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…can run JS Marshalling one FFI argument list can run JS in the middle of it: an integer or a float argument coerces through valueOf or Symbol.toPrimitive, and an object passed for "ptr" has its "ptr" property read. ffiCall wrote the argument slots left to right, so the address of a TypedArray and a "buffer_length" were read before that JS ran. The JS can detach, transfer, or resize the buffer, and then the native call receives an address and a length for memory that is gone: a write through a freed pointer, or a silent overwrite of an unrelated live typed array. ffiCall now writes the slots in two passes. The conversions that can run JS go first, in argument order. The reads off a live buffer follow. The DFG and the FTL emit the conversions in argument order as well, and each conversion has its own slow path, so an inline snapshot is stale as soon as a later conversion runs JS. Both tiers now park a JSValue copy of every argument that can take a snapshot above the slot buffer, and route such a conversion's slow path to operationFFIWriteSlotAndRetakeSnapshots. That operation writes its own slot and then reads the earlier addresses and lengths again from the parked copies. It reads a parked argument again only while that argument is still a buffer, so a "ptr" property getter never runs twice. An argument list that runs no JS after a buffer read parks nothing and keeps the plain slow path. The IC stub needs no change. Every argument that needs real conversion already goes to ffiCall, which rewrites the whole slot buffer. The exception order does not change: a read off a live buffer throws nothing, so the first pass still decides which argument's TypeError wins. * JSTests/stress/ffi-argument-buffer-snapshot.js: Added. Cold, hot and FTL-compiled callers must all answer with the address and the length the buffer has after the argument list has run. * Source/JavaScriptCore/dfg/DFGByteCodeParser.cpp: * Source/JavaScriptCore/ffi/FFIDFG.cpp: * Source/JavaScriptCore/ffi/FFIDFG.h: (JSC::FFI::conversionCanRunJS): (JSC::FFI::conversionCanTakeBufferSnapshot): * Source/JavaScriptCore/ffi/FFICallHost.cpp: (JSC::FFI::ffiCall): * Source/JavaScriptCore/ffi/FFIConversions.cpp: (JSC::FFI::slotIsBufferSnapshot): (JSC::operationFFIWriteSlotAndRetakeSnapshots): * Source/JavaScriptCore/ffi/FFIConversions.h: * Source/JavaScriptCore/ffi/FFIDFGCodegen.cpp: (JSC::DFG::SpeculativeJIT::compileCallFFI): * Source/JavaScriptCore/ffi/FFISignature.h: (JSC::FFI::argumentValueOffset): (JSC::FFI::Signature::marshalBufferBytes const): * Source/JavaScriptCore/ftl/FTLLowerDFGToB3.cpp:
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Included review availability: Your plan provides up to 5 included reviews per hour; 1 remains after this review. WalkthroughChangesThe FFI conversion paths defer buffer-backed pointer reads until JavaScript coercions complete. DFG and FTL compilation park required values and refresh snapshots before calls. A stress test covers buffer mutation across execution tiers. FFI argument buffer snapshots
Priority: ⬇️ Low Merge Risk: ⚪ Minimal · up to This change refreshes FFI buffer addresses and lengths after JavaScript-capable conversions, preventing stale pointer and length marshalling during buffer mutation. No current merge-blocking risk remains. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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In `@JSTests/stress/ffi-argument-buffer-snapshot.js`:
- Line 124: Update the count assertion in the FFI snapshot stress test to
require a separate increase in ftlCallFFI, rather than allowing dfgCallFFI
growth to satisfy the combined check; preserve any independent DFG validation
and ensure the hot caller reaches FTL when the test configuration supports it.
