node:fs: preserve special POSIX mode bits in file modes

[FuPeiJiang]

What

Change node:fs mode normalization from mode & 0o777 to mode & 0o7777.

Why

Bun currently masks file modes to 0o777. That preserves the common rwx permission bits and strips file-type bits from values like stat.mode, e.g. 0o100644 -> 0o644.

However, 0o777 also drops POSIX special mode bits:

• setuid: 0o4000
• setgid: 0o2000
• sticky: 0o1000

Node preserves these bits for chmod/fchmod and creation modes. Masking to 0o7777 keeps all POSIX permission bits while still stripping file-type bits such as S_IFREG (0o100000).

This keeps the existing stat-mode compatibility behavior while avoiding accidental loss of special mode bits.

Note

Node appears to validate file modes as uint32 values and pass them through without applying this permission mask. This PR keeps Bun's existing behavior of stripping higher file-type bits from values like stat.mode, but changes the mask from 0o777 to 0o7777 so POSIX special mode bits are preserved.

If exact Node compatibility is preferred, removing the mask entirely is another option.

Tests

Added Bun-owned coverage for:

• fs.chmod
• fs.chmodSync
• fs.fchmod
• fs.fchmodSync
• numeric and octal-string mode inputs
• fs.writeFileSync({ mode })

Local verification:

git diff --check
PATH=/root/.cargo/bin:$PATH bun run build:debug:noasan
./build/debug/bun-debug test test/js/node/fs/fs.test.ts -t 'preserves special POSIX mode bits'
./build/debug/bun-debug test/js/node/test/parallel/test-fs-chmod-mask.js

[claude]

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• src/runtime/node/types.rs
• test/js/node/fs/fs.test.ts

---

Walkthrough

mode_from_js now preserves special POSIX mode bits by masking with 0o7777. The filesystem tests add POSIX-only coverage for writeFileSync, chmod, and fchmod to verify those bits remain intact.

Changes

Special mode bits fix and tests

Layer / File(s)	Summary
Mode mask update src/runtime/node/types.rs	mode_from_js masks mode_int with 0o7777 instead of 0o777, retaining special POSIX bits in the resulting Mode.
Filesystem mode-bit tests test/js/node/fs/fs.test.ts	POSIX-only tests cover writeFileSync, chmodSync/chmod, and fchmodSync/fchmod, asserting preserved special bits via statSync and fstatSync masked by 0o7777.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: preserving special POSIX mode bits in node:fs.
Description check	✅ Passed	The description covers what changed and how it was verified, though its headings differ from the template.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@test/js/node/test/parallel/test-fs-chmod-mask.js`:
- Around line 91-141: This change adds Bun-specific special-bit assertions into
a vendored Node compatibility test, which should remain an upstream-matching
port. Remove the `testSpecialMode` block from the `fs.chmod`/`fs.fchmod` test in
`test-fs-chmod-mask` and keep that file unchanged as the Node reference. Move
the special-mode coverage into the Bun-owned `fs.test.ts` suite instead, using
the existing `fs`/`fchmod` behavior tests there to host the extra assertions.

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• src/runtime/node/types.rs
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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@test/js/node/fs/fs.test.ts`:
- Around line 1736-1745: Add a `writeFileSync({ mode })` test case for the
octal-string input alongside the existing `writeFileSyncSyncWithSpecialMode`
coverage in `fs.test.ts`. The current `it.skipIf(isWindows)` test only verifies
the numeric mode branch, so extend that same test area to call `writeFileSync`
with a string mode value and assert the preserved POSIX bits via
`fs.statSync(...).mode & 0o7777`. Keep the test near the existing special-mode
case so the `writeFileSync` parser behavior is covered by the same suite.

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Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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📒 Files selected for processing (2)

• src/runtime/node/types.rs
• test/js/node/fs/fs.test.ts

[robobun]

Thank you for catching this and sending a fix!

The & 0o777 mask that was stripping special POSIX mode bits was removed in #33072 (commit a8d0692) as part of a broader input-validation hardening pass. On current main, src/runtime/node/types.rs now returns Ok(Some(mode_int as Mode)) without masking, so setuid/setgid/sticky bits are preserved.

Closing as already fixed on main. Thanks again for the careful report and patch, and we hope to see more contributions from you!