[JSC] FFI: check that a pointer argument is a typed array view before loading its mode in FTL - #477
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… loading its mode in FTL The FTL lowering of CallFFI converts pointer-family arguments (ptr, buffer, cstring, function) inline when the value is a typed array view. For a cell argument it loaded both the cell's JSType and JSArrayBufferView::m_mode and folded the "is a view" and "is not resizable/growable-shared" tests into a single branch, so m_mode (offset 0x28) was read from every cell argument, including cells smaller than a JSArrayBufferView such as a JSArrayBuffer, JSString or JSBigInt. That read goes past the end of the cell; when the cell is the last one in its MarkedBlock and the page after the block is not committed, it faults. The symptom is an intermittent access violation in FTL-compiled code at <block end> + 8 in long-running processes that pass an ArrayBuffer (or another non-view cell) to an FFI pointer parameter, seen mostly on Windows where the page following a block is more often decommitted. The DFG lowering and the IC stub already test the type before touching m_mode. Do the same in FTL: branch to the slow path unless the cell is a typed array view, and only then load m_mode. Adds a stress test that feeds freshly allocated ArrayBuffers and other non-view values through an FTL-hot FFI pointer argument and compares the results against a noDFG reference.
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LGTM — straightforward split of the fused isView & isPlainMode branch so m_mode is only loaded after the cell is confirmed to be a typed array view.
What was reviewed:
- Block-chain sequencing (
viewCase → modeCase → vectorCase → slowCase → done) —appendToordering and branch targets are correct, semantics are unchanged for view arguments. - Cross-checked against
FFIDFGCodegen.cpp:211-212andFFIICStub.cpp:221— both already gate the mode/vector loads behindbranchIfNotType; FTL now matches. - Stress test warms the site with a real view before switching to non-view cells, so the inline path is exercised in FTL.
Extended reasoning...
Overview
The PR touches two files: a ~10-line change in FTLLowerDFGToB3.cpp inside the CallFFI argument lowering for pointer-family types, and a new stress test JSTests/stress/ffi-ptr-non-view-cell-arg.js. The code change introduces a new basic block modeCase and splits the previously fused branch(isView & isPlainMode) into two sequential branches: first branch(isView, modeCase, slowCase), then in modeCase load JSArrayBufferView::m_mode and branch(isPlainMode, vectorCase, slowCase). This guards the m_mode load (offset 0x28) so it only executes on cells whose JSType is in the typed-array range, preventing an out-of-bounds read on smaller cells like JSArrayBuffer, JSString, or JSBigInt.
Security risks
None introduced. The change strictly narrows the set of cells from which m_mode is read; it removes an OOB read rather than adding one. No new data flows, no auth/crypto/permission logic. The Gigacage caging of the vector pointer is unchanged.
Level of scrutiny
FTL codegen is a hot, correctness-critical path, so I verified the block-graph edit carefully. The transformation is purely structural: (A & B) ? fast : slow becomes A ? (B ? fast : slow) : slow, with the B computation moved inside the A guard. I confirmed the m_out.appendTo chain (viewCase, modeCase → modeCase, vectorCase → vectorCase, slowCase) is consistent and every predecessor block still terminates with a branch or jump. The usually/rarely hints match the original combined branch. I also confirmed the sibling implementations in ffi/FFIDFGCodegen.cpp (line ~211) and ffi/FFIICStub.cpp (line ~221) already use exactly this pattern — branchIfNotType before touching offsetOfMode()/offsetOfVector() — so this brings FTL in line with the other tiers rather than diverging from them.
Other factors
The bug hunting system found no issues. The new stress test is well-constructed: it warms the call site with a Uint8Array for 20k iterations to reach FTL with the inline view path emitted, then feeds many freshly allocated small ArrayBuffers (plus BigInt, string, plain object) through it after a gc() to maximize the chance of a cell landing at the end of a MarkedBlock, and diffs against a noDFG oracle. There are no prior reviewer comments on the PR to address. Given the small, mechanical nature of the fix and its exact correspondence to the existing DFG/IC-stub code, this is safe to approve without human review.
