PackFS

A statically linked, in-process virtual file system for C. PackFS treats a shipped file tree as an immutable pack image and derives writability from a mem or dir upper layer through a copy-on-write overlay. zip and tar are not part of this project and never will be — see "Status" below.

The full design rationale — why this shape, what alternatives were rejected, and the concurrency, path-containment, and integrity models this implementation follows — is specified in concept.md, which is frozen (see CLAUDE.md) and is the authoritative source for every design decision below. This README documents the implementation that followed from it, not a restatement of the rationale.

Status

This is an initial, partial implementation of the spec, not a complete one.

Implemented and tested: mount table, mem/dir/pack/overlay backends (including the standalone read-only pack backend, backend_pack_new — every mutating call against it returns VFS_ERR_PERM), copy-up, whiteouts, compaction, an append journal, path containment, and pack integrity validation.

Will never be built, by explicit project decision: the zip (miniz) and tar (USTAR) import/export backends. concept.md Section 11 recommends them, but that recommendation is superseded — see CLAUDE.md, "Project decisions that supersede concept.md." backend_overlay_new reads and writes this project's own pack format exclusively; there is no zip/tar support and none is planned. Do not open an issue or PR adding one.

Deliberately out of scope for v0 (concept.md Section 10, not gaps): full POSIX semantics, enforced permissions/symlinks/hard links, cross-process concurrency (design specified in Section 5.6, unimplemented), and content-defined chunking/delta compression.

Tested on Linux only, in one environment. The dir-mount containment fallback path for kernels without openat2 (Section 6.3) is implemented but has not been exercised on such a kernel, nor on macOS or Windows.

Building

Zero required third-party dependencies — only a C11 compiler, make, and pthread (Section 11.1 of concept.md makes this a hard constraint, not a preference).

make            # builds libpackfs.a and libpackfs.so
make test       # builds and runs the test suite
make demo       # builds and runs examples/demo.c — see "Try it" below
make bench      # builds and runs bench/bench.c — see "Benchmarks" below
make install    # installs to $PREFIX (default /usr/local), including a pkg-config file

make install also generates and installs packfs.pc, so a consuming project can build against PackFS with pkg-config --cflags --libs packfs instead of hardcoding -lpackfs -lpthread. The installed version always matches PACKFS_VERSION_STRING in include/packfs.h — pkg-config's Version: field and a runtime pfs_version() call are both derived from that one header, never maintained separately, so they cannot drift apart.

Try it

examples/demo.c is a small, runnable, human-readable program — not another automated test — that exercises the library end to end and prints what it did at each step: a pack-backed overlay (write, read, mkdir, readdir, stat, a copy-up-then-whiteout delete, compaction via vfs_sync), and a sandboxed dir mount that demonstrates a ../../../etc/passwd escape attempt being rejected. Run make demo twice in a row: the second run's first readdir shows the first run's files, proving that compaction and reload actually persist data through the pack file, not just within one process's lifetime.

Benchmarks

bench/bench.c (make bench) measures PackFS against the host filesystem across metadata operations (create/read/stat/readdir/unlink/mkdir), large sequential I/O, random-access pack reads, mount-table scaling, and concurrent mixed workloads. BENCH.md has the full results and honest analysis of both the wins and the losses, including the story of three real O(n²) findings this project's own benchmarking turned up — not just the wins. Two are fixed: bulk sequential file creation (src/upper.c's index is a persistent treap now, not a flat array — see "Resolution") and pack_write's compaction-time duplicate-content elimination, which also had a latent correctness bug now closed alongside it (see "Resolution #2"). One is confirmed and deliberately not fixed: the mount table scales O(n²) in mount count, the same way the file index used to, but mount counts are bounded by a program's own source code rather than workload-driven, so it isn't worth the added complexity — see "Finding: mount table scaling" for the reasoning and the numbers behind that call. Read the whole file before quoting a number from it: what raw fs vs raw+fsync vs dir each actually measure is not interchangeable, and it explains why.

openat2/Landlock support (Section 6) is detected automatically at compile time via <sys/syscall.h>; on kernels or platforms without them, dir mounts fall back to the weaker, documented residual-risk posture described in concept.md Section 6.3 rather than failing to build.

Reproducibility spot-check (2026-09-14)

A fresh make bench run, compared against the numbers currently documented in BENCH.md's "After" table, on the same environment described there. Not a replacement for BENCH.md — a spot-check confirming the documented numbers reproduce within normal single-run variance, per the methodology BENCH.md itself states ("illustrative of shape... not precise absolute figures").

