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

[Mauricio Faria de Oliveira]

udev builtin blkid ignores the checksum error:

nilfs_valid_sb() @ nilfs.c:96

        return blkid_probe_verify_csum(pr, crc, le32_to_cpu(sb->s_sum));

blkid_probe_verify_csum() @ probe.c:1576-1687

        if (csum != expected) {
		...
                DBG(LOWPROBE, ul_debug(
                                "incorrect checksum for type %s,"
                                " got %"PRIX64", expected %"PRIX64"",
		...
                 * Accept bad checksum if BLKID_SUBLKS_BADCSUM flags is set
		...
                        blkid_probe_set_value(pr, "SBBADCSUM", (unsigned char *) "1", 2);
                        goto accept;
                }
                return 0;       /* bad checksum */
	...
	accept:
		return 1;

from debug:

	586: libblkid: LOWPROBE: incorrect checksum for type nilfs2, got 56C7A81B, expected 2D2D206B
	586: libblkid: LOWPROBE: assigning SBBADCSUM [superblocks] 

and udev blkid uses that flag:

builtin_blkid() @ udev-builtin-blkid.c:264-268

        blkid_probe_set_superblocks_flags(pr,
		...
                BLKID_SUBLKS_BADCSUM);

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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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