Re: [RFC] fprobe call of rethook_try_get faults

[Jiri Olsa]

On Tue, Jun 13, 2023 at 05:48:44PM -0400, Steven Rostedt wrote:
> On Wed, 7 Jun 2023 09:42:30 -0700
> Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> 
> > I can't really reliable reproduce this, but while checking the code, I wonder
> > we should call rethook_free only after we call unregister_ftrace_function like
> > in the patch below
> 
> Yeah, I think you're right!
> 
> > 
> > jirka
> > 
> > 
> > ---
> > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> > index 18d36842faf5..0121e8c0d54e 100644
> > --- a/kernel/trace/fprobe.c
> > +++ b/kernel/trace/fprobe.c
> > @@ -364,19 +364,13 @@ int unregister_fprobe(struct fprobe *fp)
> >  		    fp->ops.saved_func != fprobe_kprobe_handler))
> >  		return -EINVAL;
> >  
> > -	/*
> > -	 * rethook_free() starts disabling the rethook, but the rethook handlers
> > -	 * may be running on other processors at this point. To make sure that all
> > -	 * current running handlers are finished, call unregister_ftrace_function()
> > -	 * after this.
> > -	 */
> > -	if (fp->rethook)
> > -		rethook_free(fp->rethook);
> 
> The above only waits for RCU to finish and then starts to free rethook.
> 
> This also means that something could be on the trampoline already and was
> preempted. It could be that this code path gets preempted. Anyway, I don't
> see how freeing rethook is safe before disabling all users.
> 
> Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

thanks, I'll send formal patch

jirka

> 
> -- Steve
> 
> 
> > -
> >  	ret = unregister_ftrace_function(&fp->ops);
> >  	if (ret < 0)
> >  		return ret;
> >  
> > +	if (fp->rethook)
> > +		rethook_free(fp->rethook);
> > +
> >  	ftrace_free_filter(&fp->ops);
> >  
> >  	return ret;
>