netdev - Re: [PATCH iproute2] libbpf: fix attach of prog with multiple sections

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Date:   Thu, 22 Jul 2021 21:01:43 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Martynas Pumputis <m@...bda.lt>
Cc:     David Ahern <dsahern@...il.com>,
        Networking <netdev@...r.kernel.org>,
        Hangbin Liu <haliu@...hat.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH iproute2] libbpf: fix attach of prog with multiple sections

On Wed, Jul 21, 2021 at 8:25 AM Martynas Pumputis <m@...bda.lt> wrote:
>
>
>
> On 7/21/21 4:59 PM, David Ahern wrote:
> > On 7/21/21 8:47 AM, Martynas Pumputis wrote:
> >>>> diff --git a/lib/bpf_libbpf.c b/lib/bpf_libbpf.c
> >>>> index d05737a4..f76b90d2 100644
> >>>> --- a/lib/bpf_libbpf.c
> >>>> +++ b/lib/bpf_libbpf.c
> >>>> @@ -267,10 +267,12 @@ static int load_bpf_object(struct bