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Fix x64 data breakpoint handling after CORINFO_HELP_ARRADDR_ST inlining #127251

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Fix x64 data breakpoint handling after CORINFO_HELP_ARRADDR_ST inlining #127251

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40 changes: 40 additions & 0 deletions src/coreclr/vm/excep.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5491,6 +5491,46 @@ AdjustContextForJITHelpers(
if (IsIPInMarkedJitHelper(ip))
{
Thread::VirtualUnwindToFirstManagedCallFrame(pContext);

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@jkotas jkotas Apr 21, 2026

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We have other helpers that have internal managed code between the user code and a write barrier. For example, BulkMoveWithWriteBarrier. What is the invariant that the debugger expects here?

This looks fragile to filter it here. Can this filtering be done in the higher-level debugger instead? The higher level debugger has a better idea what' a user code.

// After unwinding from the native write barrier to the first managed frame, check
// whether that frame is a managed helper that itself called the write barrier and
// keep unwinding until we reach user code.
//
// CastHelpers.StelemRef (CORINFO_HELP_ARRADDR_ST) and its callees StelemRef_Helper
// and StelemRef_Helper_NoCacheLookup all call RuntimeHelpers.WriteBarrier. The call
// chain can be up to 3 levels deep:
// user -> StelemRef -> StelemRef_Helper -> StelemRef_Helper_NoCacheLookup -> WriteBarrier
//
// Prior to the change that inlined CORINFO_HELP_ARRADDR_ST, StelemRef tail-called
// the write barrier so its frame was already destroyed and the unwind went directly
// to user code. With a regular call, the StelemRef family frames remain on the stack.
//
// We identify these helpers by their MethodTable (CastHelpers class), which is stable
// across tiered recompilation.
static MethodTable* s_pCastHelpersMT = nullptr;
while (true)
{
Comment on lines +5510 to +5512
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Copilot AI Apr 21, 2026

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static MethodTable* s_pCastHelpersMT is updated via an unsynchronized check-then-set. If multiple threads hit data breakpoints concurrently, this becomes a C++ data race. Consider removing the cache (data breakpoints are rare) or switching to an atomic/volatile pattern (e.g., VolatilePtr/VolatileLoad+VolatileStore or InterlockedCompareExchange).

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PCODE currentIP = GetIP(pContext);
if (!ExecutionManager::IsManagedCode(currentIP))
break;

EECodeInfo codeInfo(currentIP);
if (!codeInfo.IsValid())
break;

MethodDesc* pMD = codeInfo.GetMethodDesc();
if (pMD == nullptr)
break;

if (s_pCastHelpersMT == nullptr)
s_pCastHelpersMT = CoreLibBinder::GetExistingClass(CLASS__CASTHELPERS);

Comment on lines +5526 to +5527
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Copilot AI Apr 21, 2026

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CoreLibBinder::GetExistingClass(CLASS__CASTHELPERS) will _ASSERTE(pMT != NULL) if CastHelpers hasn't been loaded yet (e.g., very early startup / debugger attaches early). Since this path can run on a data breakpoint, please switch to a non-asserting lookup (e.g., CoreLibBinder::GetClassIfExist(CLASS__CASTHELPERS) or g_pCastHelpers once initialized) and handle the null case by breaking out of the loop.

Suggested change
s_pCastHelpersMT = CoreLibBinder::GetExistingClass(CLASS__CASTHELPERS);
s_pCastHelpersMT = CoreLibBinder::GetClassIfExist(CLASS__CASTHELPERS);
if (s_pCastHelpersMT == nullptr)
break;

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@jkotas jkotas Apr 21, 2026

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The local caching is useless. CoreLibBinder has similar cache already.

if (pMD->GetMethodTable() != s_pCastHelpersMT)
break;

Thread::VirtualUnwindCallFrame(pContext);
}

return TRUE;
}
#else
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