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JSTests/stress/ffi-argument-buffer-snapshot.jsSource/JavaScriptCore/dfg/DFGByteCodeParser.cppSource/JavaScriptCore/ffi/FFICallHost.cppSource/JavaScriptCore/ffi/FFIConversions.cppSource/JavaScriptCore/ffi/FFIConversions.hSource/JavaScriptCore/ffi/FFIDFG.cppSource/JavaScriptCore/ffi/FFIDFG.hSource/JavaScriptCore/ffi/FFIDFGCodegen.cppSource/JavaScriptCore/ffi/FFISignature.hSource/JavaScriptCore/ftl/FTLLowerDFGToB3.cpp
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| for (unsigned i = 0; i < index; ++i) { | ||
| FFI::Type type = signature->argumentType(i); | ||
| JSValue parked = JSValue::decode(parkedValues[i]); | ||
| if (!FFI::slotIsBufferSnapshot(type, parked)) | ||
| continue; | ||
| FFI::writeSlotFromJSValue(globalObject, context, type, parked, slots[i], &context.arena()); | ||
| OPERATION_RETURN_IF_EXCEPTION(scope); | ||
| } |
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🔴 Hot FFI callers can crash or pass a garbage pointer to native code when a snapshot argument sits between a JS-running conversion and a later non-JS-running slow path. The JIT tiers park a JSValue only for arguments followed by a conversion that can run JS, but operationFFIWriteSlotAndRetakeSnapshots (FFIConversions.cpp:569) walks every earlier argument and, for any Pointer/CString/Function/Buffer/BufferLength type, decodes the parked word whether or not it was stored. Fix: only retake from arguments the tier actually parked, e.g. set retakeSnapshots only when conversionCanRunJS(type, useKind) is true at FFIDFGCodegen.cpp:359 and FTLLowerDFGToB3.cpp:15454 (then every earlier snapshot-type argument is guaranteed parked), or pass the parked mask into the operation.
Why this was flagged
Signature (ptr, i32, buffer, buffer_length) called hot with [view, {valueOf(){...}}, view2, view3]. The backward scan in FFIDFGCodegen.cpp:107-115 and FTLLowerDFGToB3.cpp:15367-15375 parks only argument 0 (bit0): argument 2 (buffer) is not followed by any conversion that can run JS, so its parked word at argumentValueOffset(slotCount, 2) is never written and holds whatever the outgoing parameter area / locked stack slot had before. Argument 3 (buffer_length) always takes the slow path (FFIDFGCodegen.cpp:335, FTLLowerDFGToB3.cpp:15470-15472); retakeSnapshots = parkedValues & 0b111 is nonzero, so the slow path calls operationFFIWriteSlotAndRetakeSnapshots with index 3. Its loop at FFIConversions.cpp:569-576 reaches i = 2, type Buffer, and does JSValue::decode(parkedValues[2]) on uninitialized stack memory; slotIsBufferSnapshot (FFIConversions.cpp:448-451) treats any pointer-shaped word as a cell and calls dynamicDowncast on it, reading through a stale or invalid pointer (segfault), and if the garbage happens to look like a view it rewrites slots[2] with a bogus vector that the native…
Verification: normal — triggered whenever a DFG/FTL-compiled CallFFI has a signature where a JS-capable UntypedUse conversion precedes a snapshot-type argument that is itself not followed by a JS-capable conversion, and a later conversion takes the slow path (e.g. args ["ptr","ptr","buffer","buffer_length"] or ["ptr","i32"(object-profiled),"buffer","buffer_length"]; buffer_length ALWAYS takes…
| bool slotIsBufferSnapshot(Type type, JSValue value) | ||
| { | ||
| switch (type) { | ||
| case Type::Pointer: | ||
| case Type::CString: | ||
| case Type::Function: | ||
| case Type::Buffer: | ||
| case Type::BufferLength: | ||
| break; | ||
| default: | ||
| return false; | ||
| } | ||
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| if (!value.isCell()) | ||
| return false; | ||
| JSCell* cell = value.asCell(); | ||
| return dynamicDowncast<JSArrayBufferView>(cell) || dynamicDowncast<JSArrayBuffer>(cell); |
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🟡 (optional) Callers passing a plain ArrayBuffer for a 'buffer' or 'buffer_length' argument now see later arguments coerced (valueOf / Symbol.toPrimitive / 'ptr' getters run) before the TypeError, and a later argument's error can win instead. slotIsBufferSnapshot at FFIConversions.cpp:451 counts a JSArrayBuffer as a snapshot for Type::Buffer and Type::BufferLength, but writePointerSlot and writeBufferLengthSlot reject an ArrayBuffer for those types, so ffiCall defers a conversion that throws to the second pass. The JIT slow paths still throw in argument order, so the host path and the tiers disagree. …
Why this was flagged
…Fix: return true for Buffer and BufferLength only when the value is a JSArrayBufferView, so a deferred write can never throw and argument-order exceptions stay identical across the host path, the IC stub, the DFG and the FTL.
