[Bug 1637601] Re: UbuntuKVM: migration using NFS mount fails #190

[Mauricio Faria de Oliveira]

Verification steps with just attached patch.


----------------------------------------
Test 1) new user, uid available 
Result) libvirt-qemu uid is set to 64055
----------------------------------------

# id libvirt-qemu
id: ‘libvirt-qemu’: no such user

# getent passwd 64055
#

# dpkg -i libvirt0_*.deb libvirt-bin_*.deb

# id libvirt-qemu
uid=64055(libvirt-qemu) gid=117(kvm) groups=117(kvm)


----------------------------------------------
Test 2) new user, uid not available
Result) libvirt-qemu uid is _not_ set to 64055
----------------------------------------------

# apt-get purge libvirt-bin

# userdel libvirt-qemu

# id libvirt-qemu
id: ‘libvirt-qemu’: no such user

# useradd --uid 64055 uidtaken

# getent passwd 64055
uidtaken:x:64055:1001::/home/uidtaken:

# dpkg -i libvirt0_*.deb libvirt-bin_*.deb

# id libvirt-qemu
uid=113(libvirt-qemu) gid=117(kvm) groups=117(kvm)


------------------------------------------
Test 3) old user, uid available
Result) libvirt-qemu uid is _not_ changed
------------------------------------------

# apt-get purge libvirt-bin

# userdel uidtaken

# getent passwd 64055
#

# id libvirt-qemu
uid=113(libvirt-qemu) gid=117(kvm) groups=117(kvm)

# dpkg -i libvirt0_*.deb libvirt-bin_*.deb

# id libvirt-qemu
uid=113(libvirt-qemu) gid=117(kvm) groups=117(kvm)

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https://bugs.launchpad.net/bugs/1637601

Title:
  UbuntuKVM: migration using NFS mount fails #190

Status in libvirt:
  Fix Released
Status in base-passwd package in Ubuntu:
  Fix Released
Status in libvirt package in Ubuntu:
  Fix Released

Bug description:
  [Impact]

   * Users performing live migration of guests with image files
     shared over NFS between the source and target host systems
     may experience I/O errors in the guests if user id of the
     libvirt-qemu user differs between the host systems, due to
     permission errors when accessing the image files.

   * The 16.04 LTS is an important stream for KVM (at least on
     Power), and guest live migration over NFS is an important
     feature on it.

   * The proposed fix (a minimal backport from what is applied
     on Zesty/Debian, so to be very conservative for the LTS)
     simply tries to use the reserved uid for the libvirt-qemu
     user on new installations (when the user is created) if
     the reserved uid is not taken by another user (no failures
     occur if the libvirt-qemu user already exists or the uid
     is taken.)

  [Test Case]

   * Setup 2x systems with Ubuntu 16.04 LTS as KVM hosts (e.g., micro and tiny)
     (check whether the libvirt-qemu uid differs between them;
      e.g. # id libvirt-qemu )

   * Create a guest in the source KVM host system (e.g, microg5)

   * Live migrate the guest to the destination KVM host system (e.g.,
  tiny)

     root at micro:~# virsh migrate --live --domain microg5 qemu+ssh://tiny/system
                   --verbose --undefinesource --persistent --timeout 60
     Migration: [100 %]

   * Check whether the guest is now listed in the destination KVM host
  system:

      root at tiny:~# virsh list --all
      Id Name State

      12 microg5 running

   * Check whether I/O errors are seen in that guest:

      root at microg5:~# dmesg
      ...
      [ 60.818955] blk_update_request: I/O error, dev vdc, sector 96749232
      [ 60.819113] Aborting journal on device vdc2-8.
      [ 60.820121] blk_update_request: I/O error, dev vdc, sector 9084320
      [ 60.820643] EXT4-fs warning (device vdc2): ext4_end_bio:329: I/O error
                   -5 writing to inode 393279 (offset 0 size 0 ...

  [Regression Potential]

   * On installations in which the libvirt-qemu user does not exist
     (e.g., first time installation of libvirt-bin) its assigned uid
     might be different from what the user previously expected, since
     now it's assigned a number from the reserved range.

   * Overall, the fix is very conservative (the uid assignment only
     happens in case: 1) the libvirt-qemu user is being created, and
     2) if the desired uid is not taken by another user, e.g. LDAP).

  [Other Info]

   * None at this time.

  <...>

  Please see comment #8 for the problem description, and summary of
  originally bridged comments in the description in later comments.

  Sorry about the inconvenience.

  <...>

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