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WalkthroughThe FTL typed-array pointer path validates cell type before reading view mode. Native and JIT stress tests exercise optimized FFI pointer conversion with typed-array views, buffers, BigInts, objects, strings, allocation, and garbage collection. ChangesFFI pointer lowering
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…FFI call site Drives an FFI pointer-argument call site into the FTL from testFFI, then passes it cells that are not typed array views -- freshly allocated JSArrayBuffers (many of them the last cell of their MarkedBlock), a heap BigInt and an object carrying `ptr` -- and checks each result against the host-path conversion. Also asserts that the call site reached the FTL via g_ffiCompileCounts.
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Beyond the two test-file nits, I also checked the FTL block chaining — the appendTo next-block arguments are correctly rethreaded (viewCase → modeCase → vectorCase), and the split branch now matches the DFG (FFIDFGCodegen.cpp:211-212) and IC-stub orderings that already gate the m_mode load behind the type-range check.
Extended reasoning...
The inline findings are style nits in the new stress test only. The substantive change — splitting the fused isView & isPlainMode branch in FTLLowerDFGToB3.cpp so JSArrayBufferView::m_mode is loaded only after the JSType range check passes — was examined for block-ordering correctness and consistency with the other tiers. The usually/rarely weights are preserved, and the semantics for the non-view path are unchanged (still routes to slowCase → emitSlowConversion). Deferring to human review because this is FTL JIT codegen.
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Actionable comments posted: 2
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Inline comments:
In `@Source/JavaScriptCore/ffi/tests/testFFI.cpp`:
- Around line 2357-2364: Update the FTL regression test around the warm calls
and the non-view input loop to wait for and verify a signal that FTL code has
been installed or executed, rather than relying on the lowering count alone.
Ensure the ArrayBuffer, BigInt, and object cases run only after this verified
FTL state, using the existing test synchronization or validation mechanisms.
- Around line 2306-2397: Guard the Bun-specific test definition
testOptimizingTierPointerArgumentWithNonViewCells and its registration with
USE(BUN_JSC_ADDITIONS), ensuring both the function and the corresponding
test-list entry are excluded when Bun JSC additions are disabled.
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Fix all unresolved CodeRabbit comments on this PR:
- Push a commit to this branch (recommended)
- Create a new PR with the fixes
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Source/JavaScriptCore/ffi/tests/testFFI.cpp
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…on-view cells testFFI: call the site directly until the function's installed CodeBlock is FTL (checked via codeBlockForCall()->jitType()) instead of inferring it from the lowering counter, so the non-view inputs are known to run through the FTL code; re-check that after every round, and run more rounds. JSTests: use testLoopCount for the warm-up loop and don't print on success.
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I reviewed this PR and didn't find any bugs; the two earlier nits and the FTL-installation race in the C++ test have all been addressed in fc7b404. Because the core change is in FTL JIT codegen for FFI pointer arguments — a critical execution path — a human look would still be worthwhile.
What was reviewed:
- The
FTLLowerDFGToB3.cppchange: splitting the fusedisView & isPlainModebranch som_modeis only loaded after the type check — matches the existing DFG pattern inFFIDFGCodegen.cpp:211-212. - Block chaining:
viewCase → modeCase → vectorCase → slowCase → doneis threaded correctly through theappendTocontinuations; both new branches reachslowCaseon the rare edge. - The new stress test now uses
testLoopCountfor warm-up and drops the success print; the C++ test now pollscodeBlockForCall()->jitType() == JITType::FTLJITbefore and after each round.
Extended reasoning...
