revocable: Add KUnit test for provider lifetime races

Add a test to verify that revocable_alloc() correctly handles race
conditions where the provider is being released.

The test covers three scenarios:
1. Allocating from a NULL provider.
2. Allocating from a provider that has been detached (pointer is NULL).
3. Allocating from a provider that is in the process of destruction
   (refcount is 0), simulating a race between revocable_alloc() and
   revocable_provider_release().

A way to run the test:
$ ./tools/testing/kunit/kunit.py run \
	--kconfig_add CONFIG_REVOCABLE_KUNIT_TEST=y \
	--kconfig_add CONFIG_PROVE_LOCKING=y \
	--kconfig_add CONFIG_DEBUG_KERNEL=y \
	--kconfig_add CONFIG_DEBUG_INFO=y \
	--kconfig_add CONFIG_DEBUG_INFO_DWARF5=y \
	--kconfig_add CONFIG_KASAN=y \
	--kconfig_add CONFIG_DETECT_HUNG_TASK=y \
	--kconfig_add CONFIG_DEFAULT_HUNG_TASK_TIMEOUT="10" \
	--arch=x86_64 --raw_output=all \
	revocable_test

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://patch.msgid.link/20260129143733.45618-3-tzungbi@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Tzung-Bi Shih

drivers/base/revocable_test.c

@@ -14,9 +14,13 @@
  *
  * - Try Access Macro: Same as "Revocation" but uses the
  *   REVOCABLE_TRY_ACCESS_WITH() and REVOCABLE_TRY_ACCESS_SCOPED().
+ *
+ * - Provider Use-after-free: Verifies revocable_alloc() correctly handles
+ *   race conditions where the provider is being released.
  */
 
 #include <kunit/test.h>
+#include <linux/refcount.h>
 #include <linux/revocable.h>
 
 static void revocable_test_basic(struct kunit *test)
@@ -127,11 +131,48 @@ static void revocable_test_try_access_macro2(struct kunit *test)
 	revocable_free(rev);
 }
 
+static void revocable_test_provider_use_after_free(struct kunit *test)
+{
+	struct revocable_provider __rcu *rp;
+	struct revocable_provider *old_rp;
+	void *real_res = (void *)0x12345678;
+	struct revocable *rev;
+
+	rp = revocable_provider_alloc(real_res);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, rp);
+
+	rev = revocable_alloc(NULL);
+	KUNIT_EXPECT_PTR_EQ(test, rev, NULL);
+
+	/* Simulate the provider has been freed. */
+	old_rp = rcu_replace_pointer(rp, NULL, 1);
+	rev = revocable_alloc(rp);
+	KUNIT_EXPECT_PTR_EQ(test, rev, NULL);
+	rcu_replace_pointer(rp, old_rp, 1);
+
+	struct {
+		struct srcu_struct srcu;
+		void __rcu *res;
+		struct kref kref;
+		struct rcu_head rcu;
+	} *rp_internal = (void *)rp;
+
+	/* Simulate the provider is releasing. */
+	refcount_set(&rp_internal->kref.refcount, 0);
+	rev = revocable_alloc(rp);
+	KUNIT_EXPECT_PTR_EQ(test, rev, NULL);
+	refcount_set(&rp_internal->kref.refcount, 1);
+
+	revocable_provider_revoke(&rp);
+	KUNIT_EXPECT_PTR_EQ(test, unrcu_pointer(rp), NULL);
+}
+
 static struct kunit_case revocable_test_cases[] = {
 	KUNIT_CASE(revocable_test_basic),
 	KUNIT_CASE(revocable_test_revocation),
 	KUNIT_CASE(revocable_test_try_access_macro),
 	KUNIT_CASE(revocable_test_try_access_macro2),
+	KUNIT_CASE(revocable_test_provider_use_after_free),
 	{}
 };