crash_handler: report an exhausted address space during JSC initialization as an error, not a crash

[robobun]

Problem

• JSC::initialize() aborts in JSC::StructureMemoryManager::StructureMemoryManager() (WTFCrashWithInfo, abort() called) when address space is short, and bun prints "Bun has crashed". Sentry BUN-4NF7.
• JSC halves its 4 GB Structure heap request down to 32 MB. That rung always fails an assert (StructureAlignedMemoryAllocator.cpp:150), and :123 aborts if none fits. The cause is a limit such as ulimit -v.

Fix

• bun_jsc::initialize sets Action::InitializeJsc around JSCInitialize. A crash with that action first maps and unmaps 128 MB, the least JSC needs.
• If the map fails, bun prints an error with the ulimit -v value and exits 1. Otherwise the report prints, plus "Crashed while initializing JavaScriptCore". Under 96 MB is free at either assert.
• Verified: two new tests in test/cli/run/run-crash-handler.test.ts. They fail on a release build of the merge base and skip under ASAN (Notes).

Background

• A release RELEASE_ASSERT raises a signal, and bun's crash_handler() prints the report. It prints the thread local Action as "Crashed while ...".
• Fit the startup address-space reservations to RLIMIT_AS (WebKit + mimalloc bump) #41900 (open) sizes the startup reservations to RLIMIT_AS. This change then covers limits under its floor (about 180 MB) and other kinds of limit.
• Considered a check before JSC::initialize(): JSC reserves its 1 GiB JIT pool after it. Considered an error from JSC: a WebKit change per assert.

Downsides

• A real JSC initialization bug in a process with under 128 MB of address space left prints this error, with no report.
• Size (release, Linux x64): bun-profile text +512 bytes (80,702,673 to 80,703,185). Per process: two thread-local writes.

Notes

Which assert (the 1.4.0 event). In the 1.4.0 x64 binary, frame 0x17ffc20 is the return address of the one call WTFCrashWithInfo in the constructor. It is shared by the hasOneBitSet assert (needs a user set structureHeapSizeInKB) and the mi_manage_os_memory_ex assert (mov $0x93,%edi, line 147 at that pin, on the failure edge of that call). The asserts with extra arguments compile to direct call abort@plt, with no WTFCrashWithInfo frame. The Sentry trace has that frame.

Why mimalloc says no. The constructor hands it 32 MiB - 16 KiB (one block is kept for StructureID 0). mimalloc aligns the start up to its 64 KiB slice size and then needs one whole chunk of slices, MI_ARENA_MIN_SIZE = 32 MiB. With MIMALLOC_SHOW_ERRORS=1 it prints cannot use OS memory since it is not large enough (size 32704 KiB, minimum required is 32768 KiB). Every rung from 64 MB up passes.

Reproduction, release build of the merge base (bf42a52), Linux x64:

bash -c 'ulimit -v 262144; exec bun -e 1'       # every limit from 96 to 320 MiB
BUN_JSC_structureHeapSizeInKB=32768 bun -e 1    # no limit needed, 65536 works

Three ulimit -v ranges abort there: up to about 340 MB (352 MiB starts), 1,150,000 to 1,250,000 KB, and 2,200,000 to 2,300,000 KB. In the second, bun's own 1 GiB mimalloc arena has taken the space before main(). In the third, JSC's 1 GiB JIT pool takes it inside JSC::initialize(), after anything bun could check and before the Structure heap (MIMALLOC_VERBOSE=1, BUN_JSC_verboseExecutablePoolAllocation=1). A 128 MB check in bun before the call passes in that range. The JIT pool is allowed to fail, so the check cannot ask for its size too. This branch prints the error in all three ranges. bun 1.3.14 died under the same limits too (SIGILL or SIGTRAP, no output), so this is not a regression.

Output of this branch under ulimit -v 262144:

error: Bun ran out of virtual address space while initializing JavaScriptCore.

JavaScriptCore reserves address space for its heaps at startup. This process could not reserve another 128 MB.

Current limit: 262144 KB (ulimit -v)

To fix this, raise or remove the limit:

  ulimit -v unlimited

BUN_JSC_structureHeapSizeInKB=3072 and =32768, with plenty of address space, still print the crash report, plus Crashed while initializing JavaScriptCore.

The 128 MB. The last rung mimalloc accepts is 64 MB, and OSAllocator::tryReserveUncommittedAligned maps size + alignment on Linux. The probe uses the same mmap flags. At the :150 assert 32 MB is reserved and between 32 and 96 MB is free. At :123 under 64 MB is free.

