test code v1

tests/TC119-data-crc-mismatch.md

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+# Test Case: Uncompressed Data Corruption Boundary
+
+**Test ID**: TC119-data-crc-mismatch
+**Category**: Data Integrity Semantics
+**Priority**: High
+
+## Objective
+
+Verify the actual EROFS integrity boundary. FLAT_PLAIN payload has no per-file
+CRC, so one targeted raw-data bit flip is returned to the caller and detected
+only by external byte/hash comparison. A fictional kernel data-CRC error is not
+expected.
+
+## Fixture Qualification
+
+```sh
+work=$(mktemp -d /tmp/repo22-tc119.XXXXXX)
+tests/prepare_error_fixtures.sh "$work/fixtures"
+cc -O2 -Wall -Wextra -std=c17 tests/read_probe.c -o "$work/read_probe"
+python3 tests/erofs_fixture.py inspect \
+  "$work/fixtures/raw-data-corrupt.erofs" --path /plain.bin
+grep '^raw-data-corrupt ' "$work/fixtures/fixture-evidence.txt"
+wc -c "$work/fixtures/valid-plain.erofs" \
+  "$work/fixtures/raw-data-corrupt.erofs"
+sha256 -q "$work/fixtures/raw-data-corrupt.erofs"
+```
+
+The mutator resolves `/plain.bin`, proves nonempty FLAT_PLAIN layout, flips bit
+`0x80` at file offset 257, records the absolute provider byte, preserves media
+size, and verifies that the unchanged superblock checksum remains valid.
+
+## FreeBSD Procedure
+
+```sh
+mkdir "$work/mnt"
+unit=$(mdconfig -a -t vnode -f "$work/fixtures/raw-data-corrupt.erofs")
+mount -t erofs -o ro "/dev/$unit" "$work/mnt"
+source_hash=$(sha256 -q "$work/fixtures/source/plain.bin")
+mounted_hash=$(sha256 -q "$work/mnt/plain.bin")
+test "$source_hash" != "$mounted_hash"
+"$work/read_probe" pread "$work/fixtures/source/plain.bin" 257 1 \
+  "$work/source-byte.bin"
+"$work/read_probe" pread "$work/mnt/plain.bin" 257 1 \
+  "$work/mounted-byte.bin"
+! cmp -s "$work/source-byte.bin" "$work/mounted-byte.bin"
+cmp "$work/fixtures/source/control.txt" "$work/mnt/control.txt"
+umount "$work/mnt"
+mdconfig -d -u "${unit#md}"
+```
+
+## Expected Results
+
+Mount and all reads succeed. The exact target byte and complete file hash differ
+from source, while control data matches. No `EIO` or data-CRC diagnostic is
+expected. Record source, image, mounted-file hashes, target bytes, dmesg delta,
+and cleanup.