Bun.spawn: validate uid and gid as node does instead of treating NaN as id 0

[robobun]

Problem

• Bun.spawn and Bun.spawnSync treat uid: NaN and gid: NaN as id 0. A root parent that passes a NaN uid gets a root child. { uid: 65534, gid: NaN } keeps gid 0.
• Both options went through validate_integer_range with a default of 0, and it returns the default for NaN (src/jsc/JSGlobalObject.rs:1306).

Fix

• Both options call validate_int32 (src/runtime/node/util/validators.rs:120), the port of Node's validateInt32, which has no default. NaN, a fraction and ±Infinity throw ERR_OUT_OF_RANGE before the spawn: The value of "uid" is out of range. It must be an integer. Received NaN. Node 26.3.0 throws the same, with options.uid as the name.
• new ChildProcess().spawn() forwards both ids unchecked (src/js/node/child_process.ts:1473), so it had the same bug.
• Verified: 30 new cases in test/js/bun/spawn/spawn.test.ts, test/js/bun/spawn/spawnSync.test.ts and test/js/node/child_process/child_process.test.ts fail on 1.4.3-canary and pass with the fix.
• Self-reviewed: 21 concerns raised, 19 addressed, 2 rejected (listed in Notes).

Background

• spawn_maybe_sync parses the options of both functions and stores each id as Option<u32>. Some(id) makes the child set it.
• Considered: a NaN check before validate_integer_range keeps the default of 0 and two error codes for NaN and 1.5. Rejecting NaN inside the validator changes 6 other inputs, so it is separate.

Downsides

• Shapes that already threw now throw Node's errors: 1.5 moves from ERR_INVALID_ARG_TYPE to ERR_OUT_OF_RANGE, and three texts change.
• A caller that relies on NaN meaning id 0 now gets an exception. None is known.
• No cost for valid input: an id parse runs 43 instructions (was 99), and release text shrinks by 256 bytes.

Notes

Errors, before and after. Same for uid and gid, Bun.spawn and Bun.spawnSync, and both argument forms. "Before" is bun 1.4.3-canary (367d939).

value        before                                                        after
NaN          spawns, id 0 applied                                          RangeError ERR_OUT_OF_RANGE   It must be an integer. Received NaN
1.5          TypeError ERR_INVALID_ARG_TYPE  must be of type integer       RangeError ERR_OUT_OF_RANGE   It must be an integer. Received 1.5
Infinity     RangeError ERR_OUT_OF_RANGE  >= -2147483648 and <= 2147483647  RangeError ERR_OUT_OF_RANGE   It must be an integer. Received Infinity
-Infinity    RangeError ERR_OUT_OF_RANGE  >= -2147483648 and <= 2147483647  RangeError ERR_OUT_OF_RANGE   It must be an integer. Received -Infinity
2**31        RangeError ERR_OUT_OF_RANGE  >= -2147483648 and <= 2147483647  RangeError ERR_OUT_OF_RANGE   It must be >= -2147483648 && <= 2147483647
"1"          TypeError ERR_INVALID_ARG_TYPE  must be of type number. Received string   TypeError ERR_INVALID_ARG_TYPE   must be of type number, got string
true, {}, [], 0n, Symbol()   the same change as "1", with the type name of the value
-0           child runs with id 0                                          the same
-1           EINVAL from the kernel                                        the same
null         spawns, ids and groups unchanged                              the same
undefined    spawns, ids and groups unchanged                              the same
65534        child runs with that id                                       the same

Node 26.3.0, child_process.spawnSync("/bin/echo", ["ran"], { uid }): NaN, 1.5 and Infinity give ERR_OUT_OF_RANGE The value of "options.uid" is out of range. It must be an integer. Received .... 2 ** 32 gives It must be >= -2147483648 && <= 2147483647. "0" gives ERR_INVALID_ARG_TYPE.

