[PATCH v3] printf: convert test_hashed into macro

[Tamir Duberstein]

This allows the compiler to check the arguments against the __printf()
attribute on __test(). This produces better diagnostics when incorrect
inputs are passed.

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202512061600.89CKQ3ag-lkp@xxxxxxxxx/
Signed-off-by: Tamir Duberstein <tamird@xxxxxxxxxx>
---
Changes in v3:
- Added parentheses to function references.
- Changed escaped newline indentation to tabs.
- Link to v2: https://patch.msgid.link/20260116-printf-kunit-printf-attr-v2-1-3dd7fefc3207@xxxxxxxxxx

Changes in v2:
- Convert test_hashed to macro rather than try to annotate it in a way
  that is compatible with both clang and gcc.
- Link to v1: https://patch.msgid.link/20251206-printf-kunit-printf-attr-v1-1-1682808b51d0@xxxxxxxxx
---
 lib/tests/printf_kunit.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/lib/tests/printf_kunit.c b/lib/tests/printf_kunit.c
index 7617e5b8b02c..f6f21b445ece 100644
--- a/lib/tests/printf_kunit.c
+++ b/lib/tests/printf_kunit.c
@@ -266,15 +266,17 @@ hash_pointer(struct kunit *kunittest)
 	KUNIT_EXPECT_MEMNEQ(kunittest, buf, PTR_STR, PTR_WIDTH);
 }
 
-static void
-test_hashed(struct kunit *kunittest, const char *fmt, const void *p)
-{
-	char buf[PLAIN_BUF_SIZE];
-
-	plain_hash_to_buffer(kunittest, p, buf, PLAIN_BUF_SIZE);
-
-	test(buf, fmt, p);
-}
+/*
+ * This is a macro so that the compiler can compare its arguments to the
+ * __printf() attribute on __test(). This cannot be a function with a __printf()
+ * attribute because GCC requires __printf() functions to be variadic.
+ */
+#define test_hashed(kunittest, fmt, p)						\
+	do {									\
+		char buf[PLAIN_BUF_SIZE];					\
+		plain_hash_to_buffer(kunittest, p, buf, PLAIN_BUF_SIZE);	\
+		test(buf, fmt, p);						\
+	} while (0)
 
 /*
  * NULL pointers aren't hashed.

---
base-commit: 24d479d26b25bce5faea3ddd9fa8f3a6c3129ea7
change-id: 20251206-printf-kunit-printf-attr-19369fc57bf0

Best regards,
--  
Tamir Duberstein <tamird@xxxxxxxxxx>