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Pass NULL walkState to jvmti callback for JNI local reference on stack #18394

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Nov 3, 2023
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Pass NULL walkState to jvmti callback for JNI local reference on stack #18394

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2 changes: 1 addition & 1 deletion runtime/gc_base/ReferenceChainWalker.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -646,7 +646,7 @@ MM_ReferenceChainWalker::doStackSlot(J9Object **slotPtr, void *walkState, const
/* Only report heap objects */
if (isHeapObject(slotValue) && !_heap->objectIsInGap(slotValue)) {
if (J9_STACKWALK_SLOT_TYPE_JNI_LOCAL == ((J9StackWalkState *)walkState)->slotType) {
doSlot(slotPtr, J9GC_ROOT_TYPE_JNI_LOCAL, -1, (J9Object *)walkState);
doSlot(slotPtr, J9GC_ROOT_TYPE_JNI_LOCAL, -1, NULL);
} else {
doSlot(slotPtr, J9GC_ROOT_TYPE_STACK_SLOT, -1, (J9Object *)walkState);
Comment on lines +649 to 651
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I thought the cast was suspicious when I saw #18379, NULL seems more reasonable, but why should the call on line 651 not also pass NULL?

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@amicic amicic Nov 3, 2023 •
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A valid question. In theory, callbacks being aware that the type is STACK_SLOT could safely cast the referrer parameter back to walkState type and do something about that info/structure (print stack slot address etc), but I don't know if they do.

I'm ok with passing NULL, but don't have the whole picture (history and awareness of current callbacks) to say it's really safe.

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If we pass NULL for STACK_SLOT, it will break the expectation in jvmtiHeap10.c::processStackRoot() function.

}
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