[JSC] Throw instead of crashing when a Vector that grows with script input cannot grow

[robobun]

Problem

• Eight builtins end the process when a WTF::Vector that grows with script input cannot grow: panic(main thread): abort() called (exit 134) at the 2^31 byte limit, or a segfault at 0xBBADBEEF when the allocation fails, also inside try/catch. A fuzzer found three. No user has reported one.
• The cheapest on bun 1.4.3: ','.repeat(372712672).split(',') (3 s, 4 GB).
• Each site grows its Vector with the infallible append(): stringReplaceAllStringString, parseReplacementTemplate, stringSplitFast, the reviver ranges in LiteralParser::parse, JSFinalizationRegistry::registerTarget, stringListFromIterable, the WebAssembly.Tag parameters, the builtins compile option.

Fix

• Each site appends with tryAppend() and throws RangeError: Out of memory when that fails, like the sourceRanges Vectors in StringPrototypeInlines.h. A helper without a ThrowScope reports the failure and its caller throws. split() releases its scratch Vector before the throw.
• A throw from register() leaves its list full, and the end of a collection appends to that list and to the lists of held values. Nothing can throw there, so what does not fit is dropped. The specification allows a cleanup callback that never runs.
• Left out on purpose: see Notes.
• Verified with the preview build's debug jsc: the new stress test passes, and asserts on the debug shell of main (c28156899e). CI skips it (Notes).

Background

• Vector::append() calls CRASH() when the new capacity passes (2^31 - 1) / sizeof(T) entries, and crashes in fastMalloc() when the allocation fails. tryAppend() returns false in both cases.
• --maxSingleAllocationSize (debug builds only) makes every tryFastMalloc() above the size return null, so the new test needs a few hundred thousand items.
• Upstream WebKit main has the same code at all eight sites.

Notes

Where each Vector stops, with 1.5x growth. Measured on the release jsc of d3720d515e (main on 15 September), linux x64. Each line exits 134 there and throws with this PR.

Site	First append that cannot grow	Repro
split(string)	372,712,672	','.repeat(372712672).split(',')
replaceAll(string, string)	262,343,954	'a'.repeat(2 ** 28).replaceAll('a', 'c')
replace(regexp, string), one part per $ reference	174,895,970	'x'.replace(/(x)/g, '$1'.repeat(180000000))
JSON.parse with a reviver, one range per array element	51,821,029	JSON.parse('[' + '1,'.repeat(51821029) + '1]', (k, v) => v) (99 MiB of text)
FinalizationRegistry.prototype.register	116,597,314	that many register(target, 1) calls on one registry

Intl.ListFormat, WebAssembly.Tag and the builtins option hold 8 bytes per item, so they stop at 262,343,954 items like replaceAll. I ran those three only with the allocation cap. The fuzzer's Intl.ListFormat repro takes 24 s.

Left out on purpose

• JSONRanges::record(), a MarkedArgumentBuffer::appendWithCrashOnOverflow(), and the HashMap that holds the ranges of an object's properties. A HashMap has no fallible add(). The same holds for m_liveRegistrations.add() with many distinct tokens.
• new WebAssembly.Tag() with 134,217,728 to 262,343,953 parameters still aborts after the loop, in the sized Vector<TypeSlot> of rttForFunction(). The fix for that is a limit on the parameter count (Wasm::maxFunctionParams, as the section parser has), which changes behavior and belongs in its own change.
• Allocations that fail after the guarded Vector: ListFormatInput, and the replacements Vector on the function-callback paths of replace, which grows right after a sourceRanges Vector of the same size grew with tryConstructAndAppend(). The 2^31 byte limit cannot be the first failure there.
• The microtask queue ([JSC] The microtask queue aborts the process at 2^25 pending tasks #667).

