Avoid inline optimization for unwind_phase2 and unwind_phase2_forced

[xiongji90]

Summary

Currently, when building libunwind with CET enabled, we will count the number of stack frames skipped in unwind process and adjust CET shadow stack based on this number. The logic is implemented in unwind_phase2 and unwind_phase2_forced functions and these 2 functions are called by _Unwind_RaiseException and _Unwind_ForcedUnwind.
However, when counting the number of stack frames skipped, current implementation assumes "unwind_phase2" and "unwind_phase2_forced" are not inline optimized, otherwise the number of stack frames must be changed. The assumption may be incorrect when compiler inlines these 2 functions, this patch adds "attribute((noinline))" for unwind_phase2 and unwind_phase2.

[xiongji90]

Hi, @MaskRay and @compnerd
Could you help review this patch?
Thanks very much.

[MaskRay]

Can you add the stacktrace to the description?

[xiongji90]

In D136317#3881482, @MaskRay wrote:

Can you add the stacktrace to the description?

Hi, @MaskRay
Following is stack trace which current implementation assumes:
---------------------------|
unwind_phase2
---------------------------|
_Unwind_RaiseException
---------------------------|
__cxa_throw
--------------------------|
user function frames...
--------------------------|

In unwind_phase2, we will go through stacks and count the number of stack frames which needs to be skipped. We initialize variable "framesWalked" to 1 in "unwind_phase2" and "unwind_phase2_forced", this is because these 2 functions have corresponding stack frame and also have corresponding records in CET shadow stack, so we need to remove CET shadow record for "unwind_phase2" and "unwind_phase2_forced" when unwinding normal stack. However, if "unwind_phase2" or "unwind_phase2_forced" are inlined, then there will be no corresponding stack frame and corresponding CET shadow stack record, the stack trace will turn to:

---------------------------|
_Unwind_RaiseException
---------------------------|
__cxa_throw
--------------------------|
user function frames...
--------------------------|

Under such circumstance, we need to initialize "framesWalked" to 0. Although current implementation does work, we can't control whether compiler will optimize unwind_phase2 away in the future, so suggest to add "noinline" to guard.

Thanks very much.

libunwind/src/UnwindLevel1.c

if (frameInfo.handler != 0) {
         return _URC_FATAL_PHASE1_ERROR;
       }
     }
   }
   return _URC_NO_REASON;
 }
 extern int __unw_step_stage2(unw_cursor_t *);
 
+#if defined(_LIBUNWIND_USE_CET)
+__attribute__((noinline))
+#endif
 static _Unwind_Reason_Code
 unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *exception_object) {
   __unw_init_local(cursor, uc);
 
   _LIBUNWIND_TRACE_UNWINDING("unwind_phase2(ex_ojb=%p)",
                              (void *)exception_object);
 
   // uc is initialized by __unw_getcontext in the parent frame. The first stack
#endif
     }
   }
 
   // Clean up phase did not resume at the frame that the search phase
   // said it would...
   return _URC_FATAL_PHASE2_ERROR;
 }
 
+#if defined(_LIBUNWIND_USE_CET)
+__attribute__((noinline))
+#endif
 static _Unwind_Reason_Code
 unwind_phase2_forced(unw_context_t *uc, unw_cursor_t *cursor,
                      _Unwind_Exception *exception_object,
                      _Unwind_Stop_Fn stop, void *stop_parameter) {
   __unw_init_local(cursor, uc);
 
   // uc is initialized by __unw_getcontext in the parent frame. The first stack
   // frame walked is unwind_phase2_forced.