Re: perf_event_detach_bpf_prog() broken?

Re: perf_event_detach_bpf_prog() broken?

[Andrii Nakryiko]

+ bpf ML

On Tue, Oct 22, 2024 at 7:03 AM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Tue, Oct 22, 2024 at 01:16:38PM +0200, Peter Zijlstra wrote:
> > Hi guys,
> >
> > Per commit 170a7e3ea070 ("bpf: bpf_prog_array_copy() should return
> > -ENOENT if exclude_prog not found") perf_event_detach_bpf_prog() can now
> > return without doing bpf_prog_put() and leaving event->prog set.
> >
> > This is very 'unexpected' behaviour.
> >
> > I'm not sure what's sane from the BPF side of things here, but leaving
> > event->prog set is really rather unexpected.
> >
> > Help?
>
> IIUC the ENOENT should never happen in perf event context, so not

yep, if it does return an error it's a bug, right? So we can add
WARN_ONCE() or just drop the check, probably.

> sure why we have that check.. also does not seem to be used from
> lirc code, Sean?
>
> perf_event_detach_bpf_prog is called when the event is being freed
> so I think we should always put and clear the event->prog
>
> jirka

Re: perf_event_detach_bpf_prog() broken?

[Jiri Olsa]

On Tue, Oct 22, 2024 at 10:33:37AM -0700, Andrii Nakryiko wrote:
> + bpf ML
> 
> On Tue, Oct 22, 2024 at 7:03 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> >
> > On Tue, Oct 22, 2024 at 01:16:38PM +0200, Peter Zijlstra wrote:
> > > Hi guys,
> > >
> > > Per commit 170a7e3ea070 ("bpf: bpf_prog_array_copy() should return
> > > -ENOENT if exclude_prog not found") perf_event_detach_bpf_prog() can now
> > > return without doing bpf_prog_put() and leaving event->prog set.
> > >
> > > This is very 'unexpected' behaviour.
> > >
> > > I'm not sure what's sane from the BPF side of things here, but leaving
> > > event->prog set is really rather unexpected.
> > >
> > > Help?
> >
> > IIUC the ENOENT should never happen in perf event context, so not
> 
> yep, if it does return an error it's a bug, right? So we can add
> WARN_ONCE() or just drop the check, probably.

I'm now more inclined to have the WARN there, because it's possible
return value of bpf_prog_array_copy .. I'll send the patch and let's
discuss over the change

jirka

> 
> > sure why we have that check.. also does not seem to be used from
> > lirc code, Sean?
> >
> > perf_event_detach_bpf_prog is called when the event is being freed
> > so I think we should always put and clear the event->prog
> >
> > jirka