Test

• out-of-memory-when-script-sized-vector-cannot-grow.js is slow! and runDefault("--maxSingleAllocationSize=1048576") in debug builds, skipped elsewhere, like js-fixed-array-out-of-memory.js. It covers all eight sites and the three lists that drop at the end of a collection, in about 20 s on the debug ASAN shell. On the debug shell of main it stops at the first case with ASSERTION FAILED: Requested size (1420104) exceeds max single allocation size set for testing (1048576). The tested lanes of this repository are release builds, so CI skips it.
• There is no real-size test here. JSC builtins: throw instead of aborting when a script-sized Vector cannot grow (WebKit bump for oven-sh/WebKit#666) bun#42897 has one (split, 3 s, 3.7 GB) that Bun's CI runs on release builds, next to one test per site for debug builds.

Other notes

• stringReplaceAllStringString came with upstream's replaceAll fast path (https://commits.webkit.org/280990@main).
• split() and its scratch Vector: with a 512 MiB string alive, the release jsc sits at 1.91 GB ten seconds after the throw when the Vector is kept, and at 0.52 GB with the release in this PR.
• forEachInIterable() stops and closes the iterator when its callback throws. Its other callers that collect what they iterate use a MarkedArgumentBuffer and check hasOverflowed(). The three changed here were the only ones with a Vector.
• The branch is based on c28156899e, the commit bun main pins.
• Script-sized containers that still abort the process when they cannot grow: the microtask queue and the web streams request queues bun#42648 lists the Bun-side containers of the same kind.

[github-actions]

Preview build of 5074c2a: autobuild-preview-pr-666-5074c2a1

[coderabbitai]

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• JSTests/stress/out-of-memory-when-script-sized-vector-cannot-grow.js
• Source/JavaScriptCore/runtime/JSFinalizationRegistry.cpp

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Walkthrough

Changes

Out-of-memory vector handling

Layer / File(s)	Summary
String and parser allocation checks JSTests/stress/out-of-memory-when-script-sized-vector-cannot-grow.js, Source/JavaScriptCore/runtime/StringPrototype.cpp, Source/JavaScriptCore/runtime/StringPrototypeInlines.h, Source/JavaScriptCore/runtime/LiteralParser.cpp	String replacement, splitting, JSON reviver processing, and literal parsing now use fallible vector operations and report allocation failure as RangeError: Out of memory.
Finalization registration failure propagation Source/JavaScriptCore/runtime/JSFinalizationRegistry.h, Source/JavaScriptCore/runtime/JSFinalizationRegistry.cpp, Source/JavaScriptCore/runtime/FinalizationRegistryPrototype.cpp, JSTests/stress/out-of-memory-when-script-sized-vector-cannot-grow.js	Finalization registry registration now returns a success flag, prevents incomplete registrations, and handles storage failures during garbage-collection reconciliation.
Iterable and WebAssembly allocation checks Source/JavaScriptCore/runtime/IntlListFormat.cpp, Source/JavaScriptCore/wasm/js/WebAssemblyTagConstructor.cpp, Source/JavaScriptCore/wasm/js/WebAssemblyCompileOptions.cpp, JSTests/stress/out-of-memory-when-script-sized-vector-cannot-grow.js	Intl and WebAssembly vector appends now handle allocation failure. Stress tests verify out-of-memory errors, iterator closure, and valid operations.

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 5074c

The reviewed changes have no remaining actionable merge-blocking risks.

🚥 Pre-merge checks | ✅ 3 | ❌ 1

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Check name	Status	Explanation	Resolution
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Title check	✅ Passed	The title clearly states the main change: JavaScriptCore now throws instead of crashing when a script-sized Vector cannot grow.

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[claude]

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

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

• 🟡 Source/JavaScriptCore/runtime/JSFinalizationRegistry.cpp — A script that catches the new register RangeError and keeps running can now crash the process later at GC end, uncatchable, where the base aborted synchronously at register. After the throw, m_noUnregistrationLive is full (size equals capacity). One more registration with a token that dies makes reconcileWeakReferencesAtGCEnd merge it with the infallible append at JSFinalizationRegistry.cpp:143, which cannot grow. Fix: the merge at GC end must not grow a full list: keep such registrations in a dead-key bucket or move them with tryAppend and a fallback, so a caught RangeError leaves no time bomb.

