with debug we could confirm that other fields in the backup superblock are 
invalid,
such as the device size, which was not checked for not being a whole-disk 
device,
and the bytes length, which is greater than the superblock structure size (8243 
vs 1024).

so, the fix to check for the bytes field not being out-of-range resolves
this case.

primary superblock
586: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:77 :: entry                     
 
586: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:80 :: not sb or not magic

backup superblock
586: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:77 :: entry                     
 
586: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:84 :: is_bak 1                  
 
586: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:85 :: is_wholedisk 0            
 
586: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:86 :: sb->s_dev_size 
3_472_328_296_366_616_623
586: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:87 :: pr->size 1_072_693_248    
    
586: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:96 :: bytes 8243                
 
586: libblkid: LOWPROBE: incorrect checksum for type nilfs2, got 56C7A81B, 
expected 2D2D206B
586: libblkid: LOWPROBE: assigning SBBADCSUM [superblocks]                      
 
586: libblkid: LOWPROBE: nilfs2: primary=0, backup=1
586: libblkid: LOWPROBE: nilfs2: primary=0, backup=1, swap=1

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https://bugs.launchpad.net/bugs/1842437

Title:
  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:
  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'
  
https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/commit/?id=ac681a310c32319423297544833932f4d689a7a2

  $ git describe --contains ac681a310c32319423297544833932f4d689a7a2
  v2.29-rc1~172

  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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