Overview
This PR fixes an out-of-bounds read in the FTL lowering of CallFFI pointer-family arguments. The old code loaded JSArrayBufferView::m_mode from every cell argument before checking whether the cell was actually a typed array view, then folded the two predicates into a single bitAnd branch. For cells smaller than a JSArrayBufferView (e.g. JSArrayBuffer, JSString, JSBigInt), the m_mode load reads past the end of the cell — and past the end of the MarkedBlock when the cell is the last one in it, causing intermittent access violations. The fix splits the branch: check isView first, and only load m_mode in the new modeCase block. Two regression tests are added: a JS stress test that feeds many freshly allocated ArrayBuffers through an FTL-hot call site against a noDFG oracle, and a C++ testFFI case that does the same with direct heap manipulation and verifies the call site stays in FTL.
Security risks
The bug being fixed is itself a memory-safety issue (a 1-byte OOB read that can fault). The fix removes that read. I don't see any new attack surface introduced — the change strictly narrows the set of cells from which m_mode is loaded, and the semantics for the fast/slow path split are otherwise unchanged (non-view cells already went to slowCase because isView was false in the old bitAnd; now they get there without the stray load).
Level of scrutiny
High. FTLLowerDFGToB3.cpp is the code generator for the highest JIT tier; a mistake here produces incorrect machine code that runs at full speed with no runtime checks. The change itself is small (~10 net lines) and mechanically mirrors the DFG lowering in FFIDFGCodegen.cpp (which already does branchIfNotType before branchTest8 on offsetOfMode), and the block chaining looks correct. But given the tier and the memory-safety nature of the bug, a human maintainer familiar with the FTL/B3 output builder idioms should confirm the block ordering and that B3 won't hoist the load back across the branch.
Other factors
All prior review feedback is resolved: my two nits (use testLoopCount, drop the success print) were applied in fc7b404, and CodeRabbit's concern about the C++ test racing FTL installation was addressed by polling jitType() == JITType::FTLJIT and re-asserting it every round. The bug-hunting system found nothing on the current revision. The PR has good regression coverage across both the JS stress harness and the native testFFI binary.
…ding the view mode (#39829) ### What does this PR do? Bumps WebKit to `b7f217b4a657` (oven-sh/WebKit#477). That change fixes the FTL lowering of `bun:ffi` calls for pointer-family arguments (`ptr`, `buffer`, `cstring`, `function`): when the argument was a cell, the generated code loaded `JSArrayBufferView::m_mode` before checking that the cell was a typed array view. For a smaller cell — e.g. a plain `ArrayBuffer`, a heap BigInt, or an object with a `ptr` property — that read went past the end of the cell, and when the cell was the last one in its MarkedBlock and the next page wasn't committed, it faulted. In practice this showed up as intermittent segfaults in JIT code at `<64KB-aligned address> + 8` in long-running programs that pass an `ArrayBuffer` to an FFI pointer parameter once the caller tiered up to FTL, mostly on Windows. `BUN_JSC_useFTLJIT=0` avoided it. Nothing else changed between the previous pin (`0f966e81b78c`) and this one. ### How did you verify your code works? - `test/js/bun/ffi/ffi.test.js`: new subprocess test that FTL-warms an FFI `ptr` call site and then feeds it many freshly allocated `ArrayBuffer`s; on Windows with the previous WebKit it segfaults reliably, with this one it passes. - `test/js/bun/jsc-stress/fixtures/ffi/ffi-ptr-non-view-cell-arg.js`: the JSTests stress test added in oven-sh/WebKit#477, wired into `jsc-stress.test.ts` (runs where `$vm` is available). Crashes on the previous debug prebuilt, passes on this one. - The WebKit PR also adds a `testFFI` case for the same path; `test/js/bun/jsc-stress/testFFI.test.ts` picks it up from the shipped binary.
…f217b4 The previous preview was built on 0f966e81 and predates oven-sh/WebKit#477, which the ffi.test.js case from #39829 needs on Windows. This preview is b7f217b4 (what main pins) plus the #472 change.
