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tests/TC114-out-of-bounds-block.md

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+# 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.