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Sclang: PyrGC: Fix GC segfault caused by uncollected temporary function objects by placing an upper limit on un-scanned objects. #6261

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Sclang: PyrGC: Fix GC segfault caused by uncollected temporary function objects by placing an upper limit on un-scanned objects. #6261

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4 changes: 4 additions & 0 deletions lang/LangSource/GC.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -664,6 +664,10 @@ HOT void PyrGC::Collect() {
if (mNumToScan > 0) {
// post("->Collect ns %d ng %d s %d\n", mNumToScan, mNumGrey, mScans);
// DumpInfo();

// This operation can easily overflow if mNumToScan is large causing undefined behaviour.
// Right shifting a negative is also implementation defined and has led to bugs that only appear on certain
// operating systems and hardware.
mNumToScan += mNumToScan >> 3;

// post("->Collect2 ns %d ng %d s %d\n", mNumToScan, mNumGrey, mScans);
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15 changes: 13 additions & 2 deletions lang/LangSource/GC.h
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Original file line number Diff line number Diff line change
Expand Up @@ -303,9 +303,20 @@ inline void PyrGC::ToGrey2(PyrObjectHdr* obj) {
}

inline PyrObject* PyrGC::Allocate(size_t inNumBytes, int32 sizeclass, bool inRunCollection) {
if (inRunCollection && mNumToScan >= kScanThreshold)
// When allocating a large number of objects, mNumToScan (int32) can overflow.
// This usually happens when creating temporary frames, which might not be collected until later.
// This is designed to force collection when there might be a large number of temporary objects.
// The threshold is somewhat arbitrary and could be made larger, but it is better to be safe than fast,
// and sclang is not designed nor optimized to process large amounts of data.
const bool aboveTemporaryObjectLimit = mNumToScan > 5'000'000;

// All allocations should set inRunCollection to true, but some types of methods do not as a part of an
// optimization. Don't scan an insignificant amount of memory, wait until above kScanThreshold.
const bool shouldCollect = inRunCollection && mNumToScan >= kScanThreshold;

if (aboveTemporaryObjectLimit || shouldCollect) {
Collect();
else {
} else {
if (inRunCollection)
mUncollectedAllocations = 0;
else
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