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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libunwind

Author: Martin Storsjö (mstorsjo)

<details>
<summary>Changes</summary>

See https://github.com/LuaJIT/LuaJIT/issues/593#issuecomment-1717728494 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: https://github.com/llvm/llvm-project/issues/161851

---
Full diff: https://github.com/llvm/llvm-project/pull/162867.diff


1 Files Affected:

- (modified) libunwind/src/Unwind-seh.cpp (+1-1) 


``````````diff
diff --git a/libunwind/src/Unwind-seh.cpp b/libunwind/src/Unwind-seh.cpp
index 8b83f10615f22..058369acf3360 100644
--- a/libunwind/src/Unwind-seh.cpp
+++ b/libunwind/src/Unwind-seh.cpp
@@ -174,7 +174,7 @@ _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

``````````

</details>


https://github.com/llvm/llvm-project/pull/162867