linux-kernel - Re: [PATCH] tracing: Ensure module defining synth event cannot be unloaded while tracing

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Message-Id: <20250319222953.3c741b520a8686c8e0a9b1d9@kernel.org>
Date: Wed, 19 Mar 2025 22:29:53 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Douglas RAILLARD <douglas.raillard@....com>
Cc: rostedt@...dmis.org, Masami Hiramatsu <mhiramat@...nel.org>, Mathieu
 Desnoyers <mathieu.desnoyers@...icios.com>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing: Ensure module defining synth event cannot be
 unloaded while tracing

On Tue, 18 Mar 2025 18:09:05 +0000
Douglas RAILLARD <douglas.raillard@....com> wrote:

> From: Douglas Raillard <douglas.raillard@....com>
> 
> Currently, using synth_event_delete() will fail if the event is being
> used (tracing in progress), but that is normally done in the module exit
> function. At that stage, failing is problematic as returning a non-zero
> status means the module will become locked (impossible to unload or
> reload again).
> 
> Instead, ensure the module exit function does not get called in the
> first place by increasing the module refcnt when the event is enabled.
> 

OK, I think this is correct way to fix that scenario.

Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thank you,

> Signed-off-by: Douglas Raillard <douglas.raillard@....com>
> ---
>  kernel/trace/trace_events_synth.c | 30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
> index e3f7d09e5512..bb7ef6e76991 100644
> --- a/kernel/trace/trace_events_synth.c
> +++ b/kernel/trace/trace_events_synth.c
> @@ -852,6 +852,34 @@ static struct trace_event_fields synth_event_fields_array[] = {
>  	{}
>  };
>  
> +static int synth_event_reg(struct trace_event_call *call,
> +		    enum trace_reg type, void *data)
> +{
> +	struct synth_event *event = container_of(call, struct synth_event, call);
> +
> +	switch (type) {
> +	case TRACE_REG_REGISTER:
> +	case TRACE_REG_PERF_REGISTER:
> +		if (!try_module_get(event->mod))
> +			return -EBUSY;
> +		break;
> +	default:
> +		break;
> +	}
> +
> +	int ret = trace_event_reg(call, type, data);
> +
> +	switch (type) {
> +	case TRACE_REG_UNREGISTER:
> +	case TRACE_REG_PERF_UNREGISTER:
> +		module_put(event->mod);
> +		break;
> +	default:
> +		break;
> +	}
> +	return ret;
> +}
> +
>  static int register_synth_event(struct synth_event *event)
>  {
>  	struct trace_event_call *call = &event->call;
> @@ -881,7 +909,7 @@ static int register_synth_event(struct synth_event *event)
>  		goto out;
>  	}
>  	call->flags = TRACE_EVENT_FL_TRACEPOINT;
> -	call->class->reg = trace_event_reg;
> +	call->class->reg = synth_event_reg;
>  	call->class->probe = trace_event_raw_event_synth;
>  	call->data = event;
>  	call->tp = event->tp;
> -- 
> 2.43.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>