  Extended reasoning...

  The finder reasoned that base has the same append and that the needed state also exists on base. On base the state needs an exact count because register aborts at the first failed growth; with the PR a plain try/catch loop reaches it. Steps: try { for(;;) registry.register(target, 1); } catch {} - JSFinalizationRegistry.cpp:214 tryAppend fails when the 1.5x growth of the 16-byte Registration buffer passes the 2^31 limit or the allocation fails; the list is left with size == capacity (about 116M entries, matching the PR's own table). The RangeError is caught. Then registry.register(target, 1, {}) with a throwaway token succeeds via :218/:219 (a fresh 1-entry bucket). GC runs. At :110 removeAllMatching keeps every entry because target is alive. At :120 the bucket's key is dead, so :142 keyIsDead is true and :143 m_noUnregistrationLive.append(reg) must grow the full Vector; append is FailureAction::Crash so it CRASHes in the collector, with no JS frame and no way to catch. The population is any long-lived process relying on the PR's promise that register throws instead of…

  Verification: normal (incomplete fix: the base aborts synchronously on the same script, the PR defers the abort into GC where it is uncatchable) — triggered when a script catches the new RangeError from register(target, holdings) (no token) and then registers once more with an unregister token that dies while the target stays alive. Mechanism verified: -… | nit (pre-existing crash site, but the PR makes it…

• 🟣 Source/JavaScriptCore/runtime/LiteralParser.cpp — JSON.parse with a reviver still aborts the process for a large object literal, while the same call on an array of the same size now throws. The array path at LiteralParser.cpp:1708 uses tryAppend, but the object path stores each property's range with the infallible HashMap set at LiteralParser.cpp:1798 and :1893, whose rehash crashes on allocation failure. The PR describes the fix as 'JSON.parse with a reviver' without qualification. Fix: the reviver range recording must be fallible for objects too: bound the number of recorded entries or reserve with a try variant and throw RangeError: Out of memory when it fails. [also at: Source/JavaScriptCore/runtime/LiteralParser.cpp:1700 - JSON.parse of deeply nested input, with or without a reviver, still aborts the process under the same allocation failure the PR now turns into a RangeError elsewhere.]

  Extended reasoning...

  The finder accepted the author's scope statement; the PR text and test only ever mention arrays, but users read the claim as JSON.parse with a reviver. Trace: LiteralParser<...>::parse in reviver mode, sourceRanges non-null. For each object member, :1798 (primitive values) or :1893 (nested values) executes std::get<JSONRanges::Object>(m_rangesStack.last().properties).set(ident.impl(), Entry). JSONRanges::Object is UncheckedKeyHashMap<RefPtr<UniquedStringImpl>, Entry, IdentifierRepHash> (LiteralParser.h:86); its growth rehash allocates through HashTableMalloc which CRASHes when the allocator returns null, and each bucket is about 56 bytes so the table reaches the same sizes as the array Vector the PR fixed. Under the PR's own test mode (--maxSingleAllocationSize=1048576) JSON.parse('{' + '"k1":1,...' ~20,000 keys + '}', (k,v)=>v) dies with the allocation assertion while the array form of the same size throws. Population: any process parsing untrusted JSON with a reviver near its memory limit. Remedy: count entries and…

  Verification: pre-existing. Trigger: JSON.parse(text, reviver) where text is one object literal with enough distinct keys that the reviver-range HashMap's next rehash cannot be allocated (tens of millions of unique keys, multi-GB) — the process aborts instead of throwing, exactly as on the base. Mechanism verified. The diff only makes the array path fallible… | pre-existing. Trigger: `JSON.parse(text,…

