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

[Steven Rostedt]

On Tue, 25 Aug 2026 17:34:41 +0800
Haotian Zhang <vulab@xxxxxxxxxxx> wrote:

> 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@xxxxxxxxxxx>
> ---
>  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;
>  }
>  

Please simplify the code to:

  	ftrace_set_filter_ip(&direct, (unsigned long) my_ip, 0, 0);
  	ret = register_ftrace_direct(&direct, my_tramp);
  
	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;
  }