…f217b4 The previous preview was built on 0f966e81 and predates oven-sh/WebKit#477, which the ffi.test.js case from #39829 needs on Windows. This preview is b7f217b4 (what main pins) plus the #472 change.
…dlerJob fix The previous preview (e3c620df) sat one WebKit commit behind main's pin, so it lacked the FTL FFI fix from oven-sh/WebKit#477 and ffi.test.js crashed on both Windows lanes. The new preview (33161987) is oven-sh/WebKit#268 on top of current WebKit main. oven-sh/WebKit#268 gained a second commit. A promise that adopts another promise's settlement without a handler of its own (p.then(f) with p rejecting, resolve(otherPromise), an async function returning a promise) is settled by PromiseResolveWithoutHandlerJob, and JSC dropped the context then() had captured on every path that queues that job. Those derived promises are the rejections Next.js 16 filters by reading its prerender store from the unhandledRejection listener (#39847); with the bun-side change alone that app still logged two rejections per request, with this preview it logs none, the same as node. async-context-unhandled-rejection-then-passthrough.js covers the four shapes on bun and node. All four observe a null store on the previous preview.
…f217b4 The previous preview was built on 0f966e81 and predates oven-sh/WebKit#477, which the ffi.test.js case from #39829 needs on Windows. This preview is b7f217b4 (what main pins) plus the #472 change.
…dlerJob fix The previous preview (e3c620df) sat one WebKit commit behind main's pin, so it lacked the FTL FFI fix from oven-sh/WebKit#477 and ffi.test.js crashed on both Windows lanes. The new preview (33161987) is oven-sh/WebKit#268 on top of current WebKit main. oven-sh/WebKit#268 gained a second commit. A promise that adopts another promise's settlement without a handler of its own (p.then(f) with p rejecting, resolve(otherPromise), an async function returning a promise) is settled by PromiseResolveWithoutHandlerJob, and JSC dropped the context then() had captured on every path that queues that job. Those derived promises are the rejections Next.js 16 filters by reading its prerender store from the unhandledRejection listener (#39847); with the bun-side change alone that app still logged two rejections per request, with this preview it logs none, the same as node. async-context-unhandled-rejection-then-passthrough.js covers the four shapes on bun and node. All four observe a null store on the previous preview.
…dlerJob fix The previous preview (e3c620df) sat one WebKit commit behind main's pin, so it lacked the FTL FFI fix from oven-sh/WebKit#477 and ffi.test.js crashed on both Windows lanes. The new preview (33161987) is oven-sh/WebKit#268 on top of current WebKit main. oven-sh/WebKit#268 gained a second commit. A promise that adopts another promise's settlement without a handler of its own (p.then(f) with p rejecting, resolve(otherPromise), an async function returning a promise) is settled by PromiseResolveWithoutHandlerJob, and JSC dropped the context then() had captured on every path that queues that job. Those derived promises are the rejections Next.js 16 filters by reading its prerender store from the unhandledRejection listener (#39847); with the bun-side change alone that app still logged two rejections per request, with this preview it logs none, the same as node. async-context-unhandled-rejection-then-passthrough.js covers the four shapes on bun and node. All four observe a null store on the previous preview.
…dlerJob fix The previous preview (e3c620df) sat one WebKit commit behind main's pin, so it lacked the FTL FFI fix from oven-sh/WebKit#477 and ffi.test.js crashed on both Windows lanes. The new preview (33161987) is oven-sh/WebKit#268 on top of current WebKit main. oven-sh/WebKit#268 gained a second commit. A promise that adopts another promise's settlement without a handler of its own (p.then(f) with p rejecting, resolve(otherPromise), an async function returning a promise) is settled by PromiseResolveWithoutHandlerJob, and JSC dropped the context then() had captured on every path that queues that job. Those derived promises are the rejections Next.js 16 filters by reading its prerender store from the unhandledRejection listener (#39847); with the bun-side change alone that app still logged two rejections per request, with this preview it logs none, the same as node. async-context-unhandled-rejection-then-passthrough.js covers the four shapes on bun and node. All four observe a null store on the previous preview.
