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

[David Green via llvm-commits]

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@@ -207,15 +213,38 @@ 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.
+      errno = 0;
+      unsigned long PacKeys = prctl_wrapper(PR_PAC_GET_ENABLED_KEYS,
+                                            0,  // unused
+                                            0,  // unused
+                                            0,  // unused
+                                            0); // unused
+      if ((long)PacKeys < 0) {
+        if (errno == EINVAL) {
+          return "PAuth not supported on this system";
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davemgreen wrote:

Can this return nullptr because if they cannot be disabled, we can still run them (they are already disabled, or they will not be supported by the CPU which goes through the normal unsupported method)?

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