# Test Case: Superblock CRC Field Mismatch

**Test ID**: TC016-superblock-crc-invalid
**Category**: Error Handling
**Priority**: High

## Objective

Verify that changing only the stored superblock checksum field causes
`EINTEGRITY`, with all protected bytes and provider length unchanged.

## Fixture Qualification

```sh
work=$(mktemp -d /tmp/repo22-tc016.XXXXXX)
tests/prepare_error_fixtures.sh "$work/fixtures"
grep '^bad-checksum-field ' "$work/fixtures/fixture-evidence.txt"
python3 tests/erofs_fixture.py inspect \
  "$work/fixtures/bad-checksum-field.erofs"
wc -c "$work/fixtures/valid-plain.erofs" \
  "$work/fixtures/bad-checksum-field.erofs"
sha256 -q "$work/fixtures/bad-checksum-field.erofs"
```

The mutator flips bit 0 of the little-endian checksum at absolute byte 1028,
does not recompute it, asserts checksum invalidity, and changes no other byte.

## FreeBSD Procedure

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

## Expected Results

Mount returns errno 97 (`EINTEGRITY`) with a checksum diagnostic. No mount or
md consumer remains.