Comment 6 for bug 1842437

This problem can be reproduced with a synthetic test case.

Essentially,
1) create a zero-filled disk image with one partition.
2) format it as ext4.
3) insert the nilfs2 magic bytes at the right position.
4) insert the nilfs2 bytes field at the right position.

--

Step 1)

 # dd if=/dev/zero of=test.img bs=512 count=$((16 * 2048)) # 16 MiB

 # dev=$(losetup --find --show test.img)

 # parted $dev --script 'mklabel msdos'
 # parted $dev --script "mkpart primary 1s $((16 * 2048 - 1))s"

 # parted $dev --script 'unit s' --script 'print'
 Model: Loopback device (loopback)
 Disk /dev/loop0: 32768s
 Sector size (logical/physical): 512B/512B
 Partition Table: msdos
 Disk Flags:

 Number Start End Size Type File system Flags
  1 1s 32767s 32767s primary

 # hexdump -C $dev
 ... <partition table until 0x200 (first sector) > ...
 *
 000001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 aa |..............U.|
 00000200 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
 *
 01000000

 # printf '\x00\x00\x00\x00\x00\x00\x34\x34' | hexdump -C
 00000000 00 00 00 00 00 00 34 34 |......44|
 00000008

 # partprobe $dev
 # part=${dev}p1

Step 2)

 # mkfs.ext4 $part

Steps 3 and 4)

 From gdb/source:

 (gdb) ptype/o struct nilfs_super_block
 /* offset | size */ type = struct nilfs_super_block {
 ...
 /* 6 | 2 */ uint16_t s_magic;
 /* 8 | 2 */ uint16_t s_bytes;
 ...

 #define NILFS_SB_MAGIC 0x3434
 #define NILFS_SB_OFFSET 0x400
 #define NILFS_SBB_OFFSET(_sz) ((((_sz) / 0x200) - 8) * 0x200)

 Then (magic is 0x3434, bytes is 0x0401 == 1025 in litte endian -- value targeted at the fix)

 # printf '\x00\x00\x00\x00\x00\x00\x34\x34\x01\x04' | dd of=$part bs=1 count=10 conv=notrunc seek=$(( ( (16 * 2048 - 1) - 8) * 512 ))

 # hexdump -C $part
 00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
 *
 00ffee00 00 00 00 00 00 00 34 34 01 04 00 00 00 00 00 00 |......44........|
 00ffee10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
 *
 00fffe00

Original Package)

 # LIBBLKID_DEBUG=all udevadm test-builtin blkid /sys/block/$(basename $dev)/$(basename $part)
 ...
 9576: libblkid: LOWPROBE: [28] ext4:
 9576: libblkid: LOWPROBE: reuse buffer: off=1024 len=1024 pr=0x55744f2daec0
 9576: libblkid: LOWPROBE: magic sboff=56, kboff=1
 9576: libblkid: LOWPROBE: call probefunc()
 9576: libblkid: LOWPROBE: reuse buffer: off=1024 len=1024 pr=0x55744f2daec0
 9576: libblkid: PROBE: ext2_sb.compat = 0000003C:00000242:0000007B
 9576: libblkid: LOWPROBE: assigning UUID [superblocks]
 9576: libblkid: LOWPROBE: assigning VERSION [superblocks]
 9576: libblkid: LOWPROBE: assigning TYPE [superblocks]
 9576: libblkid: LOWPROBE: assigning USAGE [superblocks]
 9576: libblkid: LOWPROBE: <-- leaving probing loop (type=ext4) [SUBLKS idx=28]
 ...
 9576: libblkid: LOWPROBE: [63] nilfs2:
 9576: libblkid: LOWPROBE: call probefunc()
 9576: libblkid: LOWPROBE: reuse buffer: off=1024 len=1024 pr=0x55744f2daec0
 9576: libblkid: LOWPROBE: reuse buffer: off=16772608 len=4096 pr=0x55744f2daec0
 9576: libblkid: LOWPROBE: incorrect checksum for type nilfs2, got 2C077E8F, expected 0
 9576: libblkid: LOWPROBE: assigning SBBADCSUM [superblocks]
 9576: libblkid: LOWPROBE: nilfs2: primary=0, backup=1, swap=1
 9576: libblkid: LOWPROBE: assigning VERSION [superblocks]
 9576: libblkid: LOWPROBE: assigning TYPE [superblocks]
 9576: libblkid: LOWPROBE: assigning USAGE [superblocks]
 9576: libblkid: LOWPROBE: <-- leaving probing loop (type=nilfs2) [SUBLKS idx=63]
 ...
 9576: libblkid: LOWPROBE: ERROR: superblocks chain: ambivalent result detected (2 filesystems)!
 ...
 # echo $?
 1

Original Package with Debug Print)

 9604: libblkid: LOWPROBE: [63] nilfs2:
 9604: libblkid: LOWPROBE: call probefunc()
 9604: libblkid: LOWPROBE: reuse buffer: off=1024 len=1024 pr=0x55c076c8eec0
 9604: libblkid: LOWPROBE: reuse buffer: off=16772608 len=4096 pr=0x55c076c8eec0
 9604: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:77 :: entry
 9604: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:80 :: not sb or not magic
 9604: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:77 :: entry
 9604: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:84 :: is_bak 1
 9604: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:85 :: is_wholedisk 0
 9604: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:86 :: sb->s_dev_size 0
 9604: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:87 :: pr->size 16776704
 9604: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:96 :: bytes 1025
 9604: libblkid: LOWPROBE: incorrect checksum for type nilfs2, got 2C077E8F, expected 0
 9604: libblkid: LOWPROBE: assigning SBBADCSUM [superblocks]
 9604: libblkid: LOWPROBE: nilfs2: primary=0, backup=1
 9604: libblkid: LOWPROBE: nilfs2: primary=0, backup=1, swap=1

Modified Package)

 9691: libblkid: LOWPROBE: [63] nilfs2:
 9691: libblkid: LOWPROBE: call probefunc()
 9691: libblkid: LOWPROBE: reuse buffer: off=1024 len=1024 pr=0x55c12bac1ec0
 9691: libblkid: LOWPROBE: reuse buffer: off=16772608 len=4096 pr=0x55c12bac1ec0
 9691: libblkid: LOWPROBE: [64] exfat:

Modified Package with Debug Print)

 9632: libblkid: LOWPROBE: [63] nilfs2:
 9632: libblkid: LOWPROBE: call probefunc()
 9632: libblkid: LOWPROBE: reuse buffer: off=1024 len=1024 pr=0x557da562eec0
 9632: libblkid: LOWPROBE: reuse buffer: off=16772608 len=4096 pr=0x557da562eec0
 9632: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:78 :: entry
 9632: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:81 :: not sb or not magic
 9632: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:78 :: entry
 9632: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:85 :: is_bak 1
 9632: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:86 :: is_wholedisk 0
 9632: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:87 :: sb->s_dev_size 0
 9632: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:88 :: pr->size 16776704
 9632: libblkid: LOWPROBE: nilfs2: nilfs_valid_sb:100 :: bytes 1025
 9632: libblkid: LOWPROBE: nilfs2: primary=0, backup=0
 9632: libblkid: LOWPROBE: [64] exfat: