[Bug 1842437] Re: Xenial: libblkid: fix false-positive/misdetection of nilfs2 filesystem with udev

[Mauricio Faria de Oliveira]

modified package:
--------

# dpkg -s libblkid1 | grep ^Version:
Version: 2.27.1-6ubuntu3.8+test20190903b1

# LIBBLKID_DEBUG=all udevadm test-builtin blkid /sys/block/loop0/loop0p1
...
7289: libblkid: LOWPROBE: [28] ext4:                                                
7289: libblkid: LOWPROBE:       reuse buffer: off=1024 len=1024 pr=0x5604292b7ec0       
7289: libblkid: LOWPROBE:       magic sboff=56, kboff=1                                 
7289: libblkid: LOWPROBE:       call probefunc()                                 
7289: libblkid: LOWPROBE:       reuse buffer: off=1024 len=1024 pr=0x5604292b7ec0
7289: libblkid:    PROBE: ext2_sb.compat = 0000003C:00000246:0000007B            
7289: libblkid: LOWPROBE: assigning UUID [superblocks]                           
7289: libblkid: LOWPROBE: assigning VERSION [superblocks]                       
7289: libblkid: LOWPROBE: assigning TYPE [superblocks]                           
7289: libblkid: LOWPROBE: assigning USAGE [superblocks] 
...
7289: libblkid: LOWPROBE: [63] nilfs2:                                          
7289: libblkid: LOWPROBE:       call probefunc()                                
7289: libblkid: LOWPROBE:       reuse buffer: off=1024 len=1024 pr=0x5604292b7ec0
7289: libblkid: LOWPROBE:       reuse buffer: off=1072689152 len=4096 pr=0x5604292b7ec0
7289: libblkid: LOWPROBE: [64] exfat: 
...
7289: libblkid: LOWPROBE: returning UUID value
ID_FS_UUID=bac40f54-41c7-4a5a-a3f2-87bf23546e9c
...
# echo $?
0

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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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