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--hot: fix a use-after-free of the entry point's promise - #44350
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…a raw pointer and a flag
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Navigate logical layers of code changes, visualize relationships, and explore their blast radius. WalkthroughThe VM now stores its pending internal promise in a strong slot and provides accessors to read, set, or clear it. Entry loading, runtime hooks, hot reload, and test-runner paths use these accessors. Tests check promise-root counts during hot reload and relative to process baselines. ChangesPending Promise Retention
Suggested reviewers: Priority: ⬆️ High Merge Risk: ⚪ Minimal · up to The independent subprocess cases can run concurrently to follow testing guidance. This improvement does not block merging after normal checks. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Review comments at @test/cli/hot/hot.test.ts:
- Line 861: Update the stdout line search in the test to ignore the trailing
fragment after the last newline; only match the prefix against complete lines,
and keep the existing search behavior for those lines.
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Updated 1:13 AM PT - Oct 1st, 2026
@dylan-conway, your commit df2d5ed is building: |
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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 that the new strong::Optional field is sound in the zero-initialized VirtualMachine constructor: it is Option<NonNull<Impl>>, so all-zero bytes are None, same as the existing overridden_main slot it sits next to, and get()/set()/clear_without_deallocation()/deinit() all handle the None handle.
Extended reasoning...
The change replaces a raw *mut JSInternalPromise plus protect flag with a GC-rooted Strong slot in src/jsc/VirtualMachine.rs, updates two callers in src/runtime, and adds a heap-snapshot regression test; no security-sensitive surface. One confirmed inline finding (the slot keeps a rejected entry promise rooted across the reload GC) was posted, so a human look is warranted; the zero-init concern raised in the change summary was ruled out by reading src/jsc/Strong.rs.
dylan-conway
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This also fixes the crash in #41012: a segfault in #41146 and #44233 address the same bug. #44233 makes the same source change. #41146 is mine and this PR covers it. I leave it open for a maintainer to close. On a test for the collected case: #41146 has one that does not wait for time. After a reload, the entry runs
The collection starts from an |
…oint once it could have been collected
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Thanks, both are in.
Starting the collection from a callback of the new load was what I was missing, and df2d5ed has two tests of the behaviour that wait for nothing. I was wrong that it takes idle time. Which callback works differs between builds, on release builds too: from |
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Review comments at @test/cli/hot/hot.test.ts:
- Line 877: Update the parameterized test declaration from it.each to
it.concurrent.each so the independent subprocess cases run concurrently.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline note on the new reuse test, I also checked that no raw store, protect()/unprotect() pair or flag read of the old field survives outside the two accessors, that the slot is released in the VM's explicit deinit (needed, since the VM is raw-deallocated and field Drop does not run), that the reset path's clear_without_deallocation() leaves the block for that deinit, and that the protected() guard kept in jsc_hooks.rs load_preloads is still load-bearing for its unwrap_or(promise) fallback once HMR swaps the slot.
Extended reasoning...
The source change replaces a raw *mut JSInternalPromise plus a protected flag in VirtualMachine with a strong::Optional behind two accessors, touching the entry-point load, preload, hot-reload, reset and teardown paths of the VM; it touches no auth, injection or data-exposure surface. It is GC-rooting code in the VM core with a prior rooting-lifetime question resolved only by the author, so a human look remains worthwhile; one inline nit on the new regression test is posted separately.
| // What is left on the stack can keep it alive, and differs with where a collection starts from. | ||
| writeFileSync( | ||
| join(String(dir), "main.js"), | ||
| ` | ||
| globalThis.kept = []; | ||
| require("fs").readFile(__filename, () => { | ||
| Bun.gc(true); | ||
| setImmediate(() => { | ||
| Bun.gc(true); | ||
| require("crypto").randomBytes(8, () => { | ||
| Bun.gc(true); | ||
| for (let i = 0; i < 20000; i++) { | ||
| const promise = ${promise}; | ||
| promise.catch(() => {}); | ||
| kept.push(promise); | ||
| } | ||
| console.log("collected"); | ||
| }); | ||
| }); | ||
| }); | ||
| `, | ||
| ); | ||
| await line("collected"); |
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🟡 nit (optional): maintainers get a regression test that can pass on the unfixed build, since nothing checks the entry-point promise was actually collected before the 20k promises are made. The fixture at test/cli/hot/hot.test.ts:900-919 relies on three Bun.gc(true) calls from different stack depths to free the second load's promise, and the comment at line 900 says the result depends on what the stack holds; CLAUDE.md rule 15 names that exact approach as blaming the conservative scanner without proof. If the promise survives, the third reload is clean on base too and the toEqual at line 926 passes vacuously. Fix: assert the precondition the test depends on, or keep only the deterministic heap-edge test at line 832 and state that this one is best-effort.
Why this was flagged
The it.each test at test/cli/hot/hot.test.ts:877-930 is meant to fail on the base branch by making the collected entry-point promise's cell be reused by one of 20,000 program promises. It only asserts clean stdout and stderr after the third load (line 926-929); it never checks that the second load's promise was freed by the Bun.gc(true) calls at lines 906, 908 and 910. The comment at line 900 states the collection depends on what is left on the stack, and the PR description reports runs where the promise was not collected at all. On such a run the unfixed build reloads cleanly and the test passes, so it does not guard the fix. CLAUDE.md rule 15 says running GC from a different stack depth or more times is not a valid way to work around retention and is blaming the scanner without proof. The first new test at line 832 (counting StrongRootBlock -> Promise edges) is the deterministic one; this one needs a precondition assertion or a note that it is best-effort.
