific information:
      compat: 1.1
      lazy refcounts: false
      refcount bits: 16
      corrupt: false

  # qemu-img check test1-z.qcow2
  ERROR cluster 1191 refcount=1 reference=2
  ERROR cluster 1194 refcount=1 reference=4
  ERROR cluster 1195 refcount=1 reference=7
  ERROR cluster 1196 refcount=1 reference=7
  ERROR cluster 1197 refcount=1 reference=6
  ERROR cluster 1198 refcount=1 reference=4
  ERROR cluster 1199 refcount=1 reference=4
  ERROR cluster 1200 refcount=1 reference=5
  ERROR cluster 1201 refcount=1 reference=3
  <...> snip many errors
  Leaked cluster 94847 refcount=3 reference=0
  Leaked cluster 94848 refcount=3 reference=0
  Leaked cluster 94849 refcount=11 reference=0
  Leaked cluster 94850 refcount=14 reference=0

  20503 errors were found on the image.
  Data may be corrupted, or further writes to the image may corrupt it.

  20503 leaked clusters were found on the image.
  This means waste of disk space, but no harm to data.
  197000/327680 = 60.12% allocated, 89.32% fragmented, 88.50% compressed 
clusters
  Image end offset: 6216220672

  
  The resultant image seems to work fine in a VM when used as a backing file.

  Interestingly, if I substitute a qemu-img binary from qemu-4.0 then no
  errors are reported.

  # /tmp/qemu-img check test1-z.qcow2
  No errors were found on the image.
  197000/327680 = 60.12% allocated, 89.32% fragmented, 88.50% compressed 
clusters
  Image end offset: 6216220672

  Is the image corrupted or not? I'm guessing not.

  Just in case it matters, this is ext4 fs on rotational disk. Latest
  Arch Linux but self compiled 4.1.0 with recent QCOW2 corruption fixes
  added.

  I haven't tried latest trunk but might do so if time permits.

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