An FFI function with args ["buffer", "i32"] is called as fn(new ArrayBuffer(8), { valueOf() { sideEffect(); return 1; } }) through the C++ host path (ffiCall, Source/JavaScriptCore/ffi/FFICallHost.cpp:72). slotIsBufferSnapshot (FFIConversions.cpp:435-451) returns true for Type::Buffer with a JSArrayBuffer cell, so the first pass skips argument 0 and converts argument 1, running valueOf. The second pass then calls writeSlotFromJSValue for argument 0, and writePointerSlot (FFIConversions.cpp:313) throws "bun:ffi 'buffer' argument must be a TypedArray or DataView"; writeBufferLengthSlot (FFIConversions.cpp:305) does the same for 'buffer_length'. On the base branch the slots were written left to right, so the TypeError was thrown before valueOf ran; with a throwing second argument (a Symbol), the base threw the buffer error and the new code throws the i32…
Verification: nit — triggered when a caller passes a plain ArrayBuffer (not a view) for a 'buffer' or 'buffer_length' argument together with a later argument whose conversion runs JS, through the host path (ffiCall). Mechanism verified: slotIsBufferSnapshot (/home/claude/webkit/Source/JavaScriptCore/ffi/FFIConversions.cpp:435-451) returns true for Type::Buffer/Type::BufferLength when the cell is a… | nit.…
| @@ -0,0 +1,148 @@ | |||
| //@ requireOptions("--useDollarVM=1", "--useConcurrentJIT=0", "--jitPolicyScale=0", "--useExecutableAllocationFuzz=false") | |||
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🟡 nit (optional): the count in JSTests/BUN-TEST-DIFFERENCES.md:122 ("32 × stress/ffi-*.js" with --useExecutableAllocationFuzz=false) is now stale; this new test is the 33rd file with that directive. Fix: bump the count in that table row when adding the test, as CLAUDE.md asks that file to track every test carrying a fork-specific directive.
Why this was flagged
The new test's first line adds --useExecutableAllocationFuzz=false to requireOptions, the same opt-out that JSTests/BUN-TEST-DIFFERENCES.md:122 records for 32 files. A grep for the option across JSTests/stress/ffi-*.js now finds 33 files, so a maintainer following the root CLAUDE.md procedure for sorting a failure will find the table row undercounts. No runtime effect; documentation drift only.
Verification: nit. Triggering condition: whenever anyone reads the table row after this PR merges. Mechanism verified: /home/claude/webkit/JSTests/stress/ffi-argument-buffer-snapshot.js:1 is //@ requireOptions("--useDollarVM=1", "--useConcurrentJIT=0", "--jitPolicyScale=0", "--useExecutableAllocationFuzz=false"), and /home/claude/webkit/JSTests/BUN-TEST-DIFFERENCES.md:122 still reads `| 32 ×… | nit —…
| for (unsigned i = 0; snapshots; ++i) { | ||
| if (!(snapshots & (1u << i))) | ||
| continue; | ||
| snapshots &= ~(1u << i); | ||
| writeSlotFromJSValue(globalObject, context, signature.argumentType(i), callFrame->argument(i), slots[i], &context.stringArena()); | ||
| RETURN_IF_EXCEPTION(scope, { }); | ||
| } | ||
| slots[argumentCount] = 0; |
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🟣 pre-existing, not blocking: Pre-existing, conditional: if a Bun before-hook runs JS that detaches or resizes a buffer, native code still gets the stale address this fix was meant to remove. ffiCall now takes every buffer snapshot in its second pass (FFICallHost.cpp:79-85), but hooks->before at FFICallHost.cpp:98 runs after that pass and is allowed to run JS (its exception is caught at :101, and the in-tree test hook reads owner.hookLog through a getter). Fix: for a hooked function, take the snapshot pass after hooks->before returns, or re-take the snapshots there, so no JS can run between a buffer read and the native call, while keeping the before/after bracketing and token round trip unchanged. [also at: Source/JavaScriptCore/ffi/FFICallHost.cpp:85 - Pre-existing, conditional: an FFI function created with CallHooks whose before hook runs JS still hands native code a stale buffer address or length.]
A small fix can ride a push you are already making; otherwise a short reply is enough.
