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…, not at the block that owns the inline cache The DFG and the FTL call a native custom accessor with the realm of the code origin. GetByStatus and PutByStatus keep a CustomAccessor status only when that is the realm of the object that holds the accessor, which is the realm every other path passes. The guard compared with profiledBlock->globalObject(), and profiledBlock can be the optimized CodeBlock of a function that inlined the code origin, in another realm. Compare with profiledBlock->globalObjectFor(codeOrigin). Under ASSERT_ENABLED the variants keep the holder realm the guard computed, and the three sites that emit a direct call assert it is the realm of the node's semantic origin. $vm's JSTestCustomGetterSetter gains a DOMAttribute accessor with no DOMJIT snippet, the shape CallDOMGetter serves.
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Included review availability: This review used your included allowance. Your plan provides up to 5 included reviews per hour; 1 remain after this review. WalkthroughCustom accessor eligibility checks now compare accessor realms with the global object for the code origin. Assertion-enabled builds record realm information and check it during assignments, merges, and DFG parsing. A stress test covers accessor reads and writes across realms. ChangesCustom accessor realm handling
Priority: ➖ Normal Merge Risk: ⚪ Minimal · up to The change aims to align custom-accessor realm checks with the inlined code origin and adds cross-realm stress coverage. The supplied evidence establishes no specific user-facing regression, so no concrete merge-blocking risk is identified. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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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 on the test's iteration counts, I also checked the two other ways a CustomAccessor status reaches the DFG: the baseline-block path (GetByStatus::computeFor at GetByStatus.cpp:205 and PutByStatus.cpp:154) passes either the full origin or a bare bytecode index, and CodeBlock::globalObjectFor resolves both to the baseline block's own realm, so that path was already correct; and the recorded-status replay at GetByStatus.cpp:498 / PutByStatus.cpp:364, where ICStatusContext matching via CodeOrigin::isApproximatelyEqualTo compares the baseline CodeBlock at every inline level, so a replayed status cannot come from an origin in a different realm.
Extended reasoning...
The change narrows the realm guard in GetByStatus/PutByStatus from profiledBlock->globalObject() to profiledBlock->globalObjectFor(codeOrigin), adds ASSERT-only realm fields to GetByVariant/PutByVariant with release-size static_asserts, adds three ASSERTs at the DFG emission sites, a $vm test accessor, and a stress test. It touches no auth or injection surface, but it is JIT correctness code for cross-realm native accessor calls where the previous bug caused a segfault in an embedder. One finding is being posted inline on the test's hard-coded iteration counts and two further verified findings are unposted, so approval is not appropriate; the note records the two other status-sourcing paths examined and found sound.
Every case now loops testLoopCount times and the file requires --useConcurrentJIT=false, so each tier-up happens inside that count. It takes 12 to 110 ms of CPU per configuration and still fails in 13 of the 17 defaultRun configurations without the fix.
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Actionable comments posted: 1
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Review comments at
@JSTests/stress/custom-accessor-realm-of-inlined-code-origin.js:
- Line 103: Update the reversed-realm case around createCustomTestGetterSetter
so the property read runs in a host-defined callback that accesses object[name]
directly, rather than delegating to guest.read[name].
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JSTests/stress/custom-accessor-realm-of-inlined-code-origin.js
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I reviewed the updated PR (including the second commit that moves the stress test onto testLoopCount) and found no bugs; since this changes what the DFG/FTL are allowed to compile for native accessor calls across realms, and JSC paths are CODEOWNER-covered, a human look is still worthwhile.
What was reviewed:
- Checked the new guard against CodeBlock::globalObjectFor(): with an inlined origin it reads the inline frame's baseline block realm, and with a non-inlined origin profiledBlock is always the same function as the origin, so the baseline-block callers (CodeOrigin(bytecodeIndex)) keep their old behavior and only the ICStatusContext walk changes.
- Confirmed the structure-based GetByStatus/PutByStatus::computeFor overloads bail on CustomAccessorOrValue, so the IC paths are the only producers of CustomAccessor variants, and that DFGSpeculativeJIT/FTL already pass the node-origin realm the guard now matches.
- Worked through the member layouts: GetByVariant is 64 and PutByVariant is 72 bytes on 64-bit without assertions, so the static_asserts hold and the ASSERT-only field and ASSERT() uses compile away in release.
- The new test now runs ~130k accessor calls total under testLoopCount with the concurrent JIT off; with default thresholds that is still above the ~4k iterations the description reports for the first failure.
Extended reasoning...
The change touches GetByStatus/PutByStatus (the realm guard that decides whether the DFG/FTL may call a native custom accessor directly), adds ASSERT-only realm tracking to GetByVariant/PutByVariant with release-size static_asserts, adds three ASSERTs in DFGByteCodeParser, a $vm DOMAttribute test accessor, and a new stress test. The security-relevant surface is realm confusion for host-object accessors (the described embedder symptom was a segfault and wrong-realm objects), which the fix narrows rather than widens. Deciding against approve: Source/JavaScriptCore and JSTests are covered by CODEOWNERS (jsc-reviewers), and the change alters optimizer admission logic in the DFG/FTL tier rather than being mechanical. The second commit plausibly addressed the only prior finding (test iteration budget), and no new bugs were found this run.
…realm The callback passed to the guest delegated the read to a guest function, so the code origin and the holder were in the same realm and the case could not fail. The reader is now a function of the host realm that a guest function inlines. Without the fix its first wrong iteration is 4099.
