[clang] [clang] Function type attribute to prevent CFI instrumentation (PR #135836)

[Bruno De Fraine via cfe-commits]

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@@ -9283,8 +9283,14 @@ static AssignConvertType checkPointerTypesForAssignment(Sema &S,
       return AssignConvertType::IncompatibleFunctionPointer;
     return AssignConvertType::IncompatiblePointer;
   }
-  if (!S.getLangOpts().CPlusPlus && S.IsFunctionConversion(ltrans, rtrans))
-    return AssignConvertType::IncompatibleFunctionPointer;
+  bool DiscardingCFIUncheckedCallee, AddingCFIUncheckedCallee;
+  if (!S.getLangOpts().CPlusPlus &&
+      S.IsFunctionConversion(ltrans, rtrans, &DiscardingCFIUncheckedCallee,
+                             &AddingCFIUncheckedCallee)) {
+    // Allow conversions between CFIUncheckedCallee-ness.
+    if (!DiscardingCFIUncheckedCallee && !AddingCFIUncheckedCallee)
+      return AssignConvertType::IncompatibleFunctionPointer;
+  }
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brunodf-snps wrote:

Follow up in #160474 and #160477, please review.

https://github.com/llvm/llvm-project/pull/135836