[Bug 1858802] Re: libblkid: no bcache UUID due to ambivalent detection of bcache and xfs_external_log for regular xfs in bcache backing device

[Eric Desrochers]

** Tags added: sts-sponsor-slashd-focal

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Title:
  libblkid: no bcache UUID due to ambivalent detection of bcache and
  xfs_external_log for regular xfs in bcache backing device

Status in util-linux package in Ubuntu:
  In Progress
Status in util-linux source package in Xenial:
  In Progress
Status in util-linux source package in Bionic:
  In Progress
Status in util-linux source package in Disco:
  In Progress
Status in util-linux source package in Eoan:
  In Progress
Status in util-linux source package in Focal:
  In Progress
Status in util-linux package in Debian:
  Unknown

Bug description:
  [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

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