Credential probe on the unfixed build (1.4.3-canary 367d939, child is id):

parent                                options                child
root, groups 0,995                    (none)                 uid=0 gid=0 groups=0,995
root, groups 0,995                    uid: null              uid=0 gid=0 groups=0,995
root, groups 0,995                    uid: NaN               uid=0 gid=0 groups=0            (same as uid: 0)
root, groups 0,995                    gid: NaN               uid=0 gid=0 groups=0            (same as gid: 0)
root, groups 0,995                    uid: 65534, gid: NaN   uid=65534 gid=0 groups=0        (half dropped)
root, groups 0,995                    uid: NaN, gid: 65534   uid=0 gid=65534 groups=65534
uid 65534, no capability              uid: NaN               throws EPERM                    (same as uid: 0)
uid 65534, no capability              gid: NaN               throws EPERM                    (same as gid: 0)
uid 65534 + CAP_SETUID, CAP_SETGID    uid: NaN               uid=0 gid=65534                 (raised to uid 0)
uid 65534 + CAP_SETUID, CAP_SETGID    gid: NaN               uid=65534 gid=0
uid 65534 with gid 0, no capability   gid: NaN               spawns, ids unchanged           (gid: 65534 throws EPERM)
root with no capabilities             uid: NaN or gid: NaN   spawns, ids and groups kept     (uid: 65534 throws EPERM)

NaN became id 0, and the kernel lets a process set an id that it already holds. So the spawn succeeded wherever setuid(0) or setgid(0) is permitted: with CAP_SETUID or CAP_SETGID, or with no capability when 0 is already the id of the parent. Only a parent with neither got EPERM.

With the fix, the same probe throws ERR_OUT_OF_RANGE on every NaN row for a root parent and for a uid 65534 parent, and the null and undefined rows keep the group list. I did not run the capability rows on a fixed build. The check runs before any process exists, so it does not depend on the parent.

I have no syscall trace: strace is not available where I run.

new ChildProcess().spawn({ file, args, uid: NaN }). On the unfixed build it spawns with id 0 applied, like Bun.spawn. Node 26.3.0 aborts the process at that call. With the fix it throws the RangeError synchronously, with no syscall, no error event and no pid. spawn, spawnSync, exec* and fork from node:child_process validate both ids in JavaScript (src/js/node/child_process.ts:987) and do not change.

Not covered. Each was seen in the review of this PR and is unchanged here. #44407 tracks them.

• validate_integer_range still returns the caller's default for NaN. It has 13 other direct call sites: 7 reject NaN with their own check (valkey expire seconds, FetchSession keepAlive.maxIdleSockets, randomUUIDv7 timestamp, socket setTypeOfService, Bun.spawn timeout and cgroup, Bun.build bytecodeDepth) and 6 take the default (fetch compress.level, Bun.dns.prefetch port, socket setKeepAlive delay, Bun.CSRF expiresIn/maxAge, writeHead status, MySQL integer parameters). get_optional_int adds 4 callers that get 0 (3 RedisClient options, Bun.build minChunkSize). Open pull requests Bun.CSRF: throw on a NaN expiresIn or maxAge #41219, Bun.Image: throw ERR_OUT_OF_RANGE for out-of-range encoder options #40520 and spawn: add maxMemory and memoryUsage() for a child process tree #43825 each add one more private check. A follow-up that removes the default parameter, so that NaN cannot select a placeholder, is possible. It makes 6 inputs throw for NaN and needs a maintainer decision first.
• Bun.spawn maxBuffer: NaN (also -1 and "10") removes the output limit: 200000 bytes arrive where maxBuffer: 10 stops the child. That is a defect, and it is not changed here for three reasons. The maxBuffer branch does not use either validator. It accepts every invalid value and not only NaN, so a change must first say which values mean "no limit". Open spawn: add maxMemory and memoryUsage() for a child process tree #43825 changes the lines beside it and sets that rule for maxMemory.
• Two more inputs of the same parser take NaN without an error: killSignal: NaN uses the default signal, and stdin: NaN is read as fd 0.
• An id that a failed lookup turns into 0 (Number(""), Number(null), x | 0) still applies id 0, and undefined or null still mean "not set". Node does the same.
• child_process.spawn() and spawnSync() throw ERR_INVALID_ARG_TYPE for a NaN or fractional id. Node 26.3.0 throws ERR_OUT_OF_RANGE.
• Bun.write(dest, Bun.file(src), { mode: NaN }) writes mode 000. Open Bun.write: honour the mode option for every source type, applied before the old contents are discarded #32885 changes that function.