…dlerJob fix The previous preview (e3c620df) sat one WebKit commit behind main's pin, so it lacked the FTL FFI fix from oven-sh/WebKit#477 and ffi.test.js crashed on both Windows lanes. The new preview (33161987) is oven-sh/WebKit#268 on top of current WebKit main. oven-sh/WebKit#268 gained a second commit. A promise that adopts another promise's settlement without a handler of its own (p.then(f) with p rejecting, resolve(otherPromise), an async function returning a promise) is settled by PromiseResolveWithoutHandlerJob, and JSC dropped the context then() had captured on every path that queues that job. Those derived promises are the rejections Next.js 16 filters by reading its prerender store from the unhandledRejection listener (#39847); with the bun-side change alone that app still logged two rejections per request, with this preview it logs none, the same as node. async-context-unhandled-rejection-then-passthrough.js covers the four shapes on bun and node. All four observe a null store on the previous preview.
…dlerJob fix The previous preview (e3c620df) sat one WebKit commit behind main's pin, so it lacked the FTL FFI fix from oven-sh/WebKit#477 and ffi.test.js crashed on both Windows lanes. The new preview (33161987) is oven-sh/WebKit#268 on top of current WebKit main. oven-sh/WebKit#268 gained a second commit. A promise that adopts another promise's settlement without a handler of its own (p.then(f) with p rejecting, resolve(otherPromise), an async function returning a promise) is settled by PromiseResolveWithoutHandlerJob, and JSC dropped the context then() had captured on every path that queues that job. Those derived promises are the rejections Next.js 16 filters by reading its prerender store from the unhandledRejection listener (#39847); with the bun-side change alone that app still logged two rejections per request, with this preview it logs none, the same as node. async-context-unhandled-rejection-then-passthrough.js covers the four shapes on bun and node. All four observe a null store on the previous preview.
…dlerJob fix The previous preview (e3c620df) sat one WebKit commit behind main's pin, so it lacked the FTL FFI fix from oven-sh/WebKit#477 and ffi.test.js crashed on both Windows lanes. The new preview (33161987) is oven-sh/WebKit#268 on top of current WebKit main. oven-sh/WebKit#268 gained a second commit. A promise that adopts another promise's settlement without a handler of its own (p.then(f) with p rejecting, resolve(otherPromise), an async function returning a promise) is settled by PromiseResolveWithoutHandlerJob, and JSC dropped the context then() had captured on every path that queues that job. Those derived promises are the rejections Next.js 16 filters by reading its prerender store from the unhandledRejection listener (#39847); with the bun-side change alone that app still logged two rejections per request, with this preview it logs none, the same as node. async-context-unhandled-rejection-then-passthrough.js covers the four shapes on bun and node. All four observe a null store on the previous preview.
…dlerJob fix The previous preview (e3c620df) sat one WebKit commit behind main's pin, so it lacked the FTL FFI fix from oven-sh/WebKit#477 and ffi.test.js crashed on both Windows lanes. The new preview (33161987) is oven-sh/WebKit#268 on top of current WebKit main. oven-sh/WebKit#268 gained a second commit. A promise that adopts another promise's settlement without a handler of its own (p.then(f) with p rejecting, resolve(otherPromise), an async function returning a promise) is settled by PromiseResolveWithoutHandlerJob, and JSC dropped the context then() had captured on every path that queues that job. Those derived promises are the rejections Next.js 16 filters by reading its prerender store from the unhandledRejection listener (#39847); with the bun-side change alone that app still logged two rejections per request, with this preview it logs none, the same as node. async-context-unhandled-rejection-then-passthrough.js covers the four shapes on bun and node. All four observe a null store on the previous preview.
