# Test Case: Superblock CRC32C Validation

**Test ID**: TC002-superblock-crc32c
**Category**: Filesystem Metadata
**Priority**: Critical

## Objective

Verify the real EROFS superblock checksum field and both equivalent checksum
ranges, then prove that a valid image mounts while an equal-length image with
one covered byte changed and no checksum update is rejected.

## Fixture Qualification

Run from the repo22 root with erofs-utils 1.8.6:

```sh
work=$(mktemp -d /tmp/repo22-tc002.XXXXXX)
tests/prepare_error_fixtures.sh "$work/fixtures"
python3 tests/erofs_fixture.py inspect "$work/fixtures/valid-plain.erofs"
python3 tests/erofs_fixture.py inspect "$work/fixtures/bad-super-crc.erofs"
grep '^bad-super-crc ' "$work/fixtures/fixture-evidence.txt"
(cd "$work/fixtures" && sha256 -c SHA256SUMS)
```

For a 4096-byte block, the canonical calculation clears the little-endian
checksum at absolute byte 1028 and calculates CRC32C over bytes `[1024,4096)`
with initial value `0xffffffff`. The production verifier uses seed
`0x5045b54a` over `[1032,4096)`. The helper must print identical canonical and
kernel values, `equivalent=True`, and `valid=True` for the control. The bad
image changes byte 1088, preserves provider length, does not recompute CRC, and
must print `valid=False`.

## FreeBSD Procedure

```sh
mkdir "$work/mnt"
unit=$(mdconfig -a -t vnode -f "$work/fixtures/valid-plain.erofs")
mount -t erofs -o ro "/dev/$unit" "$work/mnt"
cmp "$work/fixtures/source/control.txt" "$work/mnt/control.txt"
umount "$work/mnt"
mdconfig -d -u "${unit#md}"

unit=$(mdconfig -a -t vnode -f "$work/fixtures/bad-super-crc.erofs")
set +e
mount -t erofs -o ro "/dev/$unit" "$work/mnt" \
  >"$work/bad-mount.out" 2>"$work/bad-mount.err"
mount_status=$?
set -e
printf 'mount_status=%s\n' "$mount_status"
test "$mount_status" -ne 0
set +e
truss -o "$work/bad-mount.truss" mount -t erofs -o ro "/dev/$unit" \
  "$work/mnt" >/dev/null 2>&1
set -e
grep -E 'nmount.*ERR#97' "$work/bad-mount.truss"
! mount -p | awk -v path="$work/mnt" '$2 == path { found=1 }
  END { exit found ? 0 : 1 }'
mdconfig -d -u "${unit#md}"
```

## Expected Results

The control mounts and reads exactly. The bad image fails with FreeBSD errno
97 (`EINTEGRITY`) and no mount remains. Record both image hashes, provider
lengths, checksum values/ranges, command status, new dmesg lines, and cleanup.