Fix preventing memory allocation in signal handler

There was a subtle bug. When the hardware exception handler returns back
to the signal handler, the exception's CONTEXT record may contain
modified registers and so the changes need to be propagated back to the
signal context. But the recent change dotnet#16384 was restoring the signal
context from the originally grabbed context instead of the one that's
pointed to by the exception, which is different.

I have also added a little optimization - the contextRecord that was
added is not needed, since the signalContextRecord can be used as the
initial context record for the exception. So we can save the
contextRecord and also copying to the signalContextRecord from it.

Author: janvorli

src/pal/src/exception/signal.cpp

@@ -845,7 +845,6 @@ static bool common_signal_handler(int code, siginfo_t *siginfo, void *sigcontext
 {
     sigset_t signal_set;
     CONTEXT signalContextRecord;
-    CONTEXT contextRecord;
     EXCEPTION_RECORD exceptionRecord;
     native_context_t *ucontext;
 
@@ -868,7 +867,7 @@ static bool common_signal_handler(int code, siginfo_t *siginfo, void *sigcontext
 
     // Pre-populate context with data from current frame, because ucontext doesn't have some data (e.g. SS register)
     // which is required for restoring context
-    RtlCaptureContext(&contextRecord);
+    RtlCaptureContext(&signalContextRecord);
 
     ULONG contextFlags = CONTEXT_CONTROL | CONTEXT_INTEGER | CONTEXT_FLOATING_POINT;
 
@@ -879,7 +878,7 @@ static bool common_signal_handler(int code, siginfo_t *siginfo, void *sigcontext
     // Fill context record with required information. from pal.h:
     // On non-Win32 platforms, the CONTEXT pointer in the
     // PEXCEPTION_POINTERS will contain at least the CONTEXT_CONTROL registers.
-    CONTEXTFromNativeContext(ucontext, &contextRecord, contextFlags);
+    CONTEXTFromNativeContext(ucontext, &signalContextRecord, contextFlags);
 
     /* Unmask signal so we can receive it again */
     sigemptyset(&signal_set);
@@ -890,17 +889,15 @@ static bool common_signal_handler(int code, siginfo_t *siginfo, void *sigcontext
         ASSERT("pthread_sigmask failed; error number is %d\n", sigmaskRet);
     }
 
-    contextRecord.ContextFlags |= CONTEXT_EXCEPTION_ACTIVE;
-
-    memcpy_s(&signalContextRecord, sizeof(CONTEXT), &contextRecord, sizeof(CONTEXT));
+    signalContextRecord.ContextFlags |= CONTEXT_EXCEPTION_ACTIVE;
 
     // The exception object takes ownership of the exceptionRecord and contextRecord
-    PAL_SEHException exception(&exceptionRecord, &contextRecord, true);
+    PAL_SEHException exception(&exceptionRecord, &signalContextRecord, true);
 
     if (SEHProcessException(&exception))
     {
         // Exception handling may have modified the context, so update it.
-        CONTEXTToNativeContext(&contextRecord, ucontext);
+        CONTEXTToNativeContext(exception.ExceptionPointers.contextRecord, ucontext);
         return true;
     }