fix(node:fs): preserve POSIX locks in realpath

[steipete]

What does this PR do?

Bun's POSIX realpath implementation opened the target, looked up its descriptor path, then closed it. Closing any descriptor for an inode releases that process's traditional POSIX record locks, so resolving a SQLite database path could silently release a live connection's lock. Opening the target also wrongly required read permission on macOS, rejecting valid unreadable files and search-only directories.

The path first passed through Bun's loose lexical normalizer. That could erase a symlink before a following .., or interpret literal POSIX backslashes as separators and resolve a different existing file.

Resolution no longer opens an ordinary descriptor for the target. Linux keeps main's O_PATH plus /proc/self/fd lookup (closing an O_PATH file never releases POSIX locks), macOS uses descriptor-free getattrlist(ATTR_CMN_FULLPATH), which matches F_GETPATH including firmlinks, and both fall back to libc realpath. Native and promise APIs pass the original component sequence to the OS. Ordinary fs.realpath and fs.realpathSync retain Node's lexical dot-segment normalization, using the existing POSIX-specific normalizer so backslashes remain literal. Filesystem policy stays with the OS canonicalizer.

It supersedes closed #42277. The locking defect was found while validating OpenClaw under Bun; the permission and backslash cases were reproduced through fs-safe's public APIs and then reduced to direct Node-compatible filesystem tests.

How did you verify your code works?

• Added regressions on macOS and Linux that hold a real POSIX fcntl lock and check sync, native, promise, and callback realpath variants from a child process. Released Bun 1.4.2 loses the lock on macOS, and an ordinary-descriptor negative control loses it on both systems; the patched implementation preserves it. A macOS test compares every variant with F_GETPATH through symlink chains and the /Users firmlink.
• Retained the symlink-before-.. collision test, including Node's intentional ordinary/native API distinction.
• Added ten regression cases across all five API variants: literal-backslash collisions, symlinks, relative paths, string/Buffer input and output, unreadable files, and search-only directories. Released Bun 1.4.2 fails all ten. The earlier PR head passed eight; this follow-up fixes the remaining two ordinary-API backslash failures.
• Patched debug+ASAN Bun: complete test/js/node/fs/fs.test.ts passed 567 tests, with 16 platform/capability skips.
• Vendored Node conformance files passed: test-fs-realpath.js (18 subtests), test-fs-realpath-native.js, test-fs-realpath-buffer-encoding.js, and test-fs-realpath-pipe.js.
• rust:check-all passed all 12 targets, with no failed or skipped targets, on the final source. Actual build/runtime proof was macOS arm64; cross-target checks are compilation checks.
• Rustfmt, Prettier, git diff --check, and independent P0–P2 review passed.

The current host's macOS 27 SDK is incompatible with the pinned LLVM 21 headers. The debug+ASAN build used an installed macOS 26.5 SDK through a task-local developer-directory view; no global Xcode configuration, source warning checks, or installed Bun runtime changed.

Earlier proof on this PR also ran OpenClaw's managed-update writer-exclusion suite with patched release Bun: 44 tests passed, with one unrelated #40005 qualification skipped.

This change was developed with AI assistance.

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Walkthrough

On POSIX, realpath_inner selects the path according to the realpath variant, checks its length, and calls Syscall::realpath directly. The promise API delegates to fs.realpathNative. Tests cover traversal, path handling, permissions, lock preservation, and error cases.

Changes

POSIX realpath behavior

Layer / File(s)	Summary
Direct POSIX realpath implementation src/runtime/node/node_fs.rs, src/js/node/fs.promises.ts	realpath_inner selects emulated or native paths, checks input length, and calls Syscall::realpath directly. Promise realpath uses fs.realpathNative.
Realpath regression coverage test/js/node/fs/fs.test.ts	Tests cover symlink traversal before .., literal backslashes, resolution without read permission, Darwin lock preservation, ENOSYS assertions, and ENAMETOOLONG.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to c6411

No actionable issue remains in the supplied review. The change is mergeable after normal checks.

Security Architecture Review

Security architecture risk: 🔵 Low · up to c6411

The change removes a lock-release hazard and leaves path resolution with the operating system. No new security bypass is established, but applications that use resolved paths for authorization may need to account for the changed promise API behavior.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — Callers supplying filesystem paths to Bun's POSIX realpath APIs can observe the changed resolution behavior, particularly through the promise API. The examined code establishes no additional filesystem privilege or cross-tenant authority.

Trust Boundaries and Controls

• inferred — Attacker-influenced path components can change the resolved filename through symlinks or literal backslashes, but the examined change sends the selected path to operating-system canonicalization. Whether a downstream caller treats the returned path as an authorization decision remains unverified.

