Re: [PATCH bpf] bpf,perf: Fix perf_event_detach_bpf_prog error handling

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, Oct 23, 2024 at 09:01:02AM -0700, Andrii Nakryiko wrote:
> On Wed, Oct 23, 2024 at 3:01 AM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > Peter reported that perf_event_detach_bpf_prog might skip to release
> > the bpf program for -ENOENT error from bpf_prog_array_copy.
> >
> > This can't happen because bpf program is stored in perf event and is
> > detached and released only when perf event is freed.
> >
> > Let's make it obvious and add WARN_ON_ONCE on the -ENOENT check and
> > make sure the bpf program is released in any case.
> >
> > Cc: Sean Young <sean@mess.org>
> > Fixes: 170a7e3ea070 ("bpf: bpf_prog_array_copy() should return -ENOENT if exclude_prog not found")
> > Closes: https://lore.kernel.org/lkml/20241022111638.GC16066@noisy.programming.kicks-ass.net/
> > Reported-by: Peter Zijlstra <peterz@infradead.org>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  kernel/trace/bpf_trace.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 95b6b3b16bac..2c064ba7b0bd 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -2216,8 +2216,8 @@ void perf_event_detach_bpf_prog(struct perf_event *event)
> >
> >         old_array = bpf_event_rcu_dereference(event->tp_event->prog_array);
> >         ret = bpf_prog_array_copy(old_array, event->prog, NULL, 0, &new_array);
> > -       if (ret == -ENOENT)
> > -               goto unlock;
> > +       if (WARN_ON_ONCE(ret == -ENOENT))
> > +               goto put;
> >         if (ret < 0) {
> >                 bpf_prog_array_delete_safe(old_array, event->prog);
> 
> seeing
> 
> if (ret < 0)
>     bpf_prog_array_delete_safe(old_array, event->prog);
> 
> I think neither ret == -ENOENT nor WARN_ON_ONCE is necessary,  tbh. So
> now I feel like just dropping WARN_ON_ONCE() is better.

heh, I was going back and forth with that and decided with 'safer' option,
but it's 2 of you now asking for that, I'll send v2 then

jirka

> 
> >         } else {
> > @@ -2225,6 +2225,7 @@ void perf_event_detach_bpf_prog(struct perf_event *event)
> >                 bpf_prog_array_free_sleepable(old_array);
> >         }
> >
> > +put:
> >         bpf_prog_put(event->prog);
> >         event->prog = NULL;
> >
> > --
> > 2.46.2
> >