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[JSC] bun:ffi: JSFFIFunction::close() makes a call throw instead of jumping to an unloaded library - #714
[JSC] bun:ffi: JSFFIFunction::close() makes a call throw instead of jumping to an unloaded library#714robobun wants to merge 7 commits into
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Navigate logical layers of code changes, visualize relationships, and explore their blast radius. WalkthroughThis change adds FFI function closure through ChangesFFI Function Closure
Priority: ➖ Normal Merge Risk: 🔵 Low · up to The closed-pointer error test can miss prefix or suffix regressions. Tighten the assertion before merging or accept this bounded test-coverage gap. 🚥 Pre-merge checks | ✅ 3 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (3 passed)
Full details: Description checkExplanation The description gives a detailed explanation of the bug, implementation, test coverage, and platform limitations. It does not include the required Bugzilla URL, explicit "Reviewed by NOBODY (OOPS!)." line, or the template's structured changed-path and function list.
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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 findings, I also checked the other tiers' close paths and found no gaps: the IC stub's watchpointLabel() sits after the frame setup and before the arity check, so the patched jump lands on the slow path with the same frame state as every other jump there; the DFG/FTL addLazily on the closed set is preceded by an isClosed() refusal and is re-validated by DesiredWatchpoints at finalize, and CallFFI clobbers the world so an invalidation point follows a close made from within an optimized caller; the host path re-tests isClosed() after argument conversion and after the before-hook, both of which can run JS.
Extended reasoning...
The change adds a closed state to JSFFIFunction across the host path, IC stub (replaceWithJump repatch), and DFG/FTL CallFFI (InlineWatchpointSet jettison), plus a new stress test; it touches no auth, crypto or injection surface but does patch JIT code and bake native targets. Inline findings were posted (the function-typed argument path still hands a closed function's raw ptr to native code, and BUN-TEST-DIFFERENCES.md is out of date), and one verified finding was not posted, so approval is not appropriate; the note records the tier paths examined and ruled out.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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Source/JavaScriptCore/ffi/FFIConversions.cpp— Users who pass a closed FFI function as afunctiontyped argument still get the segfault this PR claims to remove. writeSlotFromJSValue at FFIConversions.cpp:365 special-cases JSFFICallback only and otherwise hands the function's staleptrto native code, so after lib.close() the native callee jumps into the unmapped library. JSFFIFunction.h:82 promises none of a closed function's calls reach target(), but this path is not a JS call and is not checked. Fix: in the Type::Function conversion, reject a JSFFIFunction whose isClosed() is true with throwClosedError, and do the same for the IC stub and DFG inline Function conversions.Why this was flagged
A script does
const { fn, register } = lib.symbols; lib.close(); other.symbols.register(fn)where register's signature declares the argument asfunction. writeSlotFromJSValue at FFIConversions.cpp:365 reads the closed JSFFIFunction's ptr property (set at JSFFIFunction.cpp:153 to the raw target) and writes it into the slot; the second library then calls that address, which is unmapped after dlclose, and the process segfaults exactly as in the PR description's repro. On the base branch the same thing happens, but the base makes no closed promise; after this change JSFFIFunction.h:82 states every later call throws and none reaches target(), and the Bun side relies on close() to make unload safe. The PR text scopes .ptr and .native out, but the Type::Function conversion is a built-in JS-visible path with no raw address in the user's hands. A fix is a single isClosed() test at FFIConversions.cpp:365 plus the matching inline conversions in FFIICStub.cpp and FFIDFGCodegen.cpp.Verification: pre-existing; acknowledged in diff: the PR description says "
.ptrand.nativedo not change. A native caller that holds the raw address is outside this, the same as forJSFFICallback", and the new test pins it (JSTests/stress/ffi-function-close.js:52shouldBe(add.ptr, ptr, ".ptr after close")) — that bound is accurate as far as it goes, but it also means the object-as-argument path…
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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 the concurrent-compile race where close() fires after tryConvertCallToCallFFI's isClosed() check but before the plan is installed: DesiredWatchpoints re-tests hasBeenInvalidated() on the set when it really adds the watchpoint (SetPointerAdaptor::add), so the plan fails instead of baking in a closed target. The BUN-TEST-DIFFERENCES.md count from the earlier nit is now updated.
Extended reasoning...