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Problem
runtime/PropertySlot.cpp:39,bytecode/InlineCacheCompiler.cpp:3319).bytecode/GetByStatus.cpp:359, twinPutByStatus.cpp:268) allows the direct call only when the two realms are the same. It compared withprofiledBlock->globalObject(), andprofiledBlockcan be the optimized block of a function that inlined the code origin from another realm.node:vmfunction that readsreq.signalthen crashes:Segmentation fault at address 0x18.Fix
profiledBlock->globalObjectFor(codeOrigin), the realm the code generators pass.static_asserts pin the release size of both variants.JSTests/stress/custom-accessor-realm-of-inlined-code-origin.jsfails in 13 of the 17defaultRunconfigurations without the change and passes in all 17 with it.Background
GetByStatusturns the inline caches of a bytecode site into what the DFG compiles from. It also reads the caches of optimized code blocks, so an outer function's block can answer for an inlined access.Downsides
Notes
The mechanism.
GetByStatus::computeFor(profiledBlock, baselineMap, icContextStack, codeOrigin)walks oneICStatusContextper inline level (dfg/DFGByteCodeParser.cpp:11654). When the optimized block of an outer function has an inline cache at the inlined origin,GetByStatus.cpp:489andPutByStatus.cpp:356hand that block to the helper asprofiledBlock. Its realm is the outer function's. The code generators passGraph::globalObjectFor(origin), the realm of the inlined function (dfg/DFGSpeculativeJIT.cpp:11016,ftl/FTLLowerDFGToB3.cpp:15706,:15722,:22235). The baseline path was already right:GetByStatus.cpp:205andPutByStatus.cpp:154pass the inlinee's own block, andglobalObjectFor()returns that block's realm for both.Recorded statuses need no check of their own.
ICStatusContext::getmatches throughCodeOrigin::isApproximatelyEqualTo(bytecode/CodeOrigin.cpp:45), which compares the baseline code block at every inline level. A replayed status belongs to a code origin with the same realm.The test accessor.
$vm'sJSTestCustomGetterSettergainscustomDOMAttributeGlobalObject: a custom accessor with aDOMAttributeAnnotationand no DOMJIT snippet. That is the shape generated bindings use, and the DFG and the FTL call it throughCallDOMGetter. No JSTests case covered it. It isDontEnum, so the tests that spread or assign aJSTestCustomGetterSettersee what they saw before.Which tier. With one level of inlining the wrong call needs the FTL: the status has to come from the inline cache in the outer function's DFG code. With two levels the DFG alone does it, because the middle function's own optimized block holds the cache. The test file has both, plus the setter, a prototype-chain holder, and the reversed direction (a guest-realm holder read by a function of the host realm that a guest function inlines), each for the plain accessor and the DOMAttribute one.
The test. It requires
--useConcurrentJIT=falseand loopstestLoopCounttimes. Without the change the first wrong iteration is 4,099 or 4,073 in the FTL configurations, 984 to 1,493 in the DFG-only ones, 4 or 32 in the eager ones. With the concurrent JIT on it moved between 21,000 and 755,000 on a loaded machine, pasttestLoopCount. The file takes 12 to 110 ms of CPU per configuration. The 4 configurations that pass without the change run no JIT (lockdown), do no inlining (ftl-no-cjit-no-inline-validate), give the JIT a 50 KB pool (ftl-no-cjit-small-pool), or run the DFG alone with the LLInt inline caches off (no-cjit-validate-phases).getter-setter-globalobject-in-ic.js,getter-setter-globalobject-in-ic-2.js,runString-returns-globalThis-not-globalObject.jsandcustom-get-set-proto-chain-put.jspass in all 17 both ways.Same-realm reads. Code sizes are unchanged (Baseline 624, DFG 1360, FTL 416 bytes), the disassembly is identical after addresses are normalized, and the graph keeps
CallCustomAccessorGetterandCallDOMGetter, 2 of each before and after.Size. +352 bytes of text at the two guard sites, +672 with the
$vmaccessor (JSCOnly Release, -O3, assertions off).bin/jscof the linux-amd64-lto release grows by 1,056 bytes.sizeof(GetByVariant)andsizeof(PutByVariant)stay 64 and 72 bytes in a release build. Not measured: the instructions one more load costs the compiler thread per custom-accessor status (perf,valgrindandbloatyare not installed where I measured).Elsewhere. Upstream
WebKit/WebKitmain is byte-identical at both guards and both walkers.CallDOM, the node for DOMJIT function calls (dfg/DFGByteCodeParser.cpp:2422), passes the realm of the code origin with no guard at all; it is not changed here. The other five copies of the walker (DeleteByStatus,InByStatus,CheckPrivateBrandStatus,SetPrivateBrandStatus,CallLinkStatus) also takecontext->optimizedCodeBlockwith an inlined origin, but none passes a realm to a native function. The code generators still re-derive argument 0 from the code origin, whichwebkit.org/b/203204is about; this change makes the guard they rely on hold.In Bun. oven-sh/bun#44518 pins the preview build and adds tests for
StringDecoder#lastChar,Request#signal,Response#headers,import.meta.env, thebuffer.INSPECT_MAX_BYTESsetter and aBun.servehandler that belongs to a context. All fail on bun 1.4.3 and pass on a build with this change.