[PATCH 2/2] samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify

[Haotian Zhang <vulab@iscas.ac.cn>]

ftrace_direct_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_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: ae0cc3b7e7f5 ("ftrace/samples: Add a sample module that implements modify_ftrace_direct()")
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
---
 samples/ftrace/ftrace-direct-modify.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/samples/ftrace/ftrace-direct-modify.c b/samples/ftrace/ftrace-direct-modify.c
index da3a9f2091f5..2795ea12cdf9 100644
--- a/samples/ftrace/ftrace-direct-modify.c
+++ b/samples/ftrace/ftrace-direct-modify.c
@@ -320,8 +320,13 @@ static int __init ftrace_direct_init(void)
 	ftrace_set_filter_ip(&direct, (unsigned long) my_ip, 0, 0);
 	ret = register_ftrace_direct(&direct, my_tramp);
 
-	if (!ret)
+	if (!ret) {
 		simple_tsk = kthread_run(simple_thread, NULL, "event-sample-fn");
+		if (IS_ERR(simple_tsk)) {
+			unregister_ftrace_direct(&direct, my_tramp, true);
+			ret = PTR_ERR(simple_tsk);
+		}
+	}
 	return ret;
 }
 
-- 
2.43.0