Windows. The parser also runs on Windows. From the code (src/spawn/process.rs:1908), an id that parses makes libuv answer ENOTSUP, so NaN failed there with ENOTSUP before and throws the validation error now. I did not run Windows myself. The new cases are not platform gated: in CI build 122607 all three test files ran on the Windows 2019 x64 and Windows 11 aarch64 lanes, and those jobs passed.

CI. Build 122607 has one failed job, debian 13 x64-asan - test-bun. It fails in spawn.test.ts on two cases of stdout reader of an unref'd child and process lifetime, where LeakSanitizer prints ptrace appears to be blocked to the child's stderr. The new uid/gid cases pass in that job. The same two cases fail in the final builds of other merged pull requests (builds 122378, 122284 and 122179), so the failure does not come from this change.

Reach. This was found by fuzzing. I know of no user report and no caller that passes NaN. The options and this behavior exist since 1.4.0 (#33060). NaN comes from a failed lookup or parse. It also passes the checks a caller writes to refuse root (uid === 0, uid < 1000), because every comparison with NaN is false.

Self-review. 21 concerns were raised. 19 are addressed in the code, the tests, this description or the tracking issue #44407. 2 are rejected:

• A source lint with an inventory of call sites that take a NaN default. The design review chose to remove the default parameter instead, which makes the compiler find every call site.
• A sentence that names how common a root parent is in container images. I could not verify the numbers.

Tests. The two null/undefined cases pass with and without the fix on purpose: they guard that those values still leave the ids and the group list alone. The executable lookup runs before the option parser, so the cases use bunExe() as the command. On my debug build under heavy host load, some gcTick cases of spawn.test.ts and some cases of child_process.test.ts hit the 5 s timeout in a full run and pass when run alone. None of them uses uid or gid.

Measurements. Release builds on linux-x64, main f4d755a against this PR at 7aa9d50.

• One uid or gid parse of a valid id: 99 -> 43 instructions and 11 -> 7 conditional branches. Counted with gdb single steps from the first instruction of the validator to its return into spawn_maybe_sync (validate_integer_range::<i32> before, validate_int32::<&str> after).
• Allocations per id parse: 0 -> 0. Neither validator executes a call instruction for a valid id.
• A call with no uid and no gid runs the same code as before: two property lookups that find nothing.
• Release text: 80706155 -> 80705899 bytes (size, 256 bytes smaller). spawn_maybe_sync: 23513 -> 23366 bytes (nm -S). No new function is instantiated: validate_int32::<&str> (725 bytes) already had callers.
• Public results that change: 30 of 34 uid/gid rows (17 values for each option), plus new ChildProcess().spawn(). 4 rows go from a spawn to a throw (NaN, -NaN), 2 rows change the error code (1.5), 24 rows change only the text (±Infinity, two out-of-int32 values, two strings, two booleans, an object, an array, a bigint, a symbol). The 4 rows for -0 and -1 are identical, and so are the rows for no option, undefined, null, 0 and 65534. Unintended changes: 0.
• For a NaN id no process is created now. Before, the child started and ran the credential calls (see the probe). I could not count syscalls: strace is not available where I run.

Corrections to earlier versions of this description. (1) "18 other callers rely on its NaN behavior" was a count of grep lines that included comments. (2) "bun 1.3.14 behaves the same" was wrong in substance: the uid and gid options of Bun.spawn are new in 1.4.0 (#33060) and 1.3.14 ignores them. (3) "The harm is limited to a privileged parent that misses a drop" was too narrow, see the probe table. (4) "node:child_process never reaches this path" was wrong for new ChildProcess().spawn(). (5) The first version of the fix was a NaN check in a helper function. The review of the design replaced it with the validator call above.

