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retoorandClaude Sonnet 5 153d44ee3c

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Fix silent journal-write-failure data loss; confirm TSan blocked twice over

Prompted by "fix literally everything still open" after the previous
session's data-integrity work. Went through each open item in turn:

1. TSan: tried a genuinely different execution environment (a remote
   cloud sandbox, via a dedicated agent) rather than re-stating the local
   sandbox's limitation. Result: identical block there too --
   personality(ADDR_NO_RANDOMIZE) returns EPERM, a trivial pthread
   program fails TSan identically, and all 6 PackFS test binaries fail
   with the same FATAL: ThreadSanitizer: unexpected memory mapping
   signature. This is now confirmed in two independent environments, not
   one -- strong evidence it's a real infrastructure restriction, not a
   one-off fluke worth chasing further with the tools available here.

2. While investigating the "disk-full mid-write" gap flagged as untested
   last session, found two real, previously-unknown bugs by reading the
   journal code (not by a test catching them unprompted):

   - journal_append_record and everything that called it were void, and
     none of the fwrite/fflush/fsync calls inside had their return values
     checked. A real write failure (disk full, quota, I/O error) was
     silently reported as success to vfs_write/vfs_mkdir/vfs_unlink/
     vfs_rename -- directly contradicting Section 4.4's premise that
     success means durable.

   - Fixing that alone was not enough, confirmed by direct reproduction:
     a partial write leaves a torn record in the journal, and
     journal_replay correctly stops at the first record it can't fully
     read (Section 4.3) -- which means every record appended *after* the
     torn one, including ones that themselves wrote perfectly fine later,
     became silently unreachable on reopen. Reproduced directly before
     fixing: a forced-failed write followed by a genuinely successful one
     was unrecoverable. Fixed by rolling the journal file back to its
     exact pre-record length on any failed write.

   Both closed in src/overlay.c (journal_append_record/_put/_delete/
   _mkdir/journal_put_current now return and propagate success/failure;
   overlay_write/_mkdir/_unlink/_rename return VFS_ERR_IO on a durability
   failure without rolling back the already-applied in-memory change,
   the same asymmetry a real write()-then-failed-fsync() has). Covered
   permanently by the new tests/test_journal_failure.c, which forces a
   real failure via RLIMIT_FSIZE + ignored SIGXFSZ, not a mock.

   Also fixed in the same pass, found by inspection while touching this
   code: journal_put_current used to pass a NULL buffer into a memcpy of
   a nonzero size when malloc(size) failed (an OOM-triggered NULL-pointer
   dereference) -- closed with an explicit allocation-failure check.
   Not test-triggered (forcing malloc() failure portably isn't practical
   here); verified by code inspection instead, stated as such rather than
   claimed as tested.

3. The remaining "journal-truncation-specific crash window" gap from last
   session was investigated, not silently dropped: reliably targeting
   that narrow a window would need real concurrency (a second writer
   thread racing the kill) for benefit the existing compaction-crash test
   already gets probabilistically -- a poor trade, so left as a stated,
   deliberate non-goal (CLAUDE.md) rather than built.

4. Cross-process contention is NOT addressed here and should not be read
   as an oversight: it is concept.md's own explicit, permanent "not
   implemented in v0" scope boundary (a specified-but-unbuilt LMDB-style
   reader-table design), not a bug -- building it would be a large,
   unrequested feature addition outside this session's actual scope.

Verified: clean make all + make test (all 8 binaries), make bench and
make demo still build and the demo runs correctly end to end, and a full
ASan/UBSan sweep of all 8 binaries with zero real findings (some retries
needed for the already-documented DEADLYSIGNAL flake, which
test_crash_consistency hits more often than other tests simply because it
forks 60+ subprocesses per run -- noted in CONTRIBUTING.md so this isn't
mistaken for a regression later).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UqJpkdJ6Njnt1pw3CbghzB

2026-09-14 19:40:23 +00:00

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Security Policy

Supported versions

No version has been tagged yet (see CHANGELOG.md); this project is pre-1.0 (README.md's "Status"). Only the latest commit on the default branch is supported — there is no LTS branch and no backport policy while the project is at 0.x.

