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

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

On Sat, 10 May 2025 07:44:56 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> 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.

Oops, if user writes some commands not directly from dynevent,
(e.g. tracefs/{kprobe_events,uprobe_events} )the mutex is not held.
Hmm, we need to make another patch.

> 
> 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.

But this is still valid. We need to reroute the creation to
dynamic_events.

Thank you,

> 
> 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>


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