Comment 3 for bug 1842437

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