[PATCH] accept an empty attribute argument list

GCC and clang accept an empty argument list for attributes, e.g.
__attribute__((nonnull())), and treat it the same as the bare
__attribute__((nonnull)). sparse instead rejects it with
"an expression is expected before ')'".

ignore_attribute() and recover_unknown_attribute() consume the optional
argument list with parens_expression(), which requires a non-empty
expression, so an empty () is rejected. The bare form is fine because it
takes no parentheses and skips that path.

This turned up in the Linux kernel: the cleanup.h guard macros expand
__nonnull_args() to __attribute__((__nonnull__())).

  https://lore.kernel.org/all/[email protected]/

Reported-by: Dan Carpenter <[email protected]>
Signed-off-by: Dmitry Ilvokhin <[email protected]>
---
 parse.c                              | 17 +++++++++++++----
 validation/attribute-nonnull-empty.c | 12 ++++++++++++
 2 files changed, 25 insertions(+), 4 deletions(-)
 create mode 100644 validation/attribute-nonnull-empty.c

diff --git a/parse.c b/parse.c
index 9389079e..3b380156 100644
--- a/parse.c
+++ b/parse.c
@@ -1106,8 +1106,13 @@ static struct token *autotype_specifier(struct token *token, struct symbol *sym,
 static struct token *ignore_attribute(struct token *token, struct symbol *attr, struct decl_state *ctx)
 {
 	struct expression *expr = NULL;
-	if (match_op(token, '('))
-		token = parens_expression(token, &expr, "in attribute");
+	if (match_op(token, '(')) {
+		/* Accept an empty argument list, e.g. __attribute__((nonnull())) */
+		if (match_op(token->next, ')'))
+			token = token->next->next;
+		else
+			token = parens_expression(token, &expr, "in attribute");
+	}
 	return token;
 }
 
@@ -1362,8 +1367,12 @@ static struct token *recover_unknown_attribute(struct token *token)
 	if (Wunknown_attribute)
 		warning(token->pos, "unknown attribute '%s'", show_ident(token->ident));
 	token = token->next;
-	if (match_op(token, '('))
-		token = parens_expression(token, &expr, "in attribute");
+	if (match_op(token, '(')) {
+		if (match_op(token->next, ')'))
+			token = token->next->next;
+		else
+			token = parens_expression(token, &expr, "in attribute");
+	}
 	return token;
 }
 
diff --git a/validation/attribute-nonnull-empty.c b/validation/attribute-nonnull-empty.c
new file mode 100644
index 00000000..c4021f61
--- /dev/null
+++ b/validation/attribute-nonnull-empty.c
@@ -0,0 +1,12 @@
+/*
+ * An empty attribute argument list, like __attribute__((nonnull())), is
+ * accepted by GCC and Clang (equivalent to the bare __attribute__((nonnull)))
+ * and must be parsed by sparse too.
+ */
+extern void *bare (void *d, const void *s) __attribute__((nonnull));
+extern void *empty(void *d, const void *s) __attribute__((nonnull()));
+extern void *args (void *d, const void *s) __attribute__((nonnull(1, 2)));
+
+/*
+ * check-name: attribute with empty argument list
+ */
-- 
2.53.0-Meta