Category	Backend	Documented (BENCH.md)	New run	Delta
create 20,000 files	mem	0.0400s	0.0394s	-1.5%
create 20,000 files	raw fs	0.9943s	1.0171s	+2.3%
create 20,000 files	raw+fsync	116.2147s	116.7567s	+0.5%
read 20,000 files	mem	0.0099s	0.0091s	-8.1%
read 20,000 files	raw fs	0.1606s	0.1634s	+1.7%
stat 20,000 files	mem	0.0077s	0.0078s	+1.3%
stat 20,000 files	raw fs	0.0699s	0.0727s	+4.0%
readdir (20,000 entries)	mem	0.0041s	0.0043s	+4.9%
readdir (20,000 entries)	raw fs	0.0069s	0.0072s	+4.3%
create 20,000 files	dir	1.1418s	1.2030s	+5.4%
read 20,000 files	dir	0.1530s	0.1715s	+12.1%
stat 20,000 files	dir	0.1379s	0.1546s	+12.1%
readdir (20,000 entries)	dir	0.0037s	0.0041s	+10.8%
unlink 20,000 files	mem	0.0179s	0.0182s	+1.7%
unlink 20,000 files	dir	0.4911s	0.5978s	+21.7%
unlink 20,000 files	raw fs	0.4746s	0.5062s	+6.7%
mkdir 4,000 dirs	mem	0.0056s	0.0050s	-10.7%
rmdir 4,000 dirs	mem	0.0040s	0.0039s	-2.5%
mkdir 4,000 dirs	dir	0.1710s	0.1864s	+9.0%
rmdir 4,000 dirs	dir	0.1257s	0.1169s	-7.0%
mkdir 4,000 dirs	raw fs	0.1374s	0.1444s	+5.1%
rmdir 4,000 dirs	raw fs	0.0951s	0.1005s	+5.7%
write 1MB	mem	0.0001s	0.0001s	+0.0%
read 1MB	mem	0.0000s	0.0000s	+0.0%
write 16MB	mem	0.0110s	0.0110s	+0.0%
read 16MB	mem	0.0009s	0.0010s	+11.1%
write 64MB	mem	0.0586s	0.0621s	+6.0%
read 64MB	mem	0.0032s	0.0036s	+12.5%
write 1MB	raw fs	0.0004s	0.0004s	+0.0%
read 1MB	raw fs	0.0001s	0.0001s	+0.0%
write 16MB	raw fs	0.0044s	0.0047s	+6.8%
read 16MB	raw fs	0.0010s	0.0012s	+20.0%
write 64MB	raw fs	0.0186s	0.0189s	+1.6%
read 64MB	raw fs	0.0047s	0.0050s	+6.4%
write 1MB	raw+fsync	0.0180s	0.0340s	+88.9%
read 1MB	raw+fsync	0.0001s	0.0001s	+0.0%
write 16MB	raw+fsync	0.0231s	0.0242s	+4.8%
read 16MB	raw+fsync	0.0012s	0.0012s	+0.0%
write 64MB	raw+fsync	0.0803s	0.0835s	+4.0%
read 64MB	raw+fsync	0.0048s	0.0049s	+2.1%
compact 20,000 entries to pack	pack	0.0267s	0.0280s	+4.9%
random-read 20,000 entries	pack (mmap'd)	0.0082s	0.0085s	+3.7%
random-read 20,000 entries	raw fs	0.1629s	0.1912s	+17.4%
concurrent create+read+unlink (8×4,000×3)	mem	0.2750s	0.2791s	+1.5%
concurrent create+read+unlink (8×4,000×3)	raw fs	5.6498s	6.1583s	+9.0%
mount 500 backends	vfs	0.0054s	0.0060s	+11.1%
resolve, 500 mounts	vfs	0.0020s	0.0022s	+10.0%
unmount 500 backends	vfs	0.0046s	0.0046s	+0.0%
mount 2,000 backends	vfs	0.0831s	0.0863s	+3.9%
resolve, 2,000 mounts	vfs	0.0296s	0.0329s	+11.1%
unmount 2,000 backends	vfs	0.0758s	0.0780s	+2.9%
mount 8,000 backends	vfs	1.4739s	1.5380s	+4.3%
resolve, 8,000 mounts	vfs	0.4938s	0.5298s	+7.3%
unmount 8,000 backends	vfs	1.5172s	1.5191s	+0.1%

Almost every row sits within ±15% of the documented figures — consistent with the single-run jitter BENCH.md already warns about, not a regression. Two rows exceed that: unlink 20,000 files (dir) (+21.7%, plausible container/host I/O noise, same direction as the other dir metadata rows this run) and write 1MB (raw+fsync) (+88.9%, but this is a tiny absolute value — 18ms vs 34ms for one syscall — the single number most sensitive to one slow fsync on this container's overlay filesystem, not a meaningful regression at that scale). Total wall-clock: 4m7.7s this run vs 5m23.9s documented, itself within the same single-run variance.

