kunit: Add gnu_printf specifiers

Some KUnit functions use variable arguments to implement a printf-like
format string. Use the __printf() attribute to let the compiler warn if
invalid format strings are passed in.

If the kernel is build with W=1, it complained about the lack of these
specifiers, e.g.:
../lib/kunit/test.c:72:2: warning: function ‘kunit_log_append’ might be a candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Acked-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

include/kunit/test.h

@@ -610,7 +610,7 @@ static inline void *kunit_kzalloc(struct kunit *test, size_t size, gfp_t gfp)
 
 void kunit_cleanup(struct kunit *test);
 
-void kunit_log_append(char *log, const char *fmt, ...);
+void __printf(2, 3) kunit_log_append(char *log, const char *fmt, ...);
 
 /*
  * printk and log to per-test or per-suite log buffer.  Logging only done

lib/kunit/string-stream.h

@@ -35,9 +35,9 @@ struct string_stream *alloc_string_stream(struct kunit *test, gfp_t gfp);
 int __printf(2, 3) string_stream_add(struct string_stream *stream,
 				     const char *fmt, ...);
 
-int string_stream_vadd(struct string_stream *stream,
-		       const char *fmt,
-		       va_list args);
+int __printf(2, 0) string_stream_vadd(struct string_stream *stream,
+				      const char *fmt,
+				      va_list args);
 
 char *string_stream_get_string(struct string_stream *stream);