The change adds a closed state to JSFFIFunction and threads it through the host call path, the IC stub (repatched to its slow path), the DFG/FTL CallFFI lowering (a jettisoning watchpoint) and the operationFFIWriteSlot slow conversion, plus a $vm hook and a new stress test; it touches no auth or injection surface, but it is native-call safety code in the engine. One confirmed finding is posted inline, so this is not an approval; the note records the one further candidate ruled out by reading the DFG watchpoint registration.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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Source/JavaScriptCore/ffi/FFIConversions.cpp— pre-existing: a closed JSFFICallback passed as afunctionorptrargument still hands the native callee its entrypoint, so native code can invoke a callback JS already closed and unrooted. The new closed check at FFIConversions.cpp:379 covers only JSFFIFunction; the JSFFICallback branch at FFIConversions.cpp:365 returns nativeEntrypoint() without testing isClosed(). Fix: reject every closed FFI object handed out as a pointer, i.e. also throw forcallback->isClosed()at line 365 (the same test $vm already makes in JSDollarVM.cpp:5088), so the two closed-object paths fail alike. JSFFICallback::close() nullsptrto keep the address out of native hands; this path bypasses that.Why this was flagged
JS calls
cb.close()on a JSFFICallback, then passes the same object as afunction-typed (orptr-typed) argument of any bun:ffi call, e.g.callCb(cb, 1), through ffiCall (FFICallHost.cpp:75) or operationFFIWriteSlot (FFIConversions.cpp:531). writePointerSlot at FFIConversions.cpp:365 matchesdynamicDowncast<JSFFICallback>(cell)and storescallback->nativeEntrypoint()with noisClosed()test, while the branch this PR adds at FFIConversions.cpp:379 makes the same check for a closed JSFFIFunction. JSFFICallback::close() (JSFFICallback.cpp:156-166) calls unroot() and setsptrto null so that native code can no longer obtain the address; the trampoline's ffiCallbackDispatch (FFICallbackThunk.cpp:371) does not test m_closed either, so the native callee runs the closed callback's JS callable, and a library that stores the pointer calls a collectable cell later. JSDollarVM.cpp:5088 shows the engine treats a closed callback as invalid input for exactly this conversion. The base branch behaves the same; the PR fixes the sibling JSFFIFunction case only.Verification: pre-existing (the base takes the same route; the diff touches writePointerSlot and adds the sibling JSFFIFunction check but leaves this branch as it was). Trigger: JS calls
cb.close()on a JSFFICallback and later passes that same object as afunction/ptr-typed argument of any FFI call. Mechanism verified in /home/claude/webkit/Source/JavaScriptCore/ffi/FFIConversions.cpp: writePointerSlot…
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Inline comments:
In `@JSTests/stress/ffi-function-close.js`:
- Line 253: Update the error assertion in the pointerAdd close test to compare
error.message for exact equality with the complete expected message, while
retaining the TypeError check and existing failure behavior.
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The list was filled with JSArray::push, which is a [[Set]]: an indexed setter on a prototype was handed the array, and could leave a hole in it or take a function out of it. It is filled with putDirectIndex now, and close() skips an empty slot. The test also covers a symbol deleted from lib.symbols before close(), and a closed function passed as a pointer argument (oven-sh/WebKit#714 now throws for it).
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I reviewed the current head (through 6dfa406) and found no bugs; the BUN-TEST-DIFFERENCES count from the earlier nit is now updated. Because this patches live JIT code and adds a new jettison path across the IC stub, DFG and FTL, a human look at the JIT pieces is still worthwhile before merging.
What was reviewed:
- IC stub
close(): thewatchpointLabel()sits after the prologue, tag save andtopCallFramestore, so the patched jump lands on the slow path with the same frame state every other slow-path branch has; the padding comes fromlabelForWatchpoint, as for DFG invalidation points. - DFG/FTL:
CallFFIclobbers the world inDFGClobberize.hand the abstract interpreter, so an invalidation point follows it; theaddLazily(InlineWatchpointSet&)overload exists andareStillValid()covers a close racing the compile thread between theisClosed()check and finalization. - Host path re-checks after conversion and after the before-hook: the throw through the outer
ThrowScopeis picked up by theTopExceptionScope, cleared and rethrown aspending, matching the existing throwing-hook flow. writePointerSlotrejects a closedJSFFIFunctionbefore readingptr;operationFFIWriteSlotreads the context from the global object, and both DFG (cellOperand()) and FTL (weakPointer(ffiFunction)) pass the frozen function.
Extended reasoning...
The change adds a closed state to JSFFIFunction across the ffi/ directory (host call, IC stub, DFG conversion and codegen, FTL lowering), a $vm test hook, one new stress test and a one-line count fix in BUN-TEST-DIFFERENCES.md. It touches no injection, auth or data-exposure surface, but it does patch executable JIT memory at runtime (MacroAssembler::replaceWithJump on a live thunk) and adds a new InlineWatchpointSet that jettisons optimized code, which are the paths where a subtle mistake would be a crash rather than a wrong value. The bug hunt ran dry, the earlier inline nit was addressed by a later commit, and a coderabbit thread on the test was followed by a test-only commit that plausibly addressed it. The size (about 390 lines) and the JIT-patching nature of the change, not any open concern, are what decide defer over approve.