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no test proof · iteration 1 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/child_process/child_process.test.ts, test/js/bun/spawn/spawnSync.test.ts, test/js/bun/spawn/spawn.test.ts

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Walkthrough

The uid and gid option parsers now use int32 validation. Tests cover invalid values, unset values, and synchronous errors across the spawn APIs.

Changes

Spawn uid/gid validation

Layer / File(s)	Summary
Shared uid/gid validation src/runtime/api/bun/js_bun_spawn_bindings.rs	The parser uses validate_int32, leaves null unset, and casts validated values to u32.
Spawn validation coverage test/js/bun/spawn/spawn.test.ts, test/js/bun/spawn/spawnSync.test.ts, test/js/node/child_process/child_process.test.ts	Tests cover invalid values for both call forms, null and undefined in spawnSync, and synchronous NaN errors from ChildProcess#spawn().

Suggested reviewers: dylan-conway, jarred-sumner

Priority: ⬆️ High

Merge Risk: ⚪ Minimal · up to 7aa9d

Bun.spawn and related APIs will now throw on invalid uid/gid values instead of silently running a child as ID 0. Valid IDs, null, and undefined behave as before. No merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly identifies the main change: validating Bun.spawn uid and gid values like Node instead of treating NaN as ID 0.
Description check	✅ Passed	The description explains the problem, fix, behavior changes, verification results, test coverage, CI status, limitations, and performance impact. It does not use the exact template headings, but it pr…

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[robobun]

Status: ready for review. The design review, the self-review and the measurements are done, and their results are in the description.

How I reproduced it, on an unfixed build (bun 1.4.3-canary, 367d939):

• As root with a supplementary group, Bun.spawnSync(["id"], { uid: NaN }) prints groups=0(root). Without uid it prints groups=0(root),995(docker). So the child took the credential path with id 0, the same as uid: 0.
• As uid 65534, the same call throws EPERM, the same as uid: 0.
• From root, { uid: 65534, gid: NaN } gives a child with uid 65534 and gid 0.

With this change those calls throw ERR_OUT_OF_RANGE before the spawn. null and undefined still spawn with the ids and the group list unchanged.

Tests: 30 new cases in test/js/bun/spawn/spawn.test.ts, test/js/bun/spawn/spawnSync.test.ts and test/js/node/child_process/child_process.test.ts fail on the unfixed build and pass with this change.

CI (build 122607, finished): 180 of 181 jobs passed. The new cases pass on every lane, Windows x64 and aarch64 and macOS x64 and aarch64 included. The one red job is debian 13 x64-asan - test-bun. It fails on two other cases of spawn.test.ts, in stdout reader of an unref'd child and process lifetime. They do not use uid or gid, and the same two cases fail in the final builds of other merged pull requests (builds 122378, 122284, 122179). The new uid/gid cases pass in that job too. Every other failure in the build passed on a retry.

[claude]

LGTM — a NaN uid/gid now throws ERR_OUT_OF_RANGE instead of becoming id 0.

What was reviewed:

• The new user_or_group_id_from_js helper preserves the prior null → None and validate_integer_range::<i32> semantics exactly; only the NaN pre-check is new, and it mirrors the timeout NaN guard a few lines below.
• Confirmed throw_range_error + OutOfRangeOptions{field_name, msg: b"an integer"} formats to the exact message the tests assert (fmt.rs:3374, write_received for f64).
• Tests cover both spawn and spawnSync, both uid and gid, assert exact code+message, and add a positive null case to guard the passthrough.

Extended reasoning...

Overview

This PR adds a NaN guard to the uid/gid option parser in Bun.spawn/Bun.spawnSync. Previously, validate_integer_range mapped NaN to the caller's default (0), which for these two fields meant the child called setuid(0)/setgid(0) — a silent privilege escalation to root when a uid came from a failed lookup or a bad parseInt. The two duplicated uid/gid blocks are extracted into a single helper user_or_group_id_from_js that pre-checks NaN and throws ERR_OUT_OF_RANGE, matching how the timeout option in the same parser already handles NaN. Three files touched: the option parser and two test files (spawn.test.ts, spawnSync.test.ts).