In the handler. The check runs before PANICKING is incremented, so Global::exit does not park. The error goes through the handler's raw stderr writer, after Output::flush(), so it needs no Output source on the crashing thread. The action is cleared first: a crash while the error prints, or while the process exits, is reported as a crash. Global::exit stays because it runs the quick_exit callback (Bun__onExit), and on_jsc_invalid_env_var already exits that way from inside JSC::initialize.

Tests. The first walks ulimit -v up from 96 MiB in 32 MiB steps until bun starts. On the way, a run must end with the error, exit code 1 and no signal. It does not depend on where the ranges are, so it holds when #41900 moves the floor. On the merge base build it logs abort() called for 96 to 320 MiB and started at 352 MiB, then fails. The second test uses structureHeapSizeInKB=3072 as a crash that the address space does not explain. Both skip under ASAN: bun installs no signal handlers there (reset_on_posix), and ASAN itself reserves terabytes of address space, so it does not start under these limits. bun bd test therefore skips them. The second test also runs on Apple Silicon: the pinned WebKit uses brk #0 as the arm64 crash instruction (oven-sh/WebKit#485), which macOS delivers to the handler.

Related. oven-sh/WebKit#483 (a block bitmap fallback for the 32 MB rung) was folded into oven-sh/WebKit#586, which #41900 brings in. After that, :150 no longer aborts and :123 remains. #39985 is the Windows counterpart. It adds the same Action::InitializeJsc lines, so the second of the two to land drops its copy. The check here is cfg(unix): Windows limits commit charge, not address space. The Windows Sentry issues in the same constructor (BUN-482B, BUN-4AA0, BUN-3ZGR) are not explained by this analysis.

Size. size build/release/bun-profile, release, Linux x64. Merge base: text 80,702,673, data 110,424, bss 1,822,992. This branch: text 80,703,185, data and bss the same. The stripped bun is 80,844,360 bytes in both.

Also run. cargo clippy -p bun_sys -p bun_crash_handler. cargo check -p bun_crash_handler for x86_64-pc-windows-msvc, aarch64-apple-darwin, x86_64-unknown-freebsd, aarch64-linux-android and x86_64-unknown-linux-musl. The whole test file on the release build: 29 pass, 8 skip. Under bun bd (ASAN, the new tests skip): 28 pass, 9 skip with a 60 s test timeout. With the default 5 s, the machine I ran on (load average near 800) timed out one to three older tests of the file in each run, different ones each time.

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no test proof · iteration 3 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/cli/run/run-crash-handler.test.ts

[coderabbitai]

Navigate logical layers of code changes, visualize relationships, and explore their blast radius.

Walkthrough

The JSC initialization path now marks initialization in the crash handler. On Unix, the handler probes whether it can reserve 128 MB of address space and reports an initialization error if the probe fails. Tests cover address-space limits and unexplained initialization crashes.

Changes

JavaScriptCore initialization diagnostics

Layer / File(s)	Summary
Track initialization and diagnose address-space failures src/sys/lib.rs, src/crash_handler/lib.rs, src/jsc/lib.rs	The Unix resource enum adds AS. JSC initialization sets the InitializeJsc action. The crash handler probes a 128 MB reservation and, if it fails, reports the soft address-space limit when available and exits with status 1.
Test initialization failure outcomes test/cli/run/run-crash-handler.test.ts	POSIX tests check address-space initialization failures and unexplained initialization crashes. The test harness adds an ASAN import and a helper that disables core dumps.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: 🔵 Low · up to a0d7b

The initialization diagnostic could fail or block in constrained startup circumstances instead of reliably exiting with status 1. Use raw crash output and termination to remove this bounded reliability risk.

🚥 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 and concisely describes the main change: reporting exhausted address space during JavaScriptCore initialization as an error instead of a crash.
Description check	✅ Passed	The description provides detailed problem, fix, verification, background, and trade-off information. It does not use the template headings exactly, but it covers both required topics sufficiently.

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[robobun]

^{Updated 9:11 PM PT - Sep 30th, 2026}

✅ @robobun, your commit 4cdd1bbcbc7c307948afadc05feac8297d335b0e passed in Build #122154! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 39967

That installs a local version of the PR into your bun-39967 executable, so you can run:

bun-39967 --bun

[robobun]

Status: rework in progress. Do not merge this revision.

