objtool: Warn on zero-length functions

All callable functions should have an ELF size.
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/03d429c4fa87829c61c5dc0e89652f4d9efb62f1.1563413318.git.jpoimboe@redhat.com

tools/objtool/check.c

@@ -2357,6 +2357,12 @@ static int validate_functions(struct objtool_file *file)
 			if (func->type != STT_FUNC)
 				continue;
 
+			if (!func->len) {
+				WARN("%s() is missing an ELF size annotation",
+				     func->name);
+				warnings++;
+			}
+
 			if (func->pfunc != func || func->alias != func)
 				continue;