netdev - Re: [PATCH v4 bpf-next 2/4] bpf: fail PERF_EVENT_IOC_SET_BPF when bpf_get

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Message-ID: <CAADnVQK+xX8oKF5f=FzmE+xxbSovJ+rbZD6TRxTAtdH+-ockEw@mail.gmail.com>
Date:   Thu, 23 Jul 2020 08:22:19 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>
Subject: Re: [PATCH v4 bpf-next 2/4] bpf: fail PERF_EVENT_IOC_SET_BPF when
 bpf_get_[stack|stackid] cannot work

On Wed, Jul 22, 2020 at 11:20 PM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Jul 22, 2020, at 10:55 PM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> >
> > On Wed, Jul 22, 2020 at 11:42:08AM -0700, Song Liu wrote:
> >> diff --git a/kernel/events/core.c b/kernel/events/core.c
> >> index 856d98c36f562..f77d009fcce95 100644
> >> --- a/kernel/events/core.c
> >> +++ b/kernel/events/core.c
> >> @@ -9544,6 +9544,24 @@ static int perf_event_set_bpf_handler(struct perf_event *event, u32 prog_fd)
> >>      if (IS_ERR(prog))
> >>              return PTR_ERR(prog);
> >>
> >> +    if (event->attr.precise_ip &&
> >> +        prog->call_get_stack &&
> >> +        (!(event->attr.sample_type & __PERF_SAMPLE_CALLCHAIN_EARLY) ||
> >> +         event->attr.exclude_callchain_kernel ||
> >> +         event->attr.exclude_callchain_user)) {
> >> +            /*
> >> +             * On perf_event with precise_ip, calling bpf_get_stack()
> >> +             * may trigger unwinder warnings and occasional crashes.
> >> +             * bpf_get_[stack|stackid] works around this issue by using
> >> +             * callchain attached to perf_sample_data. If the
> >> +             * perf_event does not full (kernel and user) callchain
> >> +             * attached to perf_sample_data, do not allow attaching BPF
> >> +             * program that calls bpf_get_[stack|stackid].
> >> +             */
> >> +            bpf_prog_put(prog);
> >> +            return -EINVAL;
> >
> > I suspect this will be a common error. bpftrace and others will be hitting
> > this issue and would need to fix how they do perf_event_open.
> > But EINVAL is too ambiguous and sys_perf_event_open has no ability to
> > return a string.
> > So how about we pick some different errno here to make future debugging
> > a bit less painful?
> > May be EBADFD or EPROTO or EPROTOTYPE ?
> > I think anything would be better than EINVAL.
>
> I like EPROTO most. I will change it to EPROTO if we don't have better ideas.
>
> Btw, this is not the error code on sys_perf_event_open(). It is the ioctl()
> on the perf_event fd. So debugging this error will be less painful than
> debugging sys_perf_event_open() errors.

ahh. right. Could you also add a string hint to libbpf when it sees this errno?