A design review found that the probe in the crash handler is the wrong layer. It guesses the cause after the SIGABRT, and the guess is wrong in both directions: a real crash in JSC initialization under a tight ulimit -v loses its report, and ulimit -d keeps the crash banner.

The rework moves the report into JSC. oven-sh/WebKit#760 makes StructureMemoryManager call a bun function when the reservation fails. That function prints the error and exits with status 1. The crash handler goes back to what main has. I will push the rework to this PR after its self-review.

Reproduction on a release build of main (Linux x64): bash -c 'ulimit -v 262144; exec bun -e 1' and BUN_JSC_structureHeapSizeInKB=32768 bun -e 1. Both abort in JSC::initialize() with the crash report of BUN-4NF7.

[claude]

I reviewed this PR and didn't find any bugs. Because it adds a new early-exit path inside the signal-handler crash flow (mmap/getrlimit probe + formatted output before PANICKING is incremented), a maintainer look at the crash-handler placement would still be worthwhile.

Checked: c_int import is already cfg(not(windows))-gated so the new cfg(unix) FLAGS constants compile everywhere; bun_crash_handler is already a workspace dep of bun_jsc; the pretty_error! + Global::exit(1) shape matches the existing fd-exhaustion handling right below it; the probe runs before PANICKING.fetch_add so Global::exit won't park.

Extended reasoning...

Overview

Three files: src/crash_handler/lib.rs gains an Action::InitializeJsc variant, a hook in crash() at PANIC_STAGE == 0 that — when that action is set — probes for 128 MB of address space via mmap/munmap and, on failure, prints a ulimit -v error via pretty_error! and calls Global::exit(1) instead of producing a crash report. src/jsc/lib.rs wraps the JSCInitialize FFI call in a scoped_action(Action::InitializeJsc) guard. test/cli/run/run-crash-handler.test.ts adds two tests (Linux-only ulimit -v scan; a BUN_JSC_structureHeapSizeInKB=3072 case that must still crash-report).

Security risks

None identified. The new code reads RLIMIT_AS and does an anonymous mmap/munmap probe, all from process-local state with no external input.

Level of scrutiny

High. The crash handler is the last line of defense and this inserts new behavior on the SIGABRT path inside a signal handler — including formatted output and Global::exit (which runs atexit-style flushing). The author's reasoning is careful (single-threaded startup, synchronous abort(), check placed before PANICKING increments so Global::exit doesn't park), and the pretty_error! + Global::exit(1) pattern already exists in the same file for fd exhaustion. But a maintainer should confirm this is the right layer versus, say, having JSCInitialize return failure, and that calling mmap from the SIGABRT handler at this stage is acceptable.

Other factors

• Cross-platform cfg gating checked: c_int is imported under cfg(not(windows)) (line 461), and the new can_reserve_address_space/address_space_limit are cfg(unix) with linux/android sub-branches for MAP_NORESERVE and the vm.overcommit_memory hint. On Windows the outer fn body is empty, so the crash reports as before.
• bun_core::ffi::zeroed exists and is used for libc::rlimit.
• bun_crash_handler is already a dep of bun_jsc (Cargo.toml line 30), so the new scoped_action call introduces no dependency edge.
• Tests: the ulimit scan is Linux-only, skips ASAN (correctly — bun doesn't install signal handlers there), sets ulimit -c 0 to avoid leaked cores, and reads stderr before asserting. The scan can spawn up to ~60 short-lived shells on an unfixed build; on the fixed build it should exit on the first iteration. The second test asserts "Crashed while initializing JavaScriptCore" appears for a non-address-space crash, exercising the fall-through path.
• The PR description documents cross-target cargo check runs including windows-msvc, darwin, freebsd, android, and musl.

[robobun]

On the placement question from the review: the crash handler is the only layer that can see this. The assert is inside JSC::initialize(), which has no failure return, and a check before the call is not enough because bun's own 1 GiB mimalloc arena and JSC's 1 GiB JIT pool are reserved between any such check and the Structure heap (the upper two ulimit -v windows in the description). The WebKit half that can move to the source is oven-sh/WebKit#483.

On running the probe inside the handler: it is one anonymous mmap and one munmap, nothing is written through the mapping, and it runs before PANICKING is incremented. The crash it reacts to is a synchronous abort() on the thread that is initializing JSC, so Global::exit runs in the same situation as the fd exhaustion exits below it.

[claude]

I reviewed this PR and didn't find any bugs. Because it adds an mmap probe and formatted-output path inside the signal-driven crash handler (PANIC_STAGE == 0), a human look at the placement and signal-safety trade-off would still be worthwhile.