• 🟣 Source/JavaScriptCore/runtime/StringPrototypeInlines.h — pre-existing, low: callers of replace/replaceAll with a function callback still get a hard crash, not RangeError: Out of memory, when the process is near its memory limit. The replacements Vector at StringPrototypeInlines.h:571 grows with infallible append() right after sourceRanges grew with tryConstructAndAppend, so if that second same-sized allocation fails the process dies in fastMalloc. Fix: grow every script-sized Vector on these callback paths with tryAppend and throw the out-of-memory error, which covers the 3 sites listed. Same pattern at 3 sites (StringPrototypeInlines.h:571, StringPrototype.cpp:475, StringPrototype.cpp:567).

  Extended reasoning...

  This is the same class the PR fixes (a Vector that grows once per script-controlled match), left as it was on the base branch; the PR's list of things left out on purpose does not mention it. The 2^31-byte limit is not the trigger here: sourceRanges (Vector<Range<int32_t>, 16>) and replacements (Vector<String, 16>) have the same element size and inline capacity, and sourceRanges is appended first each iteration (StringPrototypeInlines.h:565, StringPrototype.cpp:405 and :502), so at the limit sourceRanges fails first and throws. The trigger is an allocation failure: 'a'.repeat(2**27).replaceAll('a', () => 'c') on a process whose remaining commit or address space is smaller than the next buffer (Windows commit limit, RLIMIT_AS, 32-bit). sourceRanges.tryConstructAndAppend at :565 allocates its new buffer of S bytes and succeeds. replacements.append(replaceString) at :571 then needs another S bytes; Vector::append uses FailureAction::Crash, so expandCapacity calls Malloc::realloc/malloc, which crashes on null (the 0xBBADBEEF segfault the PR describes). Same sequence in…

  Verification: pre-existing (low): the PR sets out to make every script-sized Vector append in these builtins fallible ("Eight builtins end the process when a WTF::Vector that grows with script input cannot grow ... Each site appends with tryAppend()"), but the replacements Vectors on the replace/replaceAll callback (and regexp) paths are left with infallible append(), and the PR's "left out on…

[claude]

I re-reviewed the follow-up commit (515597e) and found no bugs in it; the earlier split scratch-retention and slow! points are addressed. A human look is still worthwhile for the new GC-end behaviour of silently dropping a registration when m_noUnregistrationLive cannot grow.

What was reviewed in this push:

• stringSplitFast: the throwOutOfMemory lambda calls Vector::clear() (shrinkCapacity(0)), which frees the scratch buffer, unlike the existing shrink(0); all three throw sites go through it and the shape matches the other OUT_OF_MEMORY paths in the file.
• reconcileWeakReferencesAtGCEnd: the dropped registration is removed from its bucket (return true) and nothing else references the Registration, so no dangling pointer; only the live-list append changed, the dead-holdings appends are still infallible.
• registerTarget: the isNewEntry bucket removal keeps the ASSERT(bucket.value.size()) invariant at GC end; the test's new unique-token-then-fullGC() sequence exercises that path.

Extended reasoning...

Overview

The PR now consists of two commits. The first converts eight script-sized WTF::Vector::append sites in JSC builtins (replaceAll, replace template parsing, split, JSON.parse reviver ranges, FinalizationRegistry.prototype.register, Intl.ListFormat, WebAssembly.Tag, wasm builtins compile option) to tryAppend plus RangeError: Out of memory. The second commit, pushed after my prior review, releases vm.stringSplitIndice before the throw in stringSplitFast, makes the one append in reconcileWeakReferencesAtGCEnd that can hit a full list fallible (dropping the registration), marks the debug-only stress test slow!, extends it with a unique-token-then-GC sequence, and deletes the 6 GB real-limit replaceAll test.

Security risks

None specific to this change. The PR replaces process aborts with catchable exceptions, which is a robustness improvement; no new allocation, no user-controlled indexing, no changes to trust boundaries. The nullptr return from stringReplaceAllStringString is consumed through RELEASE_AND_RETURN(scope, ...) in the same header, consistent with the other OOM paths there.