Quick example

#include <packfs.h>

Vfs *v = vfs_new();

/* a plain in-memory writable tree */
Backend *mem = backend_mem_new();
vfs_mount(v, "/", mem);

int err = 0;
VfsFile *f = vfs_open(v, "/hello.txt", VFS_O_WRONLY | VFS_O_CREAT, &err);
vfs_write(f, "hello", 5);
vfs_close(f);

vfs_free(v);
backend_free(mem);

A shipped pack with a writable overlay on top:

Backend *mem = backend_mem_new();
int err = 0;
Backend *ov = backend_overlay_new("assets.pack", mem, &err); /* loads assets.pack if it exists */
vfs_mount(v, "/", ov);

/* ... reads served straight from the pack; writes copy-up into mem ... */

vfs_sync(v, "/"); /* compacts the overlay into a fresh assets.pack (Section 4.1, 5.3) */

A sandboxed host directory:

int derr = 0;
Backend *dir = backend_dir_new("/var/lib/myapp/data", &derr);
vfs_mount(v, "/data", dir);
/* every lookup under /data is contained to that directory (Section 6.2);
 * ".." and symlink escapes are rejected, not merely discouraged. */

A shipped pack mounted read-only, no writable layer at all:

int perr = 0;
Backend *ro = backend_pack_new("assets.pack", &perr);
vfs_mount(v, "/assets", ro);
/* vfs_write, vfs_mkdir, vfs_unlink, vfs_rename, vfs_sync against anything
 * under /assets all return VFS_ERR_PERM; there is no upper layer to
 * absorb a write into. */

API

The public API is include/packfs.h; every function and struct is documented there with a pointer to the concept.md section that specifies its behavior. In outline:

• vfs_new / vfs_free — a Vfs owns a mount table, nothing else.
• backend_mem_new / backend_dir_new / backend_pack_new / backend_overlay_new — construct a backend; vfs_mount/vfs_unmount attach or detach it at a path prefix. backend_pack_new mounts a pack read-only, with no writable upper layer at all — every mutating call against it returns VFS_ERR_PERM; wrap the same pack in backend_overlay_new instead when writability is wanted.
• vfs_open / vfs_read / vfs_write / vfs_close — file I/O.
• vfs_stat / vfs_readdir / vfs_mkdir / vfs_unlink / vfs_rename — metadata and namespace operations.
• vfs_sync — compacts an overlay mount into a fresh pack.
• vfs_harden_process_with_landlock — optional, opt-in, process-wide Landlock confinement to the process's current dir mounts. Deliberately not applied automatically by vfs_mount (Section 6.2 explains why: Landlock restrictions are irreversible and process-wide, which would be a surprising side effect for an embeddable library to trigger on its own).

Concurrency

Single-writer, wait-free-reader (Section 5): readers never take a lock and never observe a write in progress; structural changes (create/unlink/rename/ mkdir/mount/unmount) publish a new immutable snapshot via one atomic pointer swap; ordinary content writes to an already-existing file update a per-entry cell directly and never touch the snapshot. mem-backed buffer growth never mutates a buffer address a reader might be reading (Section 5.7): growth always allocates a new buffer and publishes it, never reallocates in place. tests/test_concurrency.c exercises this under concurrent reader and writer threads, and the suite is regularly run under ThreadSanitizer and AddressSanitizer (see .github/workflows/ci.yml).

Security

dir mounts are capability-scoped (Section 6.2): a mount holds an already-open directory file descriptor, not a path string, and every lookup beneath it is resolved with openat2(RESOLVE_BENEATH | RESOLVE_NO_SYMLINKS) on Linux 5.6+, which atomically rejects .. and symlink escapes in one kernel call. Where that syscall is unavailable, containment falls back to per-component O_NOFOLLOW resolution — weaker, and documented as such (Section 6.3), not silently assumed equivalent. Pack files are treated as untrusted input unless they came from this process's own compaction: every on-disk offset is bounds-checked and the pack's checksum is verified before any of it is trusted (Section 7). See tests/test_dir.c for a containment regression test and tests/test_pack_overlay.c for a corrupted-pack rejection test.

Contributing

See CONTRIBUTING.md. Read concept.md and CLAUDE.md first — they are the project's actual specification and its enforced documentation standard, respectively, and every design decision in the code traces back to one of them.

License

MIT — see LICENSE.