Security risks

The change removes a security risk rather than introducing one. The only behavioral change is that uid: NaN / gid: NaN now throw before spawn instead of running as id 0. The null passthrough, valid-integer path (i32 range, cast to u32), and all other error shapes (1.5, Infinity, 2**32, strings) are byte-identical to the previous inline code. The shared validate_integer_range helper is deliberately unchanged so its 18 other callers are unaffected.

Level of scrutiny

Medium — this is credential-adjacent option parsing, so the diff was read line-by-line against the removed code to confirm the refactor is behavior-preserving except for NaN. The NaN guard uses value.is_number() && value.as_number().is_nan(), guarded correctly, and passes value.as_number() (an f64) to throw_range_error, whose OutOfRangeValue for f64 impl produces Received NaN via the double() formatter. The OutOfRangeOptions defaults leave min/max at i64::MAX so the msg branch (an integer.) is taken — verified against src/bun_core/fmt.rs:3391-3399. The resulting message matches the test assertions exactly.

Other factors

The tests follow REVIEW.md guidance well: exact code + message assertions via expect.objectContaining, both entry points (spawn and spawnSync) covered, both fields covered, a positive test that null still spawns cleanly, and bunExe() used as the command so ENOENT can't preempt the option parser. The pattern mirrors the existing timeout NaN guard at js_bun_spawn_bindings.rs:762. The PR description documents that node:child_process already validates in JS so was never affected, and that Windows now throws ERR_OUT_OF_RANGE instead of the later ENOTSUP — hence the tests are unplatform-gated. No prior reviews on the timeline.

[claude]

LGTM — targeted validation tightening that follows the existing NaN-guard pattern already used for timeout and cgroup in the same parser.

What was reviewed:

• The extracted user_or_group_id_from_js helper is byte-identical to the two blocks it replaces, plus the new NaN guard; null, valid ids, and every other error path are unchanged.
• The NaN check (is_number() && as_number().is_nan()) and throw_range_error call mirror the timeout/cgroup handling in the same file.
• Tests cover both spawn/spawnSync × uid/gid, assert exact code+message, and add a positive case confirming null still spawns.
• comment-cop feedback was addressed (comments cut to one line each); all threads resolved.

Extended reasoning...

Overview

This PR fixes Bun.spawn/Bun.spawnSync accepting uid: NaN or gid: NaN and silently mapping it to id 0 (root). The fix extracts the two identical uid/gid parsing blocks in spawn_maybe_sync into a shared user_or_group_id_from_js helper and adds an explicit NaN check that throws ERR_OUT_OF_RANGE before validate_integer_range would map NaN to its default of 0. Tests are added to both spawn.test.ts and spawnSync.test.ts.

Security risks

The change is security-relevant in that it closes an accidental privilege-escalation path (NaN → setuid(0)). The fix is a strict tightening: it only rejects an input that was previously mishandled. It does not loosen any validation, does not touch the spawn machinery itself, and does not change behavior for any valid input. The shared validate_integer_range helper is intentionally left unchanged (18 other callers rely on its NaN→default behavior; none use a credential as default).

Level of scrutiny

Medium. While uid/gid handling is security-sensitive, the change is small (~30 net lines in Rust), mechanical (extract-helper + one guard), and follows the exact pattern already established in the same function for timeout and cgroup NaN handling. The refactor is verifiably behavior-preserving: same NULL → None mapping, same validate_integer_range::<i32> call with identical range/default/field_name, same as u32 cast.

