[Qemu-devel] [Bug 1422307] Re: qemu-nbd corrupts files

[Launchpad Bug Tracker]

This bug was fixed in the package qemu - 2.0.0+dfsg-2ubuntu1.18

---------------
qemu (2.0.0+dfsg-2ubuntu1.18) trusty-proposed; urgency=medium

  * qemu-nbd-fix-vdi-corruption.patch:
    qemu-nbd: fix corruption while writing VDI volumes (LP: #1422307)

 -- Pierre Schweitzer <address@hidden>  Mon, 17 Aug 2015 11:43:39
+0200

** Changed in: qemu (Ubuntu Trusty)
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1422307

Title:
  qemu-nbd corrupts files

Status in QEMU:
  Fix Released
Status in qemu package in Ubuntu:
  Fix Released
Status in qemu source package in Trusty:
  Fix Released

Bug description:
  [Impact]
  A race condition in the VDI block driver of Qemu leads to image (and thus 
file system) corruption under certain circumstances.
  This makes Qemu tools usage for VDI formatted images particularly dangerous 
(qemu-img, qemu-nbd).
  The bug fix introduces locks to prevent such race condition.

  
  [Test Case]
  A simple test case was provided in comment #5 
(https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1422307/comments/5):

  $ ./qemu-img create -f vdi test.vdi 2G
  Formatting 'test.vdi', fmt=vdi size=2147483648 static=off
  $ ./qemu-img create -f raw test.raw 2G
  Formatting 'test.raw', fmt=raw size=2147483648
  $ x86_64-softmmu/qemu-system-x86_64 -enable-kvm -drive 
if=virtio,file=blkverify:test.raw:test.vdi,format=raw -drive 
if=virtio,file=data.img,format=raw,format=raw -cdrom ~/tmp/arch.iso -m 512 
-boot d
  blkverify: read sector_num=810976 nb_sectors=256 contents mismatch in sector 
811008

  Operations in the guest:
  $ dd if=/dev/vdb of=/dev/vda
  $ dd if=/dev/vda of=/dev/null

  [Regression Potential]
  In case of bugs affecting the way locks are used, deadlocks could be a 
regression, but they would only affect VDI images.

  
  Original bug report:
  Dear all,

  On Trusty, in certain situations, try to copy files over a qemu-nbd
  mounted file system leads to write errors (and thus, file corruption).

  Here is the last example I tried:
  -> virtual disk is a VDI disk
  -> It has only one partition, in FAT

  Here is my mount process:
  # modprobe nbd max_part=63
  # qemu-nbd -c /dev/nbd0 "virtual_disk.vdi"
  # partprobe /dev/nbd0
  # mount /dev/nbd0p1 /tmp/mnt/

  Partition is properly mounted at that point:
  /dev/nbd0p1 on /tmp/mnt type vfat (rw)

  Now, when I copy a file (rather big, ~28MB):
  # cp file_to_copy /tmp/mnt/ ; sync
  # md5sum /tmp/mnt/file_to_copy
  2efc9f32e4267782b11d63d2f128a363  /tmp/mnt/file_to_copy
  # umount /tmp/mnt
  # mount /dev/nbd0p1 /tmp/mnt/
  # md5sum /tmp/mnt/file_to_copy
  42b0a3bf73f704d03ce301716d7654de  /tmp/mnt/file_to_copy

  The first hash was obviously the right one.

  On a previous attempt I did, I spotted thanks to vbindiff that parts of the 
file were just filed with 0s instead of actual data.
  It will randomly work after several attempts to write.

  Version information:
  # qemu-nbd --version
  qemu-nbd version 0.0.1
  Written by Anthony Liguori.

  Cheers,

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