kunit: make kunit_kfree() only work on pointers from kunit_malloc() and friends

kunit_kfree() exists to clean up allocations from kunit_kmalloc() and
friends early instead of waiting for this to happen automatically at the
end of the test.

But it can be used on *anything* registered with the kunit resource API.

E.g. the last 2 statements are equivalent:
struct kunit_resource *res = something();
kfree(res->data);
kunit_put_resource(res);

The problem is that there could be multiple resources that point to the
same `data`.

E.g. you can have a named resource acting as a pseudo-global variable in
a test. If you point it to data allocated with kunit_kmalloc(), then
calling `kunit_kfree(ptr)` has the chance to delete either the named
resource or to kfree `ptr`.
Which one it does depends on the order the resources are registered as
kunit_kfree() will delete resources in LIFO order.

So this patch restricts kunit_kfree() to only working on resources
created by kunit_kmalloc(). Calling it is therefore guaranteed to free
the memory, not do anything else.

Note: kunit_resource_instance_match() wasn't used outside of KUnit, so
it should be safe to remove from the public interface. It's also
generally dangerous, as shown above, and shouldn't be used.

Signed-off-by: Daniel Latypov <[e-mail]>
Reviewed-by: David Gow <[e-mail]>
Reviewed-by: Brendan Higgins <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

include/kunit/resource.h

···
 				       void *match_data);
 
 /**
- * kunit_resource_instance_match() - Match a resource with the same instance.
- * @test: Test case to which the resource belongs.
- * @res: The resource.
- * @match_data: The resource pointer to match against.
- *
- * An instance of kunit_resource_match_t that matches a resource whose
- * allocation matches @match_data.
- */
-static inline bool kunit_resource_instance_match(struct kunit *test,
-						 struct kunit_resource *res,
-						 void *match_data)
-{
-	return res->data == match_data;
-}
-
-/**
  * kunit_resource_name_match() - Match a resource with the same name.
  * @test: Test case to which the resource belongs.
  * @res: The resource.

lib/kunit/kunit-test.c

···
 	kunit_put_resource(res);
 }
 
+static inline bool kunit_resource_instance_match(struct kunit *test,
+						 struct kunit_resource *res,
+						 void *match_data)
+{
+	return res->data == match_data;
+}
+
 /*
  * Note: tests below use kunit_alloc_and_get_resource(), so as a consequence
  * they have a reference to the associated resource that they must release

lib/kunit/test.c

···
 }
 EXPORT_SYMBOL_GPL(kunit_kmalloc_array);
 
+static inline bool kunit_kfree_match(struct kunit *test,
+				     struct kunit_resource *res, void *match_data)
+{
+	/* Only match resources allocated with kunit_kmalloc() and friends. */
+	return res->free == kunit_kmalloc_array_free && res->data == match_data;
+}
+
 void kunit_kfree(struct kunit *test, const void *ptr)
 {
 	struct kunit_resource *res;
 
-	res = kunit_find_resource(test, kunit_resource_instance_match,
-				  (void *)ptr);
+	res = kunit_find_resource(test, kunit_kfree_match, (void *)ptr);
 
 	/*
 	 * Removing the resource from the list of resources drops the