What was reviewed:

• Action::InitializeJsc wiring: the Display match is exhaustive with the new arm; scoped_action guard drops after JSCInitialize returns.
• Probe ordering: runs before PANICKING is incremented, so Global::exit(1) takes the same non-parking path as the fd-exhaustion exits nearby.
• cfg(unix) gating: on Windows the body is empty (falls through to the normal report + "Crashed while initializing JavaScriptCore" line); libc::MAP_ANONYMOUS/RLIMIT_AS availability on the checked targets.
• Tests: hoisted noCoreCmd is only called from isPosix-gated blocks; piped stdout is undrained but bun -e 1 produces none before JSC init aborts, matching existing patterns in this file.

Extended reasoning...

Overview

Three files: src/crash_handler/lib.rs adds an Action::InitializeJsc enum variant and, when a crash arrives with that action set, an address-space probe (mmap/munmap 128 MB) that converts the crash into a user-facing ulimit -v error and Global::exit(1). src/jsc/lib.rs sets that scoped action around JSCInitialize. test/cli/run/run-crash-handler.test.ts gains two tests (Linux release: walk ulimit -v up until the error appears; POSIX non-ASAN non-Apple-Silicon: a JSC-init crash with plenty of address space is still reported) and hoists noCoreCmd to file scope for reuse.

Security risks

None identified. The new code only runs after JSC has already aborted, reads RLIMIT_AS, and prints to stderr. No user-controlled input reaches the probe or the format string.

Level of scrutiny

High. The crash handler is the last-line-of-defence code path — a mistake here can mask real crashes, hang the process during a crash, or make the handler itself re-enter. This change deliberately calls mmap/munmap and pretty_error! (formatted stderr write) from inside a SIGABRT handler before PANIC_STAGE/PANICKING are advanced. The PR justifies this convincingly (synchronous abort() on the initializing thread; mmap/munmap are async-signal-safe; Global::exit precedent in handle_root_error right below), and the fallback behavior is unchanged when the probe succeeds. But it is exactly the kind of layering call — "is the crash handler the right place for this, vs. a pre-flight check or the WebKit-side fix in oven-sh/WebKit#483 alone" — that a maintainer should sign off on. The comment-cop bot flagged the design several times (all now resolved with shortened comments and a rationale), which reinforces that a human should confirm the placement.

Other factors

• The 128 MB constant is derived from StructureAlignedMemoryAllocator.cpp internals (64 MB smallest accepted rung, reserved as size + alignment). It will drift silently if JSC's reservation strategy changes; the derivation comment is the only tether.
• The ulimit -v sweep test spawns once on the fixed build per the PR notes, but up to ~60 times on a future build where the windows shift — worth a maintainer glance for CI budget.
• Cross-target compilation was verified per the PR notes (cargo check for windows-msvc, darwin, freebsd, android, musl); I confirmed c_int, libc::MAP_ANONYMOUS, and RLIMIT_AS are in scope for the cfg-gated paths.
• No prior claude[bot] review on this PR; all inline comment-cop threads are resolved.

[robobun]

On the cost of the ulimit -v walk: the walk starts at 256 MiB and stops at the first limit that ends with the error, and the lowest window sits right above the binary's own footprint, so on a build that has the fix it stops after a few spawns wherever the windows are (the first limit on the current release build). The 60 spawn case only happens on a build without the fix, where the test fails anyway, and took about 1 second on the canary.

[robobun]

The Apple Silicon skip on a crash that the address space does not explain is reported has a fix in flight: oven-sh/WebKit#485 changes the arm64 crash instruction to brk #0, and #39978 bumps the pin and adds a test with the same structureHeapSizeInKB=3072 trigger. Once both are in, the !(isMacOS && isArm64) condition can go. The two branches touch the same harness import line in run-crash-handler.test.ts, so whichever lands second needs a one line rebase.

[robobun]

#39985 is the Windows counterpart of this PR (a commit that fails under the commit limit or a job object limit). It adds the same Action::InitializeJsc lines, so whichever of the two lands second needs to drop its copy of them.

[robobun]

Rebased on main (61edd94, force pushed). One conflict, in run-crash-handler.test.ts: #39953 also moved noCoreCmd to module scope and removed the self-kill tests. I kept main's version of both and only moved the helper above the new describe block, which uses it. The source side applied as is. On the rebased tree the file passes on a release build (28 pass, 8 skip) and under bun bd (27 pass, 9 skip).

