linux-kernel - Re: [PATCH] kernel/trace: Fix cleanup logic of enable_trace

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Date:   Thu, 29 Jun 2023 17:31:24 +0300
From:   Tzvetomir Stoyanov <tz.stoyanov@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mhiramat@...nel.org, dan.carpenter@...aro.org,
        linux-trace-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/trace: Fix cleanup logic of enable_trace_eprobe

On Wed, Jun 28, 2023 at 3:44 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 28 Jun 2023 15:18:11 +0300
> "Tzvetomir Stoyanov (VMware)" <tz.stoyanov@...il.com> wrote:
>
> > The enable_trace_eprobe() function enables all event probes, attached
> > to given trace probe. If an error occurs in enabling one of the event
> > probes, all others should be roll backed. There is a bug in that roll
> > back logic - instead of all event probes, only the failed one is
> > disabled.
> >
> > Reported-by: Dan Carpenter <dan.carpenter@...aro.org>
> > Fixes: 7491e2c44278 ("tracing: Add a probe that attaches to trace events")
> > Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@...il.com>
> > ---
> >  kernel/trace/trace_eprobe.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/trace/trace_eprobe.c b/kernel/trace/trace_eprobe.c
> > index 67e854979d53..ba9a28bc773f 100644
> > --- a/kernel/trace/trace_eprobe.c
> > +++ b/kernel/trace/trace_eprobe.c
> > @@ -702,8 +702,12 @@ static int enable_trace_eprobe(struct trace_event_call *call,
> >
> >       if (ret) {
> >               /* Failed to enable one of them. Roll back all */
> > -             if (enabled)
> > -                     disable_eprobe(ep, file->tr);
> > +             if (enabled) {
>
> If one was enabled and the second one failed, that should only happen
> if there's a bug in the kernel (unless the failure was due to a memory
> problem).
>
> I wonder if we should add:
>
>                         int cnt = 0;
>
> > +                     list_for_each_entry(pos, trace_probe_probe_list(tp), list) {
>
>                                 /*
>                                  * It's a bug if one failed for something other than memory
>                                  * not being available but another eprobe succeeded.
>                                  */
>                                 WARN_ON_ONCE(cnt++ && ret != -ENOMEM);

That makes sense, I can send v2 with it. What is the idea of this cnt
counter, why not just:
                                 WARN_ON_ONCE(ret != -ENOMEM);
outside of the loop? If enabled is true and ret is not ENOMEM, the bug
is already there.

>
> -- Steve
>
>
> > +                             ep = container_of(pos, struct trace_eprobe, tp);
> > +                             disable_eprobe(ep, file->tr);
> > +                     }
> > +             }
> >               if (file)
> >                       trace_probe_remove_file(tp, file);
> >               else
>


-- 
Tzvetomir (Ceco) Stoyanov
VMware Open Source Technology Center