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[Launchpad Bug Tracker <1858802@xxxxxxxxxxxxxxxxxx>]

You have been subscribed to a public bug by Eric Desrochers (slashd):

[Impact]

 * Users with an XFS filesystem on top of bcache
   (this is seen on some ceph, cloud deployments)
   might fail to reference the bcache device by
   UUID or other udev properties.

 * The journal of the regular XFS filesystem in
   the bcache device is incorrectly detected as
   an XFS external log; so two superblocks are
   detected (bcache and xfs_external_log).

 * Thus blkid fails with ambivalent superblocks
   detected then doesn't provide the usual udev
   properties (UUID, etc.)

 * The fix improves the probe function for XFS
   external log so it detects it's regular XFS
   and bails out.

[Test Case]

 * See test steps detailed in comment #7 and later.
   - Create an XFS filesystem with the journal/log
     in the beginning of the bcache device (< 256K).
   - Stop the bcache device.
   - Run '$ blkid -o udev -p $BCACHE_BACKING_DEVICE'.

   $ sudo make-bcache -B $BACKING_DEV
   $ sudo mkfs.xfs -d agsize=16m -l agnum=0 -f $BCACHE_DEV
   $ echo 1 | sudo tee /sys/block/$(basename $BCACHE_DEV)/bcache/stop
   $ sudo blkid -o udev -p $BACKING_DEV

[Regression Potential]

 * The patch only changes the detection function
   for XFS external log to be more general about
   the sector where the magic of regular XFS may
   be found (which is shifted inside the bcache.)

 * It still checks at sector zero (the only one
   checked previously), so this behavior didn't
   change.

 * Possible regressions are actual XFS external
   log devices that are not anymore detected as
   such. (Although that would probably indicate
   a different bug in libblkid.)

[Other Info]
 * upstream commit:
   https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/commit/?id=d756af7d640c51ce8d1414607bd3f17eeecf2424

** Affects: util-linux (Ubuntu)
     Importance: Medium
     Assignee: Mauricio Faria de Oliveira (mfo)
         Status: In Progress

** Affects: util-linux (Ubuntu Xenial)
     Importance: Medium
     Assignee: Mauricio Faria de Oliveira (mfo)
         Status: In Progress

** Affects: util-linux (Ubuntu Bionic)
     Importance: Medium
     Assignee: Mauricio Faria de Oliveira (mfo)
         Status: In Progress

** Affects: util-linux (Ubuntu Disco)
     Importance: Medium
     Assignee: Mauricio Faria de Oliveira (mfo)
         Status: In Progress

** Affects: util-linux (Ubuntu Eoan)
     Importance: Medium
     Assignee: Mauricio Faria de Oliveira (mfo)
         Status: In Progress

** Affects: util-linux (Ubuntu Focal)
     Importance: Medium
     Assignee: Mauricio Faria de Oliveira (mfo)
         Status: In Progress

** Affects: util-linux (Debian)
     Importance: Unknown
         Status: Unknown


** Tags: patch sts sts-sponsor-mfo
-- 
libblkid: no bcache UUID due to ambivalent detection of bcache and xfs_external_log for regular xfs in bcache backing device
https://bugs.launchpad.net/bugs/1858802
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