Resilience and Maintainability Implications

• inferred — Removing the target open/close transition addresses the documented process-lock failure mode on both success and syscall error paths. The Darwin case covers the public variants but is conditional on a C compiler and does not establish interruption behavior.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Issue [#42277] is closed and supplies historical context only. No active directly linked issue supplies coding requirements for this pull request. The reported changes address the historical POSIX `re…
Out of Scope Changes check	✅ Passed	The changes stay within POSIX realpath behavior. Direct Syscall::realpath calls, variant-specific path handling, promise delegation, and tests for symlink traversal, literal backslashes, permissio…
Title check	✅ Passed	The title clearly identifies the primary change: preserving POSIX locks in Node.js realpath handling.
Description check	✅ Passed	The description includes both required sections. It clearly explains the problem, implementation, scope, regression coverage, verification results, and known build limitation.

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⚠️ Outside diff range comments (1)

src/runtime/node/node_fs.rs (1)

7386-7398: 🎯 Functional Correctness | 🟠 Major | ⚡ Quick win

Account for an existing NUL terminator in the POSIX length check

PathBuffer has MAX_PATH_BYTES bytes, and POSIX PATH_MAX includes the terminating NUL. PathLikeExt::slice_z reuses an input slice that already ends in NUL. Therefore, a valid path with MAX_PATH_BYTES - 1 bytes plus its NUL has path_slice.len() == inbuf.len() and receives ENAMETOOLONG before Syscall::realpath. Allow an already terminated slice at this boundary, while still rejecting an unterminated slice that needs an additional byte.

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In `@src/runtime/node/node_fs.rs` around lines 7386 - 7398, Update the length
check before Syscall::realpath to allow path_slice.len() == inbuf.len() when the
input slice already ends with a NUL terminator, while still rejecting an
unterminated slice that would require an additional byte. Preserve the existing
ENAMETOOLONG error behavior for paths that cannot fit.

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Outside diff comments:
In `@src/runtime/node/node_fs.rs`:
- Around line 7386-7398: Update the length check before Syscall::realpath to
allow path_slice.len() == inbuf.len() when the input slice already ends with a
NUL terminator, while still rejecting an unterminated slice that would require
an additional byte. Preserve the existing ENAMETOOLONG error behavior for paths
that cannot fit.

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[steipete]

The outside-diff MAX_PATH_BYTES finding does not apply to this call path. PathLikeExt::from_js_with_allocator() rejects every embedded NUL through Valid::path_null_bytes(), then Valid::path_length() rejects path.slice().len() >= MAX_PATH_BYTES before realpath_inner() runs. Therefore path_slice here cannot already end in NUL and cannot have len() == inbuf.len(); the longest accepted input is MAX_PATH_BYTES - 1, which this guard allows and slice_z() terminates in the final buffer byte. Keeping >= also prevents the empty-path fallback that slice_z() uses for an unterminated oversized slice.

[robobun]

Thank you for this fix. It also resolves #33403 (fs.realpathSync treats a literal backslash as a separator on POSIX and throws ENOENT). A maintainer can add Fixes #33403 to the description.

I checked a debug build of main plus this PR on Linux. The repro from the issue returns /tmp/back\slash. fs.realpathSync, fs.realpath, their .native forms and fs.promises.realpath all match Node.js. I tested absolute and relative paths that contain a backslash, with string and Buffer input. I closed #33410 in favor of this PR.

#42965 edits the same realpath block in src/runtime/node/node_fs.rs. The two PRs conflict there and in test/js/node/fs/fs.test.ts, so the second one to land needs a rebase. The realpath change in #42965 handles only absolute paths. A relative path still fails with it:

const fs = require("fs");
fs.mkdirSync("/tmp/rp/back\\slash", { recursive: true });
process.chdir("/tmp/rp");
fs.realpathSync("back\\slash"); // main + #42965: ENOENT. main + this PR, and Node.js: /tmp/rp/back\slash

This PR handles both cases. If it lands first, #42965 can drop its node_fs.rs hunk and keep the module resolver change.

[robobun]

This PR also fixes a second case of the same bug, and your tests do not pin it yet.

A containment check built on realpath can pass for a file outside the root. The check is the common static-file pattern: resolve the joined path, compare the result against the root, then read the joined path.

