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[94.113.247.30]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a780a6bcc52sm379188166b.4.2024.07.12.15.39.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 12 Jul 2024 15:39:30 -0700 (PDT) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Sat, 13 Jul 2024 00:39:28 +0200 To: Jiri Olsa Cc: Joe Damato , Kyle Huey , linux-kernel@vger.kernel.org, bpf@vger.kernel.org, netdev@vger.kernel.org, acme@kernel.org, andrii.nakryiko@gmail.com, elver@google.com, khuey@kylehuey.com, mingo@kernel.org, namhyung@kernel.org, peterz@infradead.org, robert@ocallahan.org, yonghong.song@linux.dev, mkarsten@uwaterloo.ca, kuba@kernel.org, Viktor Malik Subject: Re: [bpf?] [net-next ?] [RESEND] possible bpf overflow/output bug introduced in 6.10rc1 ? Message-ID: References: Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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); > }