selftests/harness: order TEST_F and XFAIL_ADD constructors

TEST_F() allocates and registers its struct __test_metadata via mmap()
inside its constructor, and only then assigns the
_##fixture_##test##_object pointer.

XFAIL_ADD() runs in a constructor too and reads
_##fixture_##test##_object to initialize xfail->test. If XFAIL_ADD runs
first, xfail->test can be NULL and the expected failure will be reported
as FAIL.

Use constructor priorities to ensure TEST_F registration runs before
XFAIL_ADD, without adding extra state or runtime lookups.

Fixes: 2709473 ("selftests: kselftest_harness: support using xfail")
Signed-off-by: Sun Jian <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: ahjf-07

tools/testing/selftests/kselftest_harness.h

@@ -76,6 +76,9 @@ static inline void __kselftest_memset_safe(void *s, int c, size_t n)
 		memset(s, c, n);
 }
 
+#define KSELFTEST_PRIO_TEST_F  20000
+#define KSELFTEST_PRIO_XFAIL   20001
+
 #define TEST_TIMEOUT_DEFAULT 30
 
 /* Utilities exposed to the test definitions */
@@ -465,7 +468,7 @@ static inline void __kselftest_memset_safe(void *s, int c, size_t n)
 			fixture_name##_teardown(_metadata, self, variant); \
 	} \
 	static struct __test_metadata *_##fixture_name##_##test_name##_object; \
-	static void __attribute__((constructor)) \
+	static void __attribute__((constructor(KSELFTEST_PRIO_TEST_F))) \
 			_register_##fixture_name##_##test_name(void) \
 	{ \
 		struct __test_metadata *object = mmap(NULL, sizeof(*object), \
@@ -880,7 +883,7 @@ struct __test_xfail {
 		.fixture = &_##fixture_name##_fixture_object, \
 		.variant = &_##fixture_name##_##variant_name##_object, \
 	}; \
-	static void __attribute__((constructor)) \
+	static void __attribute__((constructor(KSELFTEST_PRIO_XFAIL))) \
 		_register_##fixture_name##_##variant_name##_##test_name##_xfail(void) \
 	{ \
 		_##fixture_name##_##variant_name##_##test_name##_xfail.test = \