[Qemu-devel] [Bug 1077838] Re: qemu-nbd -r -c taints device for subsequent usage, even after -d

[Serge Hallyn]

Verified on quantal.

** Tags removed: verification-needed
** Tags added: verification-done

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https://bugs.launchpad.net/bugs/1077838

Title:
  qemu-nbd -r -c taints device for subsequent usage, even after -d

Status in QEMU:
  In Progress
Status in “qemu-kvm” package in Ubuntu:
  Fix Released
Status in “qemu-kvm” source package in Precise:
  Fix Committed
Status in “qemu-kvm” source package in Quantal:
  Fix Committed

Bug description:
  Something about qemu-nbd -r -c /dev/nbd0 someimg leaves cruft behind -
  subsequent connections get marked readonly.

  This is on quantal, haven't checked precise or raring.

  To demonstrate:
  # use one image
  qemu-img create -f qcow2 /tmp/1.qcow2 100M
  sudo qemu-nbd -c /dev/nbd2 /tmp/1.qcow2
  sudo mkfs -t ext4 /dev/nbd2
  sudo qemu-nbd -d /dev/nbd2
  # use a second one on the same nbd device, shows that reuse works:
  qemu-img create -f qcow2 /tmp/2.qcow2 100M
  sudo qemu-nbd -c /dev/nbd2 /tmp/2.qcow2
  sudo mkfs -t ext4 /dev/nbd2
  sudo qemu-nbd -d /dev/nbd2
  # connect an image in read only mode
  sudo qemu-nbd -r -c /dev/nbd2 /tmp/2.qcow2
  sudo dumpe2fs /dev/nbd2 | head -n 3
  sudo qemu-nbd -d /dev/nbd2
  # now try to reuse in read-write mode again:
  qemu-img create -f qcow2 /tmp/3.qcow2 100M
  sudo qemu-nbd -c /dev/nbd2 /tmp/3.qcow2
  sudo mkfs -t ext4 /dev/nbd2
  # here it goes boom:
  mke2fs 1.42.5 (29-Jul-2012)
  /dev/nbd2: Operation not permitted while setting up superblock
  # still need to cleanup
  sudo qemu-nbd -d /dev/nbd2

  ===============================
  SRU Justification:
  1. Impact: mounting an nbd device as read-write after doing so read-only
  will cause the mount to erroneously (and quietly) be read-only.
  2. Development fix: have qemu-nbd set the device to read-write when asked,
  rather than only setting read-only.
  3. Stable fix: same as development fix
  4. Test case: See above
  5. Regression potential: The patch is localized to the handling of read-only
  flag in qemu-nbd, so any regression should not affect anything else.
  ===============================

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