Re: [PATCH] perf/bpf: Don't call bpf_overflow_handler() for tracing events

[Peter Zijlstra]

On Sat, Jul 13, 2024 at 10:32:07PM +0200, Jiri Olsa wrote:
> On Fri, Jul 12, 2024 at 09:46:45PM -0700, Kyle Huey wrote:
> > The regressing commit is new in 6.10. It assumed that anytime event->prog
> > is set bpf_overflow_handler() should be invoked to execute the attached bpf
> > program. This assumption is false for tracing events, and as a result the
> > regressing commit broke bpftrace by invoking the bpf handler with garbage
> > inputs on overflow.
> > 
> > Prior to the regression the overflow handlers formed a chain (of length 0,
> > 1, or 2) and perf_event_set_bpf_handler() (the !tracing case) added
> > bpf_overflow_handler() to that chain, while perf_event_attach_bpf_prog()
> > (the tracing case) did not. Both set event->prog. The chain of overflow
> > handlers was replaced by a single overflow handler slot and a fixed call to
> > bpf_overflow_handler() when appropriate. This modifies the condition there
> > to include !perf_event_is_tracing(), restoring the previous behavior and
> > fixing bpftrace.
> > 
> > Signed-off-by: Kyle Huey <khuey@xxxxxxxxxxxx>
> > Reported-by: Joe Damato <jdamato@xxxxxxxxxx>
> > Fixes: f11f10bfa1ca ("perf/bpf: Call BPF handler directly, not through overflow machinery")
> > Tested-by: Joe Damato <jdamato@xxxxxxxxxx> # bpftrace
> > Tested-by: Kyle Huey <khuey@xxxxxxxxxxxx> # bpf overflow handlers
> > ---
> >  kernel/events/core.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 8f908f077935..f0d7119585dc 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -9666,6 +9666,8 @@ static inline void perf_event_free_bpf_handler(struct perf_event *event)
> >   * Generic event overflow handling, sampling.
> >   */
> >  
> > +static bool perf_event_is_tracing(struct perf_event *event);
> > +
> >  static int __perf_event_overflow(struct perf_event *event,
> >  				 int throttle, struct perf_sample_data *data,
> >  				 struct pt_regs *regs)
> > @@ -9682,7 +9684,9 @@ static int __perf_event_overflow(struct perf_event *event,
> >  
> >  	ret = __perf_event_account_interrupt(event, throttle);
> >  
> > -	if (event->prog && !bpf_overflow_handler(event, data, regs))
> > +	if (event->prog &&
> > +	    !perf_event_is_tracing(event) &&
> > +	    !bpf_overflow_handler(event, data, regs))
> >  		return ret;
> 
> ok makes sense, it's better to follow the perf_event_set_bpf_prog condition
> 
> Reviewed-by: Jiri Olsa <jolsa@xxxxxxxxxx>

Urgh, so wth does event_is_tracing do with event->prog? And can't we
clean this up?

That whole perf_event_is_tracing() is a pretty gross function.

Also, I think the default return value of bpf_overflow_handler() is
wrong -- note how if !event->prog we won't call bpf_overflow_handler(),
but if we do call it, but then have !event->prog on the re-read, we
still return 0.