[PATCH v2] samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify

[Haotian Zhang]

ftrace_direct_multi_init() assigns kthread_run()'s return value to
simple_tsk without an IS_ERR() check. When kthread_run() fails it
returns ERR_PTR(-ENOMEM), but init still returns 0, so the module loads
with simple_tsk holding an error pointer. On unload,
ftrace_direct_multi_exit() then passes that ERR_PTR to kthread_stop(),
leading to a null-pointer-dereference.

Check the return value of kthread_run() with IS_ERR(); on failure,
unregister the ftrace direct call and propagate the error code.

Fixes: e1067a07cfbc ("ftrace/samples: Add module to test multi direct modify interface")
Suggested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Haotian Zhang <vulab@xxxxxxxxxxx>
---
Changes in v2:
  - Simplify the code as suggested by Steven Rostedt.
---
 samples/ftrace/ftrace-direct-multi-modify.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/samples/ftrace/ftrace-direct-multi-modify.c b/samples/ftrace/ftrace-direct-multi-modify.c
index 8f7986d698d8..70c2263d839c 100644
--- a/samples/ftrace/ftrace-direct-multi-modify.c
+++ b/samples/ftrace/ftrace-direct-multi-modify.c
@@ -364,9 +364,15 @@ static int __init ftrace_direct_multi_init(void)
 
 	ret = register_ftrace_direct(&direct, my_tramp);
 
-	if (!ret)
-		simple_tsk = kthread_run(simple_thread, NULL, "event-sample-fn");
-	return ret;
+	if (ret)
+		return ret;
+	simple_tsk = kthread_run(simple_thread, NULL, "event-sample-fn");
+	if (IS_ERR(simple_tsk)) {
+		unregister_ftrace_direct(&direct, my_tramp, true);
+		return PTR_ERR(simple_tsk);
+	}
+
+	return 0;
 }
 
 static void __exit ftrace_direct_multi_exit(void)
-- 
2.43.0