[clang] [PAC] Implement function pointer re-signing (PR #98847)

[Daniil Kovalev via cfe-commits]

================
@@ -3126,3 +3137,57 @@ CodeGenFunction::EmitPointerAuthAuth(const CGPointerAuthInfo &PointerAuth,
   return EmitPointerAuthCommon(*this, PointerAuth, Pointer,
                                llvm::Intrinsic::ptrauth_auth);
 }
+
+llvm::Value *CodeGenFunction::EmitPointerAuthSign(QualType pointeeType,
+                                                  llvm::Value *pointer) {
+  CGPointerAuthInfo pointerAuth =
+      CGM.getPointerAuthInfoForPointeeType(pointeeType);
+  return EmitPointerAuthSign(pointerAuth, pointer);
+}
+
+llvm::Value *CodeGenFunction::EmitPointerAuthAuth(QualType pointeeType,
+                                                  llvm::Value *pointer) {
+  CGPointerAuthInfo pointerAuth =
+      CGM.getPointerAuthInfoForPointeeType(pointeeType);
+  return EmitPointerAuthAuth(pointerAuth, pointer);
+}
+
+llvm::Value *
+CodeGenFunction::EmitPointerAuthResignCall(llvm::Value *value,
+                                           const CGPointerAuthInfo &curAuth,
+                                           const CGPointerAuthInfo &newAuth) {
+  assert(curAuth && newAuth);
+
+  if (curAuth.getAuthenticationMode() !=
+          PointerAuthenticationMode::SignAndAuth ||
+      newAuth.getAuthenticationMode() !=
+          PointerAuthenticationMode::SignAndAuth) {
+    auto authedValue = EmitPointerAuthAuth(curAuth, value);
+    return EmitPointerAuthSign(newAuth, authedValue);
+  }
+  // Convert the pointer to intptr_t before signing it.
+  auto origType = value->getType();
+  value = Builder.CreatePtrToInt(value, IntPtrTy);
+
+  auto curKey = Builder.getInt32(curAuth.getKey());
+  auto newKey = Builder.getInt32(newAuth.getKey());
+
+  llvm::Value *curDiscriminator = curAuth.getDiscriminator();
+  if (!curDiscriminator)
+    curDiscriminator = Builder.getSize(0);
+
+  llvm::Value *newDiscriminator = newAuth.getDiscriminator();
+  if (!newDiscriminator)
----------------
kovdan01 wrote:

Nit: it looks like that it's a pretty common practice to check returned value of `.getDiscriminator()` against nullptr and then assign discriminator to zero constant of size type if the check succeeds. Probably it's worth adding a corresponding file-local helper or member function.

Feel free to ignore.

https://github.com/llvm/llvm-project/pull/98847