[llvm] [llvm-exegesis][AArch64] Check for PAC keys before disabling them (PR #138643)

[Anatoly Trosinenko via llvm-commits]

================
@@ -207,15 +207,28 @@ class ExegesisAArch64Target : public ExegesisTarget {
 
     if (isPointerAuth(Opcode)) {
 #if defined(__aarch64__) && defined(__linux__)
+
+      // For some systems with existing PAC keys set, it is better to
+      // check the existing state of the key before setting it.
+      // For systems without PAC, this is a No-op but with PAC, it is
+      // safer to check the existing key state and then disable/enable them.
+      // Hence the guard for switching.
+      unsigned long PacKeys = 0;
+      if (prctl(PR_PAC_GET_ENABLED_KEYS, &PacKeys, 0, 0, 0) < 0) {
+        return "Failed to get PAC key status";
+      }
+
       // Disable all PAC keys. Note that while we expect the measurements to
       // be the same with PAC keys disabled, they could potentially be lower
       // since authentication checks are bypassed.
-      if (prctl(PR_PAC_SET_ENABLED_KEYS,
-                PR_PAC_APIAKEY | PR_PAC_APIBKEY | PR_PAC_APDAKEY |
-                    PR_PAC_APDBKEY, // all keys
-                0,                  // disable all
-                0, 0) < 0) {
-        return "Failed to disable PAC keys";
+      if (PacKeys != 0) {
+        if (prctl(PR_PAC_SET_ENABLED_KEYS,
+                  PR_PAC_APIAKEY | PR_PAC_APIBKEY | PR_PAC_APDAKEY |
+                      PR_PAC_APDBKEY, // all keys
+                  0,                  // disable all
+                  0, 0) < 0) {
----------------
atrosinenko wrote:

[nit] It may be more readable to define a wrapper with the default argument values in C++, as in this example these three literal 0s have different meaning: the first one is a bitmask and the rest are "the argument is unused, pass 0 now, other values to de defined in the future"
```cpp
static int prctl_wrapper(int op, long arg2 = 0, long arg3 = 0, long arg4 = 0, long arg5 = 0) {
  return prctl(op, arg2, arg3, arg4, arg5);
}
```

https://github.com/llvm/llvm-project/pull/138643