Other factors

• Test coverage is thorough: both entry points (spawn/spawnSync), both fields (uid/gid), exact error code and message asserted, plus a positive test that null still spawns unchanged. Tests use bunExe() so command lookup succeeds and the option parser is reached.
• The comment-cop bot flagged verbose comments; those were trimmed to one line each in follow-up commits and all threads are marked resolved.
• No CODEOWNERS-protected paths are touched.
• The PR description includes a full behavior matrix confirming only the NaN rows changed.

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Actionable comments posted: 1

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Inline comments:
In `@test/js/bun/spawn/spawnSync.test.ts`:
- Around line 182-185: Update the “accepts a null uid/gid” test to assert the
expected stdout with toBe(...) immediately after Bun.spawnSync returns, then
retain the existing exitCode assertion.

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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

Rebased onto main bf42a52 (was f080856, now 3c2675d). The rebase changed two source lines:

• src/runtime/api/bun/js_bun_spawn_bindings.rs. Main changed spawn_maybe_sync to take cx: &JsThread and to use cx.global() where it used global_this. The two calls that this pull request adds follow that: user_or_group_id_from_js(cx.global(), uid_value, b"uid"), and the same for gid.
• Every other hunk is identical before and after.

Checked on a debug build of the rebased branch:

• The uid/gid tests pass in spawn.test.ts (3 pass, 1 skip) and in spawnSync.test.ts (5 pass, 1 skip). On a debug build of main bf42a52, rejects a NaN uid/gid before spawning fails in both files.
• The rest of spawnSync.test.ts: 12 pass, 1 skip, 2 fail. The two failures are should not set a timeout if timeout is 0 .... They assert a wall-clock time under 1000 ms and measured about 1500 ms on my debug build. They do not use uid or gid.

[claude]

I reviewed the re-pushed version of this PR and did not find any bugs; because the change governs which uid/gid a spawned child runs as, a human look is still worthwhile.

What was reviewed:

• Traced the new NaN pre-check against validate_integer_range in src/jsc/JSGlobalObject.rs: NaN was the only number shape that reached the default (0) path; ±Infinity, fractions, strings and out-of-range still throw as before, and undefined is mapped to absent by JSValue::get so it never reaches the helper.
• Confirmed the exact error text against NewOutOfRangeFormatter and FormatDouble in src/bun_core/fmt.rs (msg: b"an integer" with default min/max yields It must be an integer. Received NaN), matching both test files word for word.
• Checked the refactor preserves the old path: null -> None, i32::MIN..=i32::MAX range, and the as u32 wrap are unchanged; both Bun.spawn and Bun.spawnSync share this parser and both get tests; node:child_process validates uid/gid in JS before reaching here.
• Test assertions use toThrow(expect.objectContaining(...)), an established pattern in this suite, so they cannot pass vacuously.

Extended reasoning...

The diff is +78/-28 across one Rust file (src/runtime/api/bun/js_bun_spawn_bindings.rs) and two test files: it replaces two duplicated uid/gid parsing blocks with a shared helper that additionally rejects NaN with ERR_OUT_OF_RANGE instead of silently coercing it to id 0. The security-sensitive surface is process credential handling (setuid/setgid of the spawned child); the change is strictly fail-closed and narrows accepted input. The code is small, uses existing in-tree helpers, and covers both sync/async entry points with exact-message tests, but because it sits on a permissions path the guidelines call for a human look rather than an automated approval. No CODEOWNERS entry covers the changed files and no third-party objections appear outstanding in the timeline metadata.

[robobun]

^{Updated 6:07 PM PT - Oct 1st, 2026}

❌ @robobun, your commit 7aa9d50 has 1 failures in Build #122607 (All Failures):

• test/js/bun/spawn/spawn.test.ts - code 1: stdout reader of an unref'd child and process lifetime > an idle reader stopped at the highwater … on 🐧 13 x64-asan

---

🧪   To try this PR locally:

bunx bun-pr 39682

That installs a local version of the PR into your bun-39682 executable, so you can run:

bun-39682 --bun

[robobun]

This pull request is no longer a draft (2026-10-02). The measured costs are in the description, and the status comment above has the CI state.

