[nmatt0]

Summary

Adds read-only extraction for ReiserFS (3.5 / 3.6). Identification landed with the legacy-block-filesystem batch (#24); until now -e on a ReiserFS image printed the summary table and wrote nothing, because extractor_for() had no reiserfs case. Closes #63.

What it does

src/extract/reiserfs.{cpp,hpp}, registered in extractor_for():

• Reads the superblock (root block, block size, block count), recovers the partition base for both the 64 KiB (3.6) and 8 KiB (3.5) superblock placements, and validates the root block before walking.
• Walks the S+ tree from the root block, collecting every leaf item. Internal nodes are traversed by child pointer only; internal-node keys are never interpreted, so the v1/v2 key-format ambiguity only has to be resolved at leaf items (per item, from ih_version).
• Rebuilds the tree under the existing SafeRoot: regular files (direct tails plus indirect block lists, with holes honored), directories, and symlinks. Objects are grouped by (dir_id, objectid); each child's stat-data gives its mode, and files are reconstructed from their direct/indirect items in key order.

Per-item key handling matters in practice: a real 3.6 image mixes a v2 stat-data item with a v1 directory item in the same object, which this handles.

Safety

• Every read goes through the bounds-checked Reader; a truncated or corrupt image yields a partial result, never an out-of-bounds read.
• Guardrails on node/item/file/entry counts and recursion depth, plus cycle guards on both tree blocks and the directory recursion.
• A declared file size larger than the image is clamped, so a crafted size field cannot force a giant zero-filled allocation.
• No journal replay, no write support, no mount, no sudo (unlike mount / debugreiserfs).

Tests

tests/test_reiserfs_extract.py (wired into run.sh and ctest):

1. Synthetic, always-on: hand-built 3.6 (two-level tree exercising internal-node traversal, v2 stat-data + v1 directory items, indirect items, a tail, a leading hole, a symlink, a nested dir) and 3.5 (single leaf, all-v1, 8 KiB superblock) images, extracted and compared byte-for-byte. The layouts are modeled on real mkreiserfs output verified with debugreiserfs.
2. Real empty image: mkreiserfs builds an empty filesystem (no mount needed) and moria must identify and extract it cleanly. Self-skips if reiserfsprogs is absent. This checks the parser against genuine on-disk bytes.
3. Corruption robustness: mangled superblock/tree inputs must not crash and must report a sane status (the ASan build is the OOB oracle).

A populated real round-trip is not possible unprivileged: mkreiserfs has no populate-from-directory mode, and the kernel driver was removed in 6.13, so neither a loop mount nor a userspace populate is available. The synthetic layer (modeled on verified real bytes) plus the real empty-image check cover every code path, and all format constants come from the kernel fs/reiserfs/reiserfs.h header.

fuzz/fuzz_reiserfs.cpp fuzzes the parse-only tree/item/directory/content walk (a fixed valid superblock, fuzzer-controlled tree region, both placements), plus the scan pipeline on the raw bytes.

Docs

ReiserFS added to the extractable-filesystem lists in README.md and moria.1.