Verification: nit. On the unfixed build, whenever one of the three Bun.gc(true) calls does not free the second load's entry-point promise, the third load prints clean output and the test passes anyway. Mechanism verified in /home/claude/bun/test/cli/hot/hot.test.ts: the only assertions are lines 926-929. Nothing asserts the precondition that the entry-point promise was reclaimed before the churn.
What does this PR do?
Fixes #41012
Fixes a use-after-free in
bun --hot: it goes on reading the promise of the entry point after the promise has been collected. What happens next depends on what the memory is used for.--hotpanic: internal error: entered unreachable code: invalid JSPromise status 255, or a segfault inisTerminationExceptionunderunhandled_rejectionThe first two reproduce every time, below. The third is 21 crash reports on 1.4.2, 11 on macOS arm64 and 10 on Windows, every one from a process with an HTTP server, and #41012: a segfault in the read of the status itself, in
report_exception_in_hot_reloaded_module_if_needed, at an address that ends in0x…B90, a cell plus the0x10of the word below. I could not reproduce that one, see the end of the next section.Cause
VirtualMachine::pending_internal_promiseis the promise of the latest load of the entry point.--hotlooks at it on every tick, inreport_exception_in_hot_reloaded_module_if_needed, and again inreload: its status, and its result if it is rejected. That goes on for as long as the process runs, long after the promise has settled.It was a
*mut JSInternalPromisewith nothing to keep the cell alive:Only the preload path called
protect(), hence the second field. Once the cell is collected and its memory is used for something else, the status is whatever is there (the panic), or it reads as rejected and the "result" is handed tounhandled_rejection(the segfaults).Reproduction
A
--hotserver whose handler callsBun.gc(true)for/gcand makes and keeps 20,000 promises for/promises. Save the file once, wait half a second, thencurl /gc,curl /promises, and save the file again. Release builds, Linux x64, no debugger:error: someone else's, and handled7fe13e1b9The 1.4.2 profile build under lldb, with a breakpoint on
report_exception_in_hot_reloaded_module_if_neededthat logs the pointer (vm + 0x7328) and the word it reads the status from (+ 0x10, bits 48 and 49), and a handler that lists the address of everyPromiseingenerateHeapSnapshotForDebugging()afterBun.gc(true):Promise0x000d000000000000Promises0x000d000000000000Promiseagain, another one0x000e0000000000000x0000000000000000Whether the promise is collected depends on where the collection starts from, because of what is left on the stack. After a reload, the new load calls
Bun.gc(true)from a callback, makes 20,000 pending promises, and the file is saved again. Runs in which that save never reloads, of 4, release builds, Linux x64:7fe13e1b9setImmediate,setTimeout(0),crypto.randomBytes,zlib.gzip, the exit of a child,fs.readFilethensetImmediatefs.readFile,fs.promises.readFile,Bun.file().text()fs.statThe first load's promise is kept alive by a local of
Run::start, found by the conservative scan: in a debug build stopped inJSC__VM__runGCunder the request handler, 4 of the 6 stack words that hold its address are in that frame.For the panic the two bits have to read 3, which no promise has, so the block has to be freed and used for something else. I tried to get there by filling the promise blocks before the reload and flooding the heap with objects of doubles, of integers, typed arrays and strings afterwards, after a reload that works and after one that fails: 25 runs on 1.4.2 and canary, no crash.
Fix
One
jsc::strong::Optionalin place of the pointer and the flag, behindpending_internal_promise()andset_pending_internal_promise(). Theprotect()/unprotect()pairs, the flag and theensure_still_alive()calls go. It is released whereoverridden_mainis.What it shows in
heapStats()Every process now has one more protected
Promise, where before only one with a preload did. The readers that go on after the load are all behindis_watcher_enabled(), so the handle could be kept to--hotand--watch. That would make each of the eight places that store it conditional, with the hand-over from an overriddenModule.runMainas a special case: the kind of state to keep in step by hand that this PR removes. So it is held always.Two tests counted strong handles in the whole process from inside the top-level await of their entry point, and expected none of the process's own:
timer-gc-roots.test.ts(at most 1StrongRootBlockleft) and a fixture ofmodule-graph-isolation.test.ts(0 protectedPromises). They now subtract what is there at their start, asbody-clone.test.ts,fetch-leak.test.ts,module-graph-gc.test.tsandserve-stream-reject-flush-leak-fixture.tsdo. Both pass on canary7fe13e1b9as well as with this PR.How did you verify your code works?
Three new tests in
hot.test.ts. None waits for time to pass.Two are of the behaviour. After a reload the entry point collects from an
fs.readFilecallback, then fromsetImmediate, then from acrypto.randomBytescallback, since which of them finds the stack clean differs between builds. It makes 20,000 promises, pending in one test, rejected and handled in the other. The file is saved again: the third load has to happen, with nothing on stderr. Three runs of the two tests each:7fe13e1b9, releaseThe third is of the cause, and does not depend on the stack. The entry point takes a heap snapshot after a full collection and prints how many
Promises aStrongRootBlockrefers to, after the first load and after a reload: 0 on 1.4.2, 1 and 1 with this PR.hot.test.ts, debug: 16 pass, 0 fail.test/cli/hot,test/cli/watch,test/cli/test/isolation.test.ts,expect-extend-preload.test.tsandnode-module-module.test.js(preloads,Module.runMain, the test runner's loads), debug, before the two tests of the behaviour were added: 130 pass, 1 skip, 0 fail.