A follow-up is possible and needs a maintainer decision first: remove the default parameter from validate_integer_range, so that NaN cannot select a placeholder for any caller. The Notes block of the description lists the callers. This pull request does not depend on it.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline finding, I also checked the other validate_integer_range callers left in src/runtime/api/bun/js_bun_spawn_bindings.rs (timeout at ~line 722 and the stdin fd parse at ~line 2191) for the same NaN-to-default pattern — both reject NaN explicitly before calling it, so the IntegerRange import is still live and those paths are not affected. validate_int32 compares in f64 before casting and has no default, so the undefined/null skip and the as u32 cast of negatives behave as before.

Extended reasoning...

The change swaps the uid/gid parse in spawn_maybe_sync to validate_int32 and adds exact-error tests across spawn, spawnSync and ChildProcess.spawn; it touches process credential handling, and a pre-existing maxBuffer NaN issue in the same parser is being posted inline, so a human look is still warranted.

Additional findings (outside the current diff — GitHub can't attach inline comments there):

• 🟣 src/runtime/api/bun/js_bun_spawn_bindings.rs — pre-existing: a caller who passes maxBuffer: NaN (or a negative number or a string) to Bun.spawn/spawnSync gets an unbounded output buffer instead of an error, exactly as on the base branch. The same options parser this PR fixes for uid/gid still lets NaN select a permissive default at js_bun_spawn_bindings.rs:755, where val.is_finite() is false for NaN so max_buffer stays None and the child is never killed for exceeding the limit. Fix: reject NaN, negatives and non-numbers for maxBuffer with ERR_OUT_OF_RANGE before the spawn, while still treating Infinity as "no limit", which the comment at line 756 documents.

  Why this was flagged

  A caller invokes Bun.spawn({cmd, stdout: "pipe", maxBuffer: limit * 1024}) where limit is undefined, producing maxBuffer: NaN. At src/runtime/api/bun/js_bun_spawn_bindings.rs:755 the guard val.is_number() && val.is_finite() is false for NaN, so the block is skipped and max_buffer remains None (line 364). The child is then allowed to write without any cap, so the parent buffers all output and the kill-on-overflow path never runs; the base branch does the same, so this is a pre-existing path the partial fix leaves failing. The same guard also silently drops maxBuffer: -1 (value > 0 is false at line 758) and maxBuffer: "10" (is_number false). The uid/gid change at lines 661-675 fixes the NaN-selects-default hazard only for those two options; the node compat layer validates maxBuffer in src/js/node/child_process.ts:1924-1926 before calling Bun.spawn, so node:child_process users are protected but direct Bun.spawn users are not. The PR description names this under "Not covered" but gives no reason the silent no-limit outcome is acceptable.

  Verification: Triggering condition: a caller of Bun.spawn/Bun.spawnSync passes a non-finite or non-numeric maxBuffer. Mechanism at src/runtime/api/bun/js_bun_spawn_bindings.rs:754-764: if val.is_number() && val.is_finite() { ... max_buffer = Some(value); } is false for NaN, so max_buffer remains None and no ERR_OUT_OF_RANGE is thrown. The base behaves the same way, so merging makes nothing worse.

[robobun]

About the maxBuffer finding in the review above: it is correct, and it is not new with this change.

On bun 1.4.3-canary, Bun.spawnSync(cmd, { maxBuffer: NaN }) returns all 200000 bytes of a child that maxBuffer: 10 stops. -1 and "10" do the same. The cause is the branch at src/runtime/api/bun/js_bun_spawn_bindings.rs:769, which sets the limit only for a finite number above 0.

This pull request does not change it, for three reasons:

• That branch does not use the validator that uid and gid move to.
• It accepts every invalid value, not only NaN. A change must first say which values mean "no limit". Infinity does today, on purpose.
• Open spawn: add maxMemory and memoryUsage() for a child process tree #43825 changes the lines beside it and sets that rule for maxMemory.

The silent "no limit" is a defect, not an accepted result. #44407 tracks it with the repro, and the description of this pull request now gives these reasons under "Not covered".