netdev - Re: [PATCH bpf-next v2] libbpf: fix probe code to return EPERM if encountered

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Date:   Tue, 5 May 2020 14:11:47 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Eelco Chaudron <echaudro@...hat.com>, bpf <bpf@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>, Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH bpf-next v2] libbpf: fix probe code to return EPERM if encountered

On Tue, May 5, 2020 at 3:07 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>
> > On Mon, May 4, 2020 at 2:13 AM Eelco Chaudron <echaudro@...hat.com> wrote:
> >>
> >> When the probe code was failing for any reason ENOTSUP was returned, even
> >> if this was due to no having enough lock space. This patch fixes this by
> >> returning EPERM to the user application, so it can respond and increase
> >> the RLIMIT_MEMLOCK size.
> >>
> >> Signed-off-by: Eelco Chaudron <echaudro@...hat.com>
> >> ---
> >> v2: Split bpf_object__probe_name() in two functions as suggested by Andrii
> >
> > Yeah, looks good, and this is good enough, so consider you have my
> > ack. But I think we can further improve the experience by:
> >
> > 1. Changing existing "Couldn't load basic 'r0 = 0' BPF program."
> > message to be something more meaningful and actionable for user. E.g.,
> >
> > "Couldn't load trivial BPF program. Make sure your kernel supports BPF
> > (CONFIG_BPF_SYSCALL=y) and/or that RLIMIT_MEMLOCK is set to big enough
> > value."
> >
> > Then even complete kernel newbies can search for CONFIG_BPF_SYSCALL or
> > RLIMIT_MEMLOCK and hopefully find useful discussions. We can/should
> > add RLIMIT_MEMLOCK examples to some FAQ, probably as well (if it's not
> > there already).
>
> Always on board with improving documentation; and yeah I agree that
> "Couldn't load basic 'r0 = 0' BPF program." could be a bit friendlier ;)
>
> > 2. I'd do bpf_object__probe_loading() before obj->loaded is set, so
> > that user can have a loop of bpf_object__load() that bump
> > RLIMIT_MEMLOCK in steps. After setting obj->loaded = true, user won't
> > be able to attemp loading again and will get "object should not be
> > loaded twice\n".
>
> In practice this is not going to be enough, though. The memlock error
> only triggers on initial load if the limit is already exceeded (by other
> BPF programs); but often what will happen is that the program being
> loaded will have a map definition that's big enough to exhaust the
> memlimit by itself. In which case the memlock error will trigger while
> creating maps, not on initial probe.
>
> Since you can't predict where the error will happen, you need to be
> prepared to close the bpf object and start over anyway, so I'm not sure
> it adds much value to move bpf_object__probe_loading() earlier?
>

True that. Ok, sounds fine to me (error message would be still nice to
improve, though).

> -Toke
>