# Test Case: Out-of-Bounds FLAT_PLAIN Start Block

**Test ID**: TC114-out-of-bounds-block
**Category**: Error Handling
**Priority**: Critical

## Objective

Verify checked data mapping rejects `/plain.bin` when its resolved
`startblk_lo` equals the declared filesystem block count, while valid files in
the same mounted image remain readable.

## Fixture Qualification

```sh
work=$(mktemp -d /tmp/repo22-tc114.XXXXXX)
tests/prepare_error_fixtures.sh "$work/fixtures"
cc -O2 -Wall -Wextra -std=c17 tests/read_probe.c -o "$work/read_probe"
python3 tests/erofs_fixture.py inspect \
  "$work/fixtures/oob-start-block.erofs" --path /plain.bin
grep '^oob-start-block ' "$work/fixtures/fixture-evidence.txt"
sha256 -q "$work/fixtures/oob-start-block.erofs"
```

The mutator proves a nonempty FLAT_PLAIN path, records its NID and inode field
offset, sets `startblk_lo` to `blocks_lo`, recomputes CRC32C, and preserves
provider length.

## FreeBSD Procedure

```sh
mkdir "$work/mnt"
unit=$(mdconfig -a -t vnode -f "$work/fixtures/oob-start-block.erofs")
mount -t erofs -o ro "/dev/$unit" "$work/mnt"
cmp "$work/fixtures/source/control.txt" "$work/mnt/control.txt"
truss -o "$work/read.truss" "$work/read_probe" expect-error \
  "$work/mnt/plain.bin" 97 | tee "$work/error.out"
grep -q 'expected_errno=97' "$work/error.out"
grep -E 'read.*ERR#97' "$work/read.truss"
cmp "$work/fixtures/source/control.txt" "$work/mnt/control.txt"
umount "$work/mnt"
mdconfig -d -u "${unit#md}"
```

## Expected Results

The target read returns errno 97 (`EINTEGRITY`) without issuing bytes from a
different range. Control data remains exact and cleanup is complete.