[libunwind] Fix aarch64 SEH unwinding with a debugger attached (llvm#162867)

See LuaJIT/LuaJIT#593 (comment)
for the original explanation of the problem.

In short; when a debugger is attached, there's a
function KiUserExceptionDispatcher in the stack that is being unwound.
The function KiUserExceptionDispatcher contains a CONTEXT, with a copy
of the context from where the exception was raised. When unwinding
through this function, this whole CONTEXT gets restored.

This CONTEXT is what we receive a pointer to in the callbacks, as the
ms_ctx pointer.

When we unwind manually using RtlUnwindEx, the unwinding overwrites the
CONTEXT that is passed to it. Thus, to avoid clobbering the CONTEXT that
needs to be restored by KiUserExceptionDispatcher, we could either
declare a new temporary CONTEXT on the stack before calling RtlUnwindEx,
or just use disp->ContextRecord as we already have available.

Fixes: llvm#161851

Co-authored-by: Peter Cawley <[email protected]>
Co-authored-by: Hannes Domani <[email protected]>

Author: 3 people

libunwind/src/Unwind-seh.cpp

@@ -174,7 +174,8 @@ _GCC_specific_handler(PEXCEPTION_RECORD ms_exc, PVOID frame, PCONTEXT ms_ctx,
     }
     // FIXME: Indicate target frame in foreign case!
     // phase 2: the clean up phase
-    RtlUnwindEx(frame, (PVOID)disp->ControlPc, ms_exc, exc, ms_ctx, disp->HistoryTable);
+    RtlUnwindEx(frame, (PVOID)disp->ControlPc, ms_exc, exc, disp->ContextRecord,
+                disp->HistoryTable);
     _LIBUNWIND_ABORT("RtlUnwindEx() failed");
   case _URC_INSTALL_CONTEXT: {
     // If we were called by __libunwind_seh_personality(), indicate that