Level of scrutiny

Medium. The mechanical tryAppend conversions follow the existing sourceRanges pattern and were covered in the prior run. What is new and merits a maintainer's judgement rather than more bug hunting is the policy choice at GC end: a registration whose unregister token died before its target is now silently dropped when m_noUnregistrationLive is full, so its cleanup callback never runs. The spec permits not calling cleanup callbacks, and the implementation is memory-safe (the registration is removed from its bucket and nothing else points at it), but it is a behaviour decision the fork maintainers should consciously accept, and it is reachable only after a prior register has already thrown OOM on the same registry.

Other factors

The three "pre-existing, not blocking" inline notes from the prior review (the second-stage crash-on-failure allocations in WebAssembly.Tag, Intl.ListFormat, and the HashMap::add in registerTarget) are unchanged by this push and remain open; they were explicitly scoped as outside the PR's Vector-only fix and do not block it. This run's bug hunt ran dry with no new findings. The test only runs in debug builds, which CI does not test, so the covered paths rely on local runs; the PR description reports both shells were exercised, and Bun's own suite (oven-sh/bun#42897) is said to carry per-site tests.

Still open from earlier reviews (3):

• Unresolved: 3 minor or pre-existing.

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@Source/JavaScriptCore/runtime/JSFinalizationRegistry.cpp`:
- Line 146: Update the GC reconciliation logic around m_deadRegistrations and
m_noUnregistrationLive so every insertion is fallible: avoid creating
token-specific dead buckets during GC or use a fallible map insertion path,
replace both dead-vector append operations with tryAppend(), and continue
reconciliation while dropping only the affected registration when bucket
creation or append fails.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Essentials

Run ID: a3b35ad8-acc6-4f77-8fe3-2e4315afaac0

📥 Commits

Reviewing files that changed from the base of the PR and between c281568 and 515597e.

📒 Files selected for processing (10)

• JSTests/stress/out-of-memory-when-script-sized-vector-cannot-grow.js
• Source/JavaScriptCore/runtime/FinalizationRegistryPrototype.cpp
• Source/JavaScriptCore/runtime/IntlListFormat.cpp
• Source/JavaScriptCore/runtime/JSFinalizationRegistry.cpp
• Source/JavaScriptCore/runtime/JSFinalizationRegistry.h
• Source/JavaScriptCore/runtime/LiteralParser.cpp
• Source/JavaScriptCore/runtime/StringPrototype.cpp
• Source/JavaScriptCore/runtime/StringPrototypeInlines.h
• Source/JavaScriptCore/wasm/js/WebAssemblyCompileOptions.cpp
• Source/JavaScriptCore/wasm/js/WebAssemblyTagConstructor.cpp

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[claude]

Code review found no issues

No high-confidence issues detected in this change.

Still open from earlier reviews (3):

• Unresolved: 3 minor or pre-existing.

[robobun]

Answers to the findings in the review body (the inline threads have their own replies):

• A caught RangeError from register() and the append at the end of a collection. Fixed in 515597e and 5074c2a. Every Vector that reconcileWeakReferencesAtGCEnd() grows now uses tryAppend() and drops what does not fit. Nothing can throw there, and the specification allows a cleanup callback that never runs. The stress test fills each of the three lists. On the preview builds without the change it asserts inside fullGC().
• JSON.parse with a reviver and a large object literal. Left as it is. The ranges of an object's properties live in a HashMap, and WTF::HashTable has no fallible insertion. The PR body now says "one range per array element" and lists the HashMap under "Left out on purpose".
• The replacements Vectors on the function-callback paths of replace. Left as they are, and listed. sourceRanges has the same entry size and grows first with tryConstructAndAppend(), so the 2^31 byte limit throws there before replacements can reach it. What remains is an allocation that fails right after one of the same size succeeded.