Re: [bpf?] [net-next ?] [RESEND] possible bpf overflow/output bug introduced in 6.10rc1 ?

[Jiri Olsa]

On Sat, Jul 13, 2024 at 12:18:26AM +0200, Jiri Olsa wrote:
> On Fri, Jul 12, 2024 at 09:53:53AM -0700, Joe Damato wrote:
> > Greetings:
> > 
> > (I am reposting this question after 2 days and to a wider audience
> > as I didn't hear back [1]; my apologies it just seemed like a
> > possible bug slipped into 6.10-rc1 and I wanted to bring attention
> > to it before 6.10 is released.)
> > 
> > While testing some unrelated networking code with Martin Karsten (cc'd on
> > this email) we discovered what appears to be some sort of overflow bug in
> > bpf.
> > 
> > git bisect suggests that commit f11f10bfa1ca ("perf/bpf: Call BPF handler
> > directly, not through overflow machinery") is the first commit where the
> > (I assume) buggy behavior appears.
> 
> heya, nice catch!
> 
> I can reproduce.. it seems that after f11f10bfa1ca we allow to run tracepoint
> program as perf event overflow program 
> 
> bpftrace's bpf program returns 1 which means that perf_trace_run_bpf_submit
> will continue to execute perf_tp_event and then:

also bpftrace should perhaps return 0 in tracepoint programs
and cut the extra processing in any case

cc-ing Viktor

jirka


> 
>   perf_tp_event
>     perf_swevent_event
>       __perf_event_overflow
>         bpf_overflow_handler
> 
> bpf_overflow_handler then executes event->prog on wrong arguments, which
> results in wrong 'work' data in bpftrace output
> 
> I can 'fix' that by checking the event type before running the program like
> in the change below, but I wonder there's probably better fix
> 
> Kyle, any idea?
> 
> > 
> > Running the following on my machine as of the commit mentioned above:
> > 
> >   bpftrace -e 'tracepoint:napi:napi_poll { @[args->work] = count(); }'
> > 
> > while simultaneously transferring data to the target machine (in my case, I
> > scp'd a 100MiB file of zeros in a loop) results in very strange output
> > (snipped):
> > 
> >   @[11]: 5
> >   @[18]: 5
> >   @[-30590]: 6
> >   @[10]: 7
> >   @[14]: 9
> > 
> > It does not seem that the driver I am using on my test system (mlx5) would
> > ever return a negative value from its napi poll function and likewise for
> > the driver Martin is using (mlx4).
> > 
> > As such, I don't think it is possible for args->work to ever be a large
> > negative number, but perhaps I am misunderstanding something?
> > 
> > I would like to note that commit 14e40a9578b7 ("perf/bpf: Remove #ifdef
> > CONFIG_BPF_SYSCALL from struct perf_event members") does not exhibit this
> > behavior and the output seems reasonable on my test system. Martin confirms
> > the same for both commits on his test system, which uses different hardware
> > than mine.
> > 
> > Is this an expected side effect of this change? I would expect it is not
> > and that the output is a bug of some sort. My apologies in that I am not
> > particularly familiar with the bpf code and cannot suggest what the root
> > cause might be.
> > 
> > If it is not a bug:
> >   1. Sorry for the noise :(
> 
> your report is great, thanks a lot!
> 
> jirka
> 
> 
> >   2. Can anyone suggest what this output might mean or how the
> >      script run above should be modified? AFAIK this is a fairly
> >      common bpftrace that many folks run for profiling/debugging
> >      purposes.
> > 
> > Thanks,
> > Joe
> > 
> > [1]: https://lore.kernel.org/bpf/Zo64cpho2cFQiOeE@LQ3V64L9R2/T/#u
> 
> ---
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index c6a6936183d5..0045dc754ef7 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -9580,7 +9580,7 @@ static int bpf_overflow_handler(struct perf_event *event,
>  		goto out;
>  	rcu_read_lock();
>  	prog = READ_ONCE(event->prog);
> -	if (prog) {
> +	if (prog && prog->type == BPF_PROG_TYPE_PERF_EVENT) {
>  		perf_prepare_sample(data, event, regs);
>  		ret = bpf_prog_run(prog, &ctx);
>  	}