Re: [RFC][PATCH 3/4] tracing/kprobes: Fail to unregister if probeevent files are open

[Masami Hiramatsu]

(2013/07/04 12:33), Steven Rostedt wrote:
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index 7ed6976..ffcaf42 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -95,7 +95,7 @@ static __kprobes bool trace_probe_is_on_module(struct trace_probe *tp)
>  }
>  
>  static int register_probe_event(struct trace_probe *tp);
> -static void unregister_probe_event(struct trace_probe *tp);
> +static int unregister_probe_event(struct trace_probe *tp);
>  
>  static DEFINE_MUTEX(probe_lock);
>  static LIST_HEAD(probe_list);
> @@ -340,9 +340,12 @@ static int unregister_trace_probe(struct trace_probe *tp)
>  	if (trace_probe_is_enabled(tp))
>  		return -EBUSY;
>  
> +	/* Will fail if probe is being used by ftrace or perf */
> +	if (unregister_probe_event(tp))
> +		return -EBUSY;
> +
>  	__unregister_trace_probe(tp);
>  	list_del(&tp->list);
> -	unregister_probe_event(tp);
>  
>  	return 0;
>  }

This may cause an unexpected access violation at kprobe handler because
unregister_probe_event frees event_call/event_files and it will be
accessed until kprobe is *completely* disabled.
Actually disable_kprobe() doesn't ensure to finish the current running
kprobe handlers. Thus, even if trace_probe_is_enabled() returns false,
we must do synchronize_sched() for waiting, before unregister_probe_event().
OTOH, unregister_kprobe() waits for that. That's why I do
__unregister_trace_probe(tp) first here.

I think there are 2 solutions, one is adding a wait after disable_k*probe
at disable_trace_probe(), another is adding a wait before unregister_probe_event()
at unregister_trace_probe().

Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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