…dlerJob fix The previous preview (e3c620df) sat one WebKit commit behind main's pin, so it lacked the FTL FFI fix from oven-sh/WebKit#477 and ffi.test.js crashed on both Windows lanes. The new preview (33161987) is oven-sh/WebKit#268 on top of current WebKit main. oven-sh/WebKit#268 gained a second commit. A promise that adopts another promise's settlement without a handler of its own (p.then(f) with p rejecting, resolve(otherPromise), an async function returning a promise) is settled by PromiseResolveWithoutHandlerJob, and JSC dropped the context then() had captured on every path that queues that job. Those derived promises are the rejections Next.js 16 filters by reading its prerender store from the unhandledRejection listener (#39847); with the bun-side change alone that app still logged two rejections per request, with this preview it logs none, the same as node. async-context-unhandled-rejection-then-passthrough.js covers the four shapes on bun and node. All four observe a null store on the previous preview.
…dlerJob fix The previous preview (e3c620df) sat one WebKit commit behind main's pin, so it lacked the FTL FFI fix from oven-sh/WebKit#477 and ffi.test.js crashed on both Windows lanes. The new preview (33161987) is oven-sh/WebKit#268 on top of current WebKit main. oven-sh/WebKit#268 gained a second commit. A promise that adopts another promise's settlement without a handler of its own (p.then(f) with p rejecting, resolve(otherPromise), an async function returning a promise) is settled by PromiseResolveWithoutHandlerJob, and JSC dropped the context then() had captured on every path that queues that job. Those derived promises are the rejections Next.js 16 filters by reading its prerender store from the unhandledRejection listener (#39847); with the bun-side change alone that app still logged two rejections per request, with this preview it logs none, the same as node. async-context-unhandled-rejection-then-passthrough.js covers the four shapes on bun and node. All four observe a null store on the previous preview.
…dlerJob fix The previous preview (e3c620df) sat one WebKit commit behind main's pin, so it lacked the FTL FFI fix from oven-sh/WebKit#477 and ffi.test.js crashed on both Windows lanes. The new preview (33161987) is oven-sh/WebKit#268 on top of current WebKit main. oven-sh/WebKit#268 gained a second commit. A promise that adopts another promise's settlement without a handler of its own (p.then(f) with p rejecting, resolve(otherPromise), an async function returning a promise) is settled by PromiseResolveWithoutHandlerJob, and JSC dropped the context then() had captured on every path that queues that job. Those derived promises are the rejections Next.js 16 filters by reading its prerender store from the unhandledRejection listener (#39847); with the bun-side change alone that app still logged two rejections per request, with this preview it logs none, the same as node. async-context-unhandled-rejection-then-passthrough.js covers the four shapes on bun and node. All four observe a null store on the previous preview.
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The FTL lowering of
CallFFIconverts pointer-family arguments (ptr,buffer,cstring,function) inline when the value is a typed array view. For a cell argument it loaded both the cell's JSType andJSArrayBufferView::m_modeand folded the "is a view" and "is not resizable/growable-shared" tests into a single branch:so
m_mode(offset 0x28) was read from every cell argument, including cells smaller than aJSArrayBufferViewsuch as aJSArrayBuffer,JSStringorJSBigInt. That read goes past the end of the cell; when the cell is the last one in its MarkedBlock and the page after the block is not committed, it faults. The symptom is an intermittent access violation in FTL-compiled code at<block end> + 8in long-running processes that pass anArrayBuffer(or another non-view cell) to an FFI pointer parameter, seen mostly on Windows where the page following a block is more often decommitted.The DFG lowering (
FFIDFGCodegen.cpp) and the IC stub (FFIICStub.cpp) already test the type before touchingm_mode. This does the same in FTL: branch to the slow path unless the cell is a typed array view, and only then loadm_mode.Adds
JSTests/stress/ffi-ptr-non-view-cell-arg.js, which feeds freshly allocatedArrayBuffers and other non-view values through an FTL-hot FFI pointer argument and compares the results against anoDFGreference.