Compiler Attributes: Add __diagnose_as for Clang

Clang will perform various compile-time diagnostics on uses of various
functions (e.g. simple bounds-checking on strcpy(), etc). These
diagnostics can be assigned to other functions (for example, new
implementations of the string functions under CONFIG_FORTIFY_SOURCE)
using the "diagnose_as_builtin" attribute. This allows those functions
to retain their compile-time diagnostic warnings.

Cc: Nathan Chancellor <nathan@kernel.org>
Cc: llvm@lists.linux.dev
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220208225350.1331628-5-keescook@chromium.org

include/linux/compiler_attributes.h

@@ -100,6 +100,19 @@
 # define __copy(symbol)
 #endif
 
+/*
+ * Optional: not supported by gcc
+ * Optional: only supported since clang >= 14.0
+ * Optional: not supported by icc
+ *
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#diagnose_as_builtin
+ */
+#if __has_attribute(__diagnose_as_builtin__)
+# define __diagnose_as(builtin...)	__attribute__((__diagnose_as_builtin__(builtin)))
+#else
+# define __diagnose_as(builtin...)
+#endif
+
 /*
  * Don't. Just don't. See commit 771c035372a0 ("deprecate the '__deprecated'
  * attribute warnings entirely and for good") for more information.