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The review here found that a closed |
…stead of reaching the target An embedder unloads a library while function objects that point into it are still reachable. The target is baked into the IC stub and into DFG and FTL CallFFI code, and the host path reads it with no closed state to test, so a call after the unload jumps to an unmapped address. close() costs an open function nothing. It repatches the start of the IC stub's fast path with a jump to its slow path, and it fires a watchpoint that DFG and FTL code with a CallFFI of the function watches. ffiCall() tests isClosed() on entry and again after the arguments are converted, because a conversion can run JS that closes the function. operationFFIWriteSlot does the same for the DFG and FTL slow path.
writePointerSlot() read the `ptr` of a closed JSFFIFunction and handed the native callee its dangling address. It throws a TypeError now. The stress test also covers a close from inside the call (a callback), a close inside the before-hook, and that a caller compiled again after the close gets no CallFFI node. The call from C++ is now Array#sort: Array#map is a JS builtin.
…ion has not returned JS that the target called back into can close the function. The call in flight returns into the code at the target, so an embedder must not unload that code then. isRunning() walks the stack once: the host path and the IC stub give the call a frame of its own, and a CallFFI is made by the DFG or FTL frame that was on top when the VM was entered again. The stress test gives each case a source text of its own. Compiled code is specialized on the function it calls, so the cases that shared a source text ran through the IC stub.
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Beyond the inline finding, I also checked the window between the isClosed() test in tryConvertCallToCallFFI (compiler thread) and a close() on the main thread before the plan lands: DesiredWatchpoints::reallyAdd rejects the plan when the lazily-added InlineWatchpointSet is no longer valid, so a CallFFI compiled against a function closed mid-compilation is discarded rather than installed. I also checked the IC-stub patch against an in-flight call to the same stub (a callback that closes the function): the replaceWithJump site precedes the arity check, the running frame is already past it, and that call completes into the still-mapped target as isRunning() documents.
Extended reasoning...
The change adds a closed state to JSFFIFunction across the host path, the IC stub (runtime repatch of a watchpoint label to the slow path), and DFG/FTL CallFFI (watchpoint-driven jettison), plus a new isRunning() stack scan and a $vm test hook; it touches JIT code patching but no auth, crypto, or injection surface. The two items above were examined and ruled out from the code; the inline finding on isRunning() missing bound/constant-folded callees stands as posted.
The list was filled with JSArray::push, which is a [[Set]]: an indexed setter on a prototype was handed the array, and could leave a hole in it or take a function out of it. It is filled with putDirectIndex now, and close() skips an empty slot. The test also covers a symbol deleted from lib.symbols before close(), and a closed function passed as a pointer argument (oven-sh/WebKit#714 now throws for it).
A JSCallback that the library calls can close the library. The call in flight returns into the library, so dlclose() there made that return a jump to an unmapped address. close() asks JSFFIFunction::isRunning() (oven-sh/WebKit#714) for each function, and skips the unload when one of them is running. The functions are closed either way: the call finishes, and the next one throws.
…s to strongly A cell that something froze strongly (the function of a bound function, an operand of a folded ===) is a constant of the optimized code block, not one of its weak references. isRunning() looked at the weak references only, and said no for a CallFFI that was on the stack. The stress test did not see it: its function took the callback as an argument, and a pointer argument has a slow conversion that refers to the function weakly. The function under test now takes an int32 and calls a callback that a new fixture stored before.
…ed the frame it called isRunning() took the frame after one whose caller is the entry frame for the frame that made the native call. The stack walk does not visit a frame that optimized code replaced with an inlined tail call, so a strict callback that ends in a call (x => handle(x)) hid the entry, and isRunning() said no for the CallFFI under it. It now compares the entry frame of each visited frame with the one of the frame above, which holds for every frame the walk visits.
…ck that makes a tail call A strict callback that ends in a call makes a tail call, and optimized code inlines the callee in place of the callback's frame. The stack walk of JSFFIFunction::isRunning() then did not see the VM entry, close() unloaded the library, and the call in flight returned to an unmapped address. oven-sh/WebKit#714 at d4f2d385 has the fix.
A
bun:ffilibrary can be unloaded while function objects that point into it are still reachable: a destructured symbol, or a caller the JIT compiled.JSFFIFunctionhas no closed state. The host path (ffiCall) readsm_target, and the IC stub and the DFG / FTLCallFFIcode have the target baked in. So a call after the unload jumps to an unmapped address:This is the piece of oven-sh/bun#31960 that the engine-native FFI did not pick up (see the last two comments there). The bun side is oven-sh/bun#43764. It calls
close()on every function of a library before it unloads the library.Change.
