Strip PAC bits for return addresses in stack traces

PiperOrigin-RevId: 705945725
Change-Id: I0a87d81244999849b163480758247b2e87c2d430

absl/debugging/internal/stacktrace_aarch64-inl.inc

@@ -178,6 +178,20 @@ static void **NextStackFrame(void **old_frame_pointer, const void *uc,
   return new_frame_pointer;
 }
 
+// When PAC-RET (-mbranch-protection=pac-ret) is enabled, return addresses
+// stored on the stack will be signed, which means that pointer bits outside of
+// the VA range are potentially set. Since the stacktrace code is expected to
+// return normal code pointers, this function clears those bits.
+inline void* ClearPacBits(void* ptr) {
+  register void* x30 __asm__("x30") = ptr;
+  // The normal instruction for clearing PAC bits is XPACI, but for
+  // compatibility with ARM platforms that do not support pointer
+  // authentication, we use the hint space instruction XPACLRI instead. Hint
+  // space instructions behave as NOPs on unsupported platforms.
+  asm("xpaclri" : "+r"(x30));
+  return x30;
+}
+
 template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
 // We count on the bottom frame being this one. See the comment
 // at prev_return_address
@@ -219,7 +233,7 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
     if (skip_count > 0) {
       skip_count--;
     } else {
-      result[n] = prev_return_address;
+      result[n] = ClearPacBits(prev_return_address);
       if (IS_STACK_FRAMES) {
         sizes[n] = static_cast<int>(
             ComputeStackFrameSize(prev_frame_pointer, frame_pointer));