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  Xenial: libblkid: fix false-positive/misdetection of nilfs2 filesystem
  with udev

Status in util-linux package in Ubuntu:
  Fix Released
Status in util-linux source package in Xenial:
  In Progress

Bug description:

   * Users / systemd can fail to mount a filesystem by UUID
     (e.g., during boot, triggering emergency shell prompt)
     if the magic bytes for the nilfs filesystem are written
     to the right place in a partition of another filesystem,
     (for whatever reason or coincidence).

   * Note this can happen after the filesystem/mount is working
     correctly, so a change of behavior/problem can potentially
     be noticed when trying to mount the filesystem again, which
     can very well be the next time the system boots.

   * This happens because if udev blkid detects more than one
     filesystem, it does not print the UUID env vars required
     to create the /dev/disk/by-id symlinks and other things.

   * The fix enhances the check for valid nilfs superblock by
     specifically checking a value read from disk to be valid/
     within a value range, which addresses this one occurrence
     and prevents a lot more.

  [Test Case]

   * Synthetic test case written for this problem on comment #6.

  [Regression Potential]

   * Low.  The code is contained in the probe for the nilfs filesystem.

   * This just makes it be more restrictive about the possibly valid
     values for a few bytes read from disk (that now need to be within
     the acceptable range of valid values) so this only decreases false-
     positives, and cannot increase false-negatives of valid filesystems.

  [Original Description]

  The nilfs filesystem has a backup superblock at the end of the device.

  If the magic number is coincidentally found at the right position
  and the filesystem is on a partition/not-wholedisk device,
  the only check left is for checksum verification,
  which is explicitly ignored in 'udev built-in blkid'.

  This causes blkid to detect one actually valid filesystem with a
  superblock at the beginning of the device (e.g., ext4), and then
  an invalid nilfs2 filesystem due to a coincidental magic number
  at the end of the device.

  And this causes blkid to break out of the safeprobe routine
  (which expects a single filesystem to be detected), and not
  print the UUIDs, thus not creating /dev/disk/by-uuid/ links
  which prevent mounting the partition by-uuid at boot time,
  causing emergency shell/boot failures.

  This upstream fix resolved the problem by introducing a check
  for the 'bytes' paramenters in the superblock, which is read
  from disk, and turns out to have an out-of-range value.

  - 'liblkid: Add length check in probe_nilfs2 before crc32'

  $ git describe --contains ac681a310c32319423297544833932f4d689a7a2

  Xenial, which is v2.27.1-based, is the only release that needs it.
  Bionic is v2.31.1, so all post-Xenial supported releases have it.

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