What PackFS actually claims as a security boundary

Stated precisely, per this project's documentation standard (CLAUDE.md), rather than a generic "we take security seriously":

Path containment for dir mounts is a hard requirement (concept.md Section 6, CLAUDE.md's "Load-bearing constraints"). On Linux 5.6+, every lookup beneath a dir mount uses openat2(RESOLVE_BENEATH | RESOLVE_NO_SYMLINKS) to atomically block .. escapes, symlink escapes, and the canonicalize-then-open TOCTOU window in one kernel call. vfs_harden_process_with_landlock (Section 6.2) is an additional, opt-in kernel-level layer — not applied automatically, since Landlock restrictions are cumulative, irreversible, and process-wide, which would be a surprising side effect for an embeddable library to trigger on its own.
On platforms or kernels without openat2, containment falls back to per-component O_NOFOLLOW plus canonicalize-and-verify (Section 6.3) — an accepted, explicitly weaker residual-risk posture, not a claim of equal strength. Code and documentation that describe this fallback as equivalent to the openat2 path would themselves be a bug worth reporting.
The VFS core canonicalizes ./.. in the virtual namespace itself, before any backend is reached, so a crafted path cannot escape a mount's prefix even when no host directory is involved.
A pack file is treated as untrusted input unless it came from this run's own compaction (Section 7). Every index entry's offsets and sizes are bounds-checked against the actual file/strings-region length before being trusted; a failed check invalidates the whole pack load. An externally-sourced pack additionally needs a checksum verified at load. Loading an attacker-supplied .img file that PackFS accepts without detecting corruption, or that causes an out-of-bounds read/write, is a security bug. That checksum (FNV-1a64, src/hash.c) is explicitly non-cryptographic and not collision-resistant — stated plainly here because this is the one place in this document where it matters most: it is a real, effective check against accidental corruption (a truncated copy, a bit flip, a bad transfer), but it is not a tamper- evident guarantee against a deliberate adversary who can choose the bytes of a .img file — someone with that capability could in principle construct a corrupted pack whose checksum still matches. Do not treat "checksum verified at load" as a substitute for verifying the source of an externally-supplied pack through some other channel if the threat model includes a deliberate adversary, not just accidental corruption.
Journal records are length/checksum self-describing (Section 4.4), so replay stops cleanly at the first torn or corrupted record rather than reading past it.
The crash-safety design above is verified by actual fault injection, not only by reasoning about the design. tests/test_crash_consistency.c forks a real child process and SIGKILLs it at randomized points during journaled writes and during compaction, then checks what a fresh reopen recovers, across dozens of trials per run — not a hand-truncated file standing in for a crash. See CLAUDE.md's "Known reliability characteristics" for the methodology and results, including a real bug the test itself had on its first draft (conflating "never attempted before the kill" with "lost after completing") — fixed before treating the crash-safety claim as verified.
A real, previously-unknown data-loss bug in this exact area was found and fixed: a failed journal write (disk full, quota, an I/O error) used to be silently reported as success, and even after fixing that, a partial write left behind a torn record that made every later, individually-successful write unrecoverable on reopen too — not just the one that actually failed. Both are fixed (src/overlay.c, journal_append_record) and covered by tests/test_journal_failure.c, which forces a real write failure via RLIMIT_FSIZE rather than simulating one. See CLAUDE.md's "Known reliability characteristics" for the full story, including how each half of the bug was confirmed by direct reproduction before and after the fix, not just inspection.

What PackFS explicitly does not claim

Reporting these as vulnerabilities is not necessary — they are documented, known limitations, not silent gaps:

mode (permission bits, symlinks, hard links) is stored and restored but not enforced. Nothing in the VFS core interprets it as an access control boundary in v0 (concept.md Section 10).
Cross-process concurrency is specified (an LMDB-style reader-table design, Section 5.6) but not implemented. Two uncoordinated processes writing the same host directory through a dir mount are not synchronized against each other.
zip/tar are permanently out of scope (CLAUDE.md, "Project decisions that supersede concept.md") — there is no archive-format parsing code in this project to have a vulnerability in.

Reporting a vulnerability

This project is hosted on Gitea, not GitHub (see CONTRIBUTING.md). Please do not open a public issue on the issue tracker for a suspected security vulnerability — report it privately, by email, to the maintainer:

retoor — retoor@molodetz.nl

When reporting, include: the affected function(s) or file(s), a minimal reproduction (ideally as a tests/test_*.c-style program using only the public API in include/packfs.h), and which of the guarantees above (or which concept.md section) you believe is violated — this project's own verification discipline (make test, ASan/UBSan/TSan per CONTRIBUTING.md) depends on being able to turn a report into a permanent regression test.

Verification this project already does

Every change to src/upper.c, src/overlay.c, or src/vfs.c is required to pass under -fsanitize=address,undefined and -fsanitize=thread before being considered done (CLAUDE.md) — this process has already caught and fixed a real heap-use-after-free in the snapshot-reclamation logic during development (see the reclaim_gate note in src/internal.h). .gitea/workflows/ci.yml (Gitea Actions) runs the full test suite plus both sanitizer passes on every push.

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