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Ensure syncblock is cleared with GC_ALLOC_ZEROING_OPTIONAL #97174

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Jan 19, 2024
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Ensure syncblock is cleared with GC_ALLOC_ZEROING_OPTIONAL #97174

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Ensure syncblock is cleared with GC_ALLOC_ZEROING_OPTIONAL
jkotas Jan 18, 2024
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jkotas Jan 19, 2024
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15 changes: 13 additions & 2 deletions src/coreclr/vm/gchelpers.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -434,7 +434,8 @@ OBJECTREF AllocateSzArray(MethodTable* pArrayMT, INT32 cElements, GC_ALLOC_FLAGS
// dummy object.
// If the GC gives us a 8 byte aligned address, we use it for the array and place the dummy
// object after the array, otherwise we put the dummy object first, shifting the base of
// the array to an 8 byte aligned address.
// the array to an 8 byte aligned address. Also, we need to make sure that the syncblock of the
// second object is zeroed. GC won't take care of zeroing it out with GC_ALLOC_ZEROING_OPTIONAL.
//
// Note: on 64 bit platforms, the GC always returns 8 byte aligned addresses, and we don't
// execute this code because DATA_ALIGNMENT < sizeof(double) is false.
Expand All @@ -447,14 +448,24 @@ OBJECTREF AllocateSzArray(MethodTable* pArrayMT, INT32 cElements, GC_ALLOC_FLAGS
orArray = (ArrayBase*)Alloc(totalSize + MIN_OBJECT_SIZE, flags);

Object* orDummyObject;
if ((size_t)orArray % sizeof(double))
if (((size_t)orArray % sizeof(double)) != 0)
{
orDummyObject = orArray;
orArray = (ArrayBase*)((size_t)orArray + MIN_OBJECT_SIZE);
if (flags & GC_ALLOC_ZEROING_OPTIONAL)
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{
// clean the syncblock of the aligned array.
*(((void**)orArray)-1) = 0;
}
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}
else
{
orDummyObject = (Object*)((size_t)orArray + totalSize);
if (flags & GC_ALLOC_ZEROING_OPTIONAL)
{
// clean the syncblock of the dummy object.
*(((void**)orDummyObject)-1) = 0;
}
}
_ASSERTE(((size_t)orArray % sizeof(double)) == 0);
orDummyObject->SetMethodTable(g_pObjectClass);
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