Re: [PATCH v2 2/2] tracing: protect trace_probe_log with mutex

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Fri, 02 May 2025 15:15:53 +0200
Paul Cacheux via B4 Relay <devnull+paulcacheux.gmail.com@kernel.org> wrote:

> From: Paul Cacheux <paulcacheux@gmail.com>
> 
> The shared trace_probe_log variable can be accessed and modified
> by multiple processes using tracefs at the same time, this new
> mutex will guarantee it's always in a coherent state.

Actually that's not happen. For the create part (and currently the
event_log is used only in the creation), `dyn_event_ops_mutex` is already
held in create_dyn_event(). Thus, it is already serialized.

And even if there are multiple events are done in parallel, this
is not enough because trace_probe_log::argc/argv can be changed
before other user finishs to use the log.

Thank you,

> 
> There is no guarantee that multiple errors happening at the same
> time will each have the correct error message, but at least this
> won't crash.
> 
> Fixes: ab105a4fb894 ("tracing: Use tracing error_log with probe events")
> 
> Signed-off-by: Paul Cacheux <paulcacheux@gmail.com>
> ---
>  kernel/trace/trace_probe.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
> index 2eeecb6c95eea55502b83af6775b7b6f0cc5ab94..14a7a0b59cd20a8bc43e3e7c653e986081f924c8 100644
> --- a/kernel/trace/trace_probe.c
> +++ b/kernel/trace/trace_probe.c
> @@ -154,9 +154,11 @@ static const struct fetch_type *find_fetch_type(const char *type, unsigned long
>  }
>  
>  static struct trace_probe_log trace_probe_log;
> +static DEFINE_MUTEX(trace_probe_log_lock);
>  
>  void trace_probe_log_init(const char *subsystem, int argc, const char **argv)
>  {
> +	guard(mutex)(&trace_probe_log_lock);
>  	trace_probe_log.subsystem = subsystem;
>  	trace_probe_log.argc = argc;
>  	trace_probe_log.argv = argv;
> @@ -165,11 +167,13 @@ void trace_probe_log_init(const char *subsystem, int argc, const char **argv)
>  
>  void trace_probe_log_clear(void)
>  {
> +	guard(mutex)(&trace_probe_log_lock);
>  	memset(&trace_probe_log, 0, sizeof(trace_probe_log));
>  }
>  
>  void trace_probe_log_set_index(int index)
>  {
> +	guard(mutex)(&trace_probe_log_lock);
>  	trace_probe_log.index = index;
>  }
>  
> @@ -178,6 +182,8 @@ void __trace_probe_log_err(int offset, int err_type)
>  	char *command, *p;
>  	int i, len = 0, pos = 0;
>  
> +	guard(mutex)(&trace_probe_log_lock);
> +
>  	if (!trace_probe_log.argv)
>  		return;
>  
> 
> -- 
> 2.49.0
> 
> 


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