Fix x86 exception handling edge case

When a NULL reference exception occurs in a JIT helper or a VSD stub,
runtime pretends the exception occured in the managed caller. There is a
bug on x86 Windows where the COMPlusThrowCallback considers that frame
to be the frame where the exception actually occurred (based on the
m_crawl.isFirst). In case the call to the helper is the last instruction
in a try region, the exception handler lookup would reject that address
and the exception may not get handled at the right place or at all.

This change fixes it by ensuring that the m_crawl.isFirst is not set
when the frame is not the frame of the failure.

Close dotnet#113106

Author: janvorli

src/coreclr/vm/excep.cpp

@@ -6779,15 +6779,15 @@ VEH_ACTION WINAPI CLRVectoredExceptionHandlerPhase3(PEXCEPTION_POINTERS pExcepti
                 // the subsequent logic here sees a managed fault.
                 //
                 // On 64-bit, some additional work is required..
-#ifdef FEATURE_EH_FUNCLETS
                 pContext->ContextFlags &= ~CONTEXT_EXCEPTION_ACTIVE;
+#ifdef FEATURE_EH_FUNCLETS
                 return VEH_EXECUTE_HANDLE_MANAGED_EXCEPTION;
 #endif // defined(FEATURE_EH_FUNCLETS)
             }
             else if (AdjustContextForVirtualStub(pExceptionRecord, pContext))
             {
-#ifdef FEATURE_EH_FUNCLETS
                 pContext->ContextFlags &= ~CONTEXT_EXCEPTION_ACTIVE;
+#ifdef FEATURE_EH_FUNCLETS
                 return VEH_EXECUTE_HANDLE_MANAGED_EXCEPTION;
 #endif
             }
@@ -7118,9 +7118,7 @@ LONG WINAPI CLRVectoredExceptionHandlerShim(PEXCEPTION_POINTERS pExceptionInfo)
     // WARNING WARNING WARNING WARNING WARNING WARNING WARNING
     //
 
-#ifdef FEATURE_EH_FUNCLETS
     pExceptionInfo->ContextRecord->ContextFlags |= CONTEXT_EXCEPTION_ACTIVE;
-#endif // FEATURE_EH_FUNCLETS
 
     // WARNING
     //

src/coreclr/vm/stackwalk.cpp

@@ -3207,8 +3207,17 @@ void StackFrameIterator::PostProcessingForNoFrameTransition()
 
     // Flags the same as from a FaultingExceptionFrame.
     m_crawl.isInterrupted = true;
-    m_crawl.hasFaulted = true;
+    m_crawl.hasFaulted = (pContext->ContextFlags & CONTEXT_EXCEPTION_ACTIVE) != 0;
     m_crawl.isIPadjusted = false;
+    if (!m_crawl.hasFaulted)
+    {
+        // If the context is from a hardware exception that happened in a helper where we have unwound
+        // the exception location to the caller of the helper, the frame needs to be marked as not
+        // being the first one. The COMPlusThrowCallback uses this information to decide whether
+        // the current IP should or should not be included in the try region range. The call to
+        // the helper that has fired the exception may be the last instruction in the try region.
+        m_crawl.isFirst = false;
+    }
 
 #if defined(STACKWALKER_MAY_POP_FRAMES)
     // If Frames would be unlinked from the Frame chain, also reset the UseExInfoForStackwalk bit