// bun x.mjs | node x.mjs (Linux)
import fs from "node:fs"; import os from "node:os"; import path from "node:path"; import http from "node:http";
const J = fs.mkdtempSync(path.join(os.tmpdir(), "jail-")), root = path.join(J, "pub");
fs.mkdirSync(path.join(root, "up\\x"), { recursive: true });   // a directory whose NAME holds a backslash, inside the root
fs.writeFileSync(path.join(root, "data.txt"), "inside the root");
fs.writeFileSync(path.join(J, "data.txt"), "OUTSIDE THE ROOT");  // one level above the root
const srv = http.createServer((req, res) => {
  const p = root + decodeURIComponent(new URL(req.url, "http://x").pathname);
  let rp; try { rp = fs.realpathSync(p); } catch { res.writeHead(404); return res.end("no such file"); }
  if (!rp.startsWith(fs.realpathSync(root) + path.sep)) { res.writeHead(403); return res.end("outside: " + rp.replace(J, "<jail>")); }
  res.end(`realpath says ${rp.replace(J, "<jail>")} -> served: ${fs.readFileSync(p, "utf8")}`);
});
srv.listen(0, "127.0.0.1", async () => {
  const r = await fetch(`http://127.0.0.1:${srv.address().port}/up%5Cx%2f..%2f..%2fdata.txt`);
  console.log(r.status, await r.text()); srv.close(); fs.rmSync(J, { recursive: true, force: true });
});

Results, three runs each, Linux x64:

build	output
main b52d513481, debug + ASAN	200 realpath says <jail>/pub/data.txt -> served: OUTSIDE THE ROOT
main b52d513481 plus this PR, debug + ASAN	403 outside: <jail>/data.txt
node v26.3.0	403 outside: <jail>/data.txt

The cause is the one this PR fixes. realpath resolves pub/up\x/../../data.txt as pub/up/x/../../data.txt, which stays under the root, so the check passes. open(2) on the same string walks one directory named up\x, so the two parent steps land one level above the root.

Your tests pin a directory\name collision. They do not pin a .. step that follows a name which holds a backslash. Here is a case for the describe.each block you added in test/js/node/fs/fs.test.ts:

  // A containment check (realpath, then startsWith(root)) is only safe when realpath
  // counts the same components the kernel counts. A name that holds a backslash must
  // stay one component, so a following `..` leaves the directory that holds it.
  it.skipIf(!isPosix)("counts a name that holds a backslash as one parent traversal component", async () => {
    using dir = tempDir("fs-realpath-backslash-parent", {});
    const root = String(dir);
    const jail = join(root, "jail");
    mkdirSync(join(jail, "up\\x"), { recursive: true });
    writeFileSync(join(root, "data.txt"), "outside");
    writeFileSync(join(jail, "data.txt"), "inside");
    const escapes = `${jail}/up\\x/../../data.txt`;

    // The kernel walks one directory named `up\x`, so two parent steps reach the root.
    expect(readFileSync(escapes, "utf8")).toBe("outside");
    expect(await realpath(escapes)).toBe(join(root, "data.txt"));

    // A name that starts with `..` is also one component, not a parent step.
    const literal = join(jail, "..\\secret.txt");
    const attempt = async (input: string) => realpath(input);
    await expect(attempt(literal)).rejects.toMatchObject({ code: "ENOENT" });
    writeFileSync(literal, "literal");
    expect(await realpath(literal)).toBe(literal);
  });

I verified it both ways with a debug + ASAN build of main plus your diff. All five variants fail without the src/ change (realpath returns <root>/jail/data.txt, the expected value is <root>/data.txt). All five pass with it. Prettier reports no change. Add the case if you want it.

[Jarred-Sumner]

Is there a way we can keep the performance optimization from using get fd path here? libc realpath is O(N) symlink traversal where this get_fd_path is O(1)

[steipete]

Yes. The fd-path lookup is back, without a descriptor whose close can drop the process's POSIX locks:

• Linux: same as main: O_PATH open plus /proc/self/fd readlink. filp_close() skips locks_remove_posix() for FMODE_PATH files, so this path never released locks; the lock bug only hit macOS, where main opened the target O_RDONLY for F_GETPATH. If /proc is unavailable it falls back to libc realpath, never to an ordinary descriptor.
• macOS: getattrlist(ATTR_CMN_FULLPATH): one call, no descriptor, and the same result as F_GETPATH, including firmlinks (where libc realpath differs).
• Windows: unchanged.

The lock tests hold a real fcntl lock and check from a child process that it survives every realpath API; an ordinary-descriptor negative control fails them on both systems. A new test also pins the backslash plus .. containment case robobun pointed out.

Median µs per realpathSync call (9 runs × 10,000 calls, release builds, LTO off):

OS	Path	libc realpath (previous head)	this change
macOS arm64	plain	21.99	2.46
macOS arm64	24 symlinks	136.09	17.56
Linux x64	plain	0.53	1.27
Linux x64	24 symlinks	4.88	2.13

On Linux, glibc is faster for a plain path and slower for symlink chains; the new code has the same shape as main there. On macOS the descriptor-free call is about 8× faster than libc throughout.