Why this was flagged
A JSFFIFunction created with non-null CallHooks is host-path-only (JSFFIFunction.h:77), so every call goes through ffiCall in FFICallHost.cpp. The two passes at FFICallHost.cpp:69-85 write every slot, including view->vector() and byteLength() for 'ptr', 'buffer' and 'buffer_length' arguments. Only then does hooks->before(globalObject, callFrame) run at FFICallHost.cpp:98, with the native call at :100 following it. The hook type (JSFFIFunction.h:42-45) takes a JSGlobalObject and a CallFrame and its exception is checked at :99-103, so hooks are permitted to run JS; the only in-tree implementation, dollarVMTestHookBefore in JSDollarVM.cpp:5203-5215, does owner->get("hookLog") and JSArray::push, which run a user getter. If such a hook (or a Bun hook outside this checkout) transfers, detaches or resizes a buffer that was passed for a pointer or length argument, slots still holds the old address and length and the thunk at :100 hands them to C, the same freed-memory write the PR describes. The base branch…
Verification: pre-existing (the base already takes every slot before hooks->before by the same route; this change fixes the conversion-ordering hazard on the non-hooked paths and does not reach the hook path). Trigger: a JSFFIFunction created with non-null CallHooks whose before-hook runs JS that detaches, transfers or resizes a buffer already marshalled for a… | pre-existing; triggered when a JSFFIFunction…
…d a nested FFI call The cstring case runs the retaking slow path together with the string arena bracket that the DFG and the FTL emit for an untyped cstring argument. The 32-argument case uses the largest parked area a signature can need. The nested case makes a whole FFI call inside a ptr getter and checks that the outer call still marshals its own live view and the getter's value. * JSTests/stress/ffi-argument-buffer-snapshot.js:
500 calls already take every case through the DFG and the FTL with --jitPolicyScale=0, so 1000 keeps a 2x margin and makes the file about three times faster in a debug build. * JSTests/stress/ffi-argument-buffer-snapshot.js:
…llFFI node The nested case made FFI calls in its setUp and in its ptr getter. Both were compiled with a CallFFI node of their own while the case ran, so the compile count grew even if the hot caller had stayed on the host path. Those calls now go through two helpers that are never inlined and are compiled in every tier before the first case reads the counts. Each case now adds exactly one DFG and one FTL CallFFI node. The header comment no longer mentions a caller pinned below the DFG: the file has no such pass. * JSTests/stress/ffi-argument-buffer-snapshot.js:
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Code review found no new issues
No new issues were found in this update; 4 findings from earlier reviews are still open above.
Still open from earlier reviews (4):
- 🔴
Source/JavaScriptCore/ffi/FFIConversions.cpp:576—Hot FFI callers can crash or pass a garbage pointer to native code when a snapshot argument sits between a JS-running c… - Also unresolved: 3 minor or pre-existing.
If you have decided not to act on one of these findings, resolve its thread (a reply alone leaves it open) and the next review stops counting it. To review this commit again now, use Re-run on its "Claude Code Review" check.
Problem
TypedArrayand abuffer_lengthbefore it coerces the other arguments.ffiCallwrites the slots left to right (Source/JavaScriptCore/ffi/FFICallHost.cpp:62), and a conversion in that loop can run JS: an integer or a float argument coerces throughvalueOforSymbol.toPrimitive, and an object passed forptrhas itsptrproperty read.bun:ffiand libcmemsetthat overwrites an unrelated liveUint8Array(10 of 10 runs), and with the documentedbufferplusbuffer_lengthpair it writes 1 MiB into a 16-byte buffer (SEGV ... WRITE memory access, 10 of 10 runs).Fix
ffiCallwrites the slots in two passes. The conversions that can run JS go first, in argument order. The reads off a live buffer follow, so they cannot be stale.operationFFIWriteSlotAndRetakeSnapshots. That operation writes its own slot and then reads the earlier addresses and lengths again. It reads a parked argument again only while that argument is still a buffer, so aptrproperty getter never runs twice.ffiCall, which rewrites the whole slot buffer.fill(view, view),add(1, 2)) pay nothing.JSTests/stress/ffi-argument-buffer-snapshot.jsfails before this change (got 123634705809728 (number), expected null) for the cold and the hot caller, and passes after it. It asserts with$vm.ffiCompileCounts()that the DFG and the FTL compiled each case, and it also coverscstring(the string arena bracket), a 32-argument signature and a nested FFI call inside aptrgetter. All otherJSTests/stress/ffi-*.jspass, and so does bun'stest/js/bun/ffiagainst a local build.Background
writeSlotFromJSValuefills one slot (Source/JavaScriptCore/ffi/FFIConversions.cpp).ptr,bufferandbuffer_length, that write readsview->vector()orview->byteLength()off the live object. The value holds only until the next JS runs, which is why this PR calls it a snapshot.Signature::marshalBufferBytes()is the slot buffer plus the parked values. The DFG keeps both in the outgoing parameter area, and the FTL keeps them in one locked stack slot.TypeErrorwins.The bun side (the
WEBKIT_VERSIONmove and abun:ffiregression test) is oven-sh/bun#43740.