JSFFIFunction::close(VM&)makes every later call throwTypeError: bun:ffi: cannot call '<name>' because its library was closed. An open function pays nothing for it:IC stub: the stub is now an
FFI::ICStubCode(aDirectJITCodethat remembers two labels).generateICStubCodetakes awatchpointLabel()at the start of the fast path.close()overwrites it with a jump to the stub's slow path (MacroAssembler::replaceWithJump, what a DFG invalidation point uses). The slow path reachesffiCall(), which throws. The instruction stream of an open stub is identical to before.DFG / FTL:
tryConvertCallToCallFFIregisters the function'sInlineWatchpointSetwith the plan and refuses a closed function.close()fires the set, which jettisons the code.CallFFIclobbers the world, so an invalidation point follows it.Host path:
ffiCall()testsisClosed()on entry, and again after the arguments are converted.The second test exists because a conversion can run JS (
valueOf, aptrgetter), and that JS can close the function after the call has started. The DFG and FTL slow conversions all go throughoperationFFIWriteSlot, so it now takes the function (in place of theFFIContext*it can get from the global object) and throws if the function is closed when the conversion returns.Pointer arguments:
writePointerSlot()read theptrof a closed function that was passed as aptrorfunctionargument, and the native callee then called the dangling address. It throws aTypeErrornow. Object arguments always take this C++ path, so one test covers every tier.A close inside the call. JS that the target called back into can close the function, and the embedder then unloads the library under a call that has not returned.
JSFFIFunction::isRunning(VM&)tells the embedder so, and Bun keeps the library loaded then. It walks the stack once, at close time only:CallFFIhas no frame. The native call is made by the DFG or FTL frame that was on top when the VM was entered again. That is the first visited frame whose entry frame differs from the one of the frame above. (The frame that the entry called cannot say so: the walk does not visit it when optimized code replaced it with an inlined tail call.) That frame's machine code block is looked up for the function: among its weak references, and among its constants, where a cell is when something froze it strongly (the function of a bound function, an operand of a folded===). An inlined frame reports the inlinee's baseline code block, so the machine frame's is used.CallFFIit can say yes when the optimized code calls the function somewhere and is inside another native call that entered JS again. The embedder then only keeps a library loaded.CallFFIof the function and closes it itself, with no VM entry in between, gets no. Without that,for (...) fn(); lib.close();in one hot function would never unload..ptrand.nativedo not change. A native caller that holds the raw address is outside this, the same as forJSFFICallback, and so is a raw.ptrnumber the script kept. The cell stays in its 80-byte size class (72 bytes before, 80 now).Tested with a debug + ASan build of
jscand of Bun against this branch (Linux x64):JSTests/stress/ffi-function-close.js: cold calls,call/apply/bind/ a call from C++ (Array#sort), a caller compiled withCallFFIbefore the close, a close inside a hot loop, a close insidevalueOfand inside aptrgetter on the host path and in an optimized caller, a close from a callback while the call is on the stack (withisRunning()asked from inside it: cold, DFG, FTL, inlined into an FTL caller, through a bound function, and from a strict callback that ends in a tail call), a hooked function and a close inside its before-hook, a closed function as a pointer argument. The fixtures stay mapped, so a missed path shows up as a call that returns a value.operationFFIWriteSlottest, the secondffiCall()test, or theisClosed()test intryConvertCallToCallFFIremoved, the new test fails at the matching case. (Without the last one a caller compiled after the close converts toCallFFIevery time and the plan is thrown away at the end.) The same forisRunning(): without the optimized-frame lookup, without the VM-entry condition, with the inlinee's code block, without the weak references, or without the constants, the test fails at the matching case. So it does when the VM entry is taken fromcallerIsEntryFrame()of the frame visited before.JSTests/stress/ffi-*.jspass. The new test also passes with--useFTLJIT=0,--useDFGJIT=0,--useFFIICStub=0,--forceICFailure=1,--useLLInt=0, eager thresholds,--validateGraphAtEachPhase=1,--collectContinuously=1,--validateExceptionChecks=1and the concurrent JIT on.run-jsc-stress-testson Linux x64 and arm64.int32and calls a callback that a new fixture stored before (ffi_store_cb_i32,ffi_call_stored_cb_i32): a pointer argument has a slow conversion that refers to the function weakly, which hid the strongly frozen case.TypeError. The new case intest/js/bun/ffi/ffi.test.jspasses, and it fails on 1.4.3-canary with exit code 139. A library that a callback closes inside one of its calls finishes the call, and with theisRunning()answer ignored the call returns to an unmapped address (AddressSanitizer: SEGV), cold and warm. Windows and macOS are not tested locally.#713 edits the same
ffiCallloop and addsoperationFFIWriteSlotAndRetakeSnapshotsnext tooperationFFIWriteSlot. The second of the two to land needs a rebase, and the new operation needs the same closed test.Rebased onto f20ce77 (the upstream upgrade). The one conflict was
utf8().data()toutf8().legacyCStringPointer()ingenerateICStubCode.