Noted the two follow-ups above: when #39978 lands, the !(isMacOS && isArm64) condition on the second test can go, and #39985 shares the Action::InitializeJsc lines with this PR.

[claude]

I reviewed this PR and didn't find any bugs. Because it changes the crash handler's signal-path behavior (running an mmap probe inside the SIGABRT handler and suppressing the crash report in favor of a user-facing error), and the 128 MB threshold is derived from JSC internals, a human look would still be worthwhile.

What was reviewed:

• Signal-handler safety of the mmap/munmap probe and Global::exit — both are async-signal-safe and match the existing fd-exhaustion pattern in the same stage-0 path.
• The Action::InitializeJsc scoping in bun_jsc::initialize and its restoration on drop — the guard is held across JSCInitialize only.
• Windows / non-unix: the check is a no-op and falls through to the normal report plus the new "Crashed while initializing JavaScriptCore" line.
• The ulimit -v walk test terminates early on a fixed build and drains stdout is not needed (only stderr piped and read).

Extended reasoning...

Overview

The PR adds a new Action::InitializeJsc variant to the crash handler's CURRENT_ACTION thread-local, sets it around JSCInitialize in src/jsc/lib.rs, and hooks the crash handler's stage-0 path so that when a crash arrives with that action set, it first probes whether 128 MB of address space can still be reserved via mmap. If not, it prints a user-facing error naming ulimit -v and exits 1 without a crash report; otherwise the crash is reported as before with an added "Crashed while initializing JavaScriptCore" line. Two tests exercise both branches on release lanes (they skip under ASAN and the second skips on Apple Silicon). The existing noCoreCmd helper is hoisted above its new use site.

Security risks

None identified. The change only affects how a process that is already aborting inside JSC::initialize() reports its own death. The mmap is anonymous, private, and immediately unmapped; nothing is written through it. getrlimit(RLIMIT_AS) reads a process-local value. No user input reaches the new code.

Level of scrutiny

High. This is the crash handler — the last-resort diagnostic surface — and the new code runs inside a signal handler after abort() from C++. The change also introduces a design decision: suppressing the Sentry crash report entirely when the probe fails, on the reasoning that under 128 MB free JSC could not have started regardless, so the crash is the environment's doing, not Bun's. That reasoning is well-argued and the 128 MB derivation (64 MB smallest usable rung × size+alignment on Linux) is documented, but the constant tracks JSC internals (StructureAlignedMemoryAllocator.cpp) that could shift. The user-facing error wording and the choice to exit(1) rather than still emit a trace string are product decisions worth a human eye.

Other factors

• The pattern (pretty_error! + Global::exit(1) inside crash-handler stage 0, before PANICKING is incremented) mirrors the existing fd-exhaustion handling directly below it, so precedent for running these helpers here is established.
• mmap/munmap/getrlimit are async-signal-safe per POSIX; the crash the handler is reacting to is a synchronous abort() on the initializing thread, not an async fault, so re-entrancy concerns are limited.
• On Windows and other non-unix targets the function body is empty, so behavior is unchanged there apart from the extra "Crashed while" line.
• The tests are platform-gated and hermetic (no network), use ulimit -c 0 to avoid core-file leaks on the Alpine lanes, and the walk test's cost is bounded (the author notes it stops after the first spawn on a fixed build). The second test uses --debug-crash-handler-use-trace-string to avoid the slow symbolizer path.
• There is cross-PR coordination noted in the timeline (oven-sh/WebKit#483, #39978 for the Apple Silicon skip removal, #39985 for the Windows counterpart) — whichever lands second needs a small rebase.
• CI on the last full run passed the touched test file on every lane; the only red is an unrelated gitlab.com 403 outage affecting install tests.
• The comment-cop bot flagged long comments earlier; those were shortened and the threads are resolved.

[claude]

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

[coderabbitai]

Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @src/crash_handler/lib.rs:
- Line 1266: In the Action::InitializeJsc branch reached by
handle_segfault_posix, replace pretty_error! with diagnostics written through
stderr_writer(), then exit with status 1 using the platform-specific raw exit
helper. Avoid the Output subsystem and Global::exit so this path does not
require Source initialization, flush writers, or wait for crash handling.

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📒 Files selected for processing (4)

• src/crash_handler/lib.rs
• src/jsc/lib.rs
• src/sys/lib.rs
• test/cli/run/run-crash-handler.test.ts

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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

Code review found no issues

No high-confidence issues detected in this change.