[PATCHv2 1/1] lib/tests: test_ratelimit: fix stress test thread lifecycle and leak

[Jia He <justin.he@arm.com>]

The stress test checks the kthread pointer in the child thread, but the
thread was created with kthread_run(). Since kthread_run() wakes the
thread before returning, the child can run before sktp[i].tp is assigned,
triggering the WARN_ON_ONCE() check on an arm64 N2 server with 128 cores.

Use kthread_create() instead, assign sktp[i].tp after successful
creation, and then wake the thread explicitly. This guarantees that the
child observes an initialized task pointer.

Also add a common cleanup path for thread creation failures. If creating
one of the later threads fails, stop all threads that were already
started and free the allocated array instead of leaving orphan kthreads
and leaked memory behind.

Finally, replace the module-static doneflag with kthread_should_stop().
With the doneflag, child threads may exit before the parent calls
kthread_stop(), so the task lifetime is no longer guaranteed when the
parent later tries to stop them. Using kthread_should_stop() keeps each
child alive until kthread_stop() synchronously terminates it.

Signed-off-by: Jia He <justin.he@arm.com>
---
v2: replace the module-static doneflag with kthread_should_stop() to avoid
UAF (sashiko-bot)
 lib/tests/test_ratelimit.c | 37 ++++++++++++++++++++++++++-----------
 1 file changed, 26 insertions(+), 11 deletions(-)

diff --git a/lib/tests/test_ratelimit.c b/lib/tests/test_ratelimit.c
index 33cea5f3d28b..d95c49e914c7 100644
--- a/lib/tests/test_ratelimit.c
+++ b/lib/tests/test_ratelimit.c
@@ -68,7 +68,6 @@ static void test_ratelimit_smoke(struct kunit *test)
 static struct ratelimit_state stressrl = RATELIMIT_STATE_INIT_FLAGS("stressrl", HZ / 10, 3,
 								    RATELIMIT_MSG_ON_RELEASE);
 
-static int doneflag;
 static const int stress_duration = 2 * HZ;
 
 struct stress_kthread {
@@ -86,7 +85,7 @@ static int test_ratelimit_stress_child(void *arg)
 	set_user_nice(current, MAX_NICE);
 	WARN_ON_ONCE(!sktp->tp);
 
-	while (!READ_ONCE(doneflag)) {
+	while (!kthread_should_stop()) {
 		sktp->nattempts++;
 		if (___ratelimit(&stressrl, __func__))
 			sktp->nunlimited++;
@@ -105,26 +104,42 @@ static void test_ratelimit_stress(struct kunit *test)
 	const int n_stress_kthread = cpumask_weight(cpu_online_mask);
 	struct stress_kthread skt = { 0 };
 	struct stress_kthread *sktp = kzalloc_objs(*sktp, n_stress_kthread);
+	int n_started = 0;
 
-	KUNIT_EXPECT_NOT_NULL_MSG(test, sktp, "Memory allocation failure");
+	KUNIT_ASSERT_NOT_NULL_MSG(test, sktp, "Memory allocation failure");
 	for (i = 0; i < n_stress_kthread; i++) {
-		sktp[i].tp = kthread_run(test_ratelimit_stress_child, &sktp[i], "%s/%i",
-					 "test_ratelimit_stress_child", i);
-		KUNIT_EXPECT_NOT_NULL_MSG(test, sktp, "kthread creation failure");
+		struct task_struct *tp;
+
+		tp = kthread_create(test_ratelimit_stress_child, &sktp[i],
+				    "%s/%i", "test_ratelimit_stress_child", i);
+		if (IS_ERR(tp)) {
+			KUNIT_FAIL(test, "kthread_create failed: %ld", PTR_ERR(tp));
+			goto out_stop;
+		}
+
+		sktp[i].tp = tp;
+		wake_up_process(tp);
+		n_started++;
 		pr_alert("Spawned test_ratelimit_stress_child %d\n", i);
 	}
 	schedule_timeout_idle(stress_duration);
-	WRITE_ONCE(doneflag, 1);
-	for (i = 0; i < n_stress_kthread; i++) {
+
+out_stop:
+	for (i = 0; i < n_started; i++) {
 		kthread_stop(sktp[i].tp);
 		skt.nattempts += sktp[i].nattempts;
 		skt.nunlimited += sktp[i].nunlimited;
 		skt.nlimited += sktp[i].nlimited;
 		skt.nmissed += sktp[i].nmissed;
 	}
-	KUNIT_ASSERT_EQ_MSG(test, skt.nunlimited + skt.nlimited, skt.nattempts,
-			    "Outcomes not equal to attempts");
-	KUNIT_ASSERT_EQ_MSG(test, skt.nlimited, skt.nmissed, "Misses not equal to limits");
+	if (n_started == n_stress_kthread) {
+		KUNIT_ASSERT_EQ_MSG(test, skt.nunlimited + skt.nlimited, skt.nattempts,
+				    "Outcomes not equal to attempts");
+		KUNIT_ASSERT_EQ_MSG(test, skt.nlimited, skt.nmissed,
+				    "Misses not equal to limits");
+	}
+
+	kfree(sktp);
 }
 
 static struct kunit_case ratelimit_test_cases[] = {
-- 
2.34.1