kasan: test: prevent cache merging in kmem_cache_double_destroy

With HW_TAGS KASAN and kasan.stacktrace=off, the cache created in the
kmem_cache_double_destroy() test might get merged with an existing one.
Thus, the first kmem_cache_destroy() call won't actually destroy it but
will only decrease the refcount.  This causes the test to fail.

Provide an empty constructor for the created cache to prevent the cache
from getting merged.

Link: https://lkml.kernel.org/r/b597bd434c49591d8af00ee3993a42c609dc9a59.1644346040.git.andreyknvl@google.com
Fixes: f98f966 ("kasan: test: add test case for double-kmem_cache_destroy()")
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/test_kasan.c

@@ -869,11 +869,14 @@ static void kmem_cache_invalid_free(struct kunit *test)
 	kmem_cache_destroy(cache);
 }
 
+static void empty_cache_ctor(void *object) { }
+
 static void kmem_cache_double_destroy(struct kunit *test)
 {
 	struct kmem_cache *cache;
 
-	cache = kmem_cache_create("test_cache", 200, 0, 0, NULL);
+	/* Provide a constructor to prevent cache merging. */
+	cache = kmem_cache_create("test_cache", 200, 0, 0, empty_cache_ctor);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, cache);
 	kmem_cache_destroy(cache);
 	KUNIT_EXPECT_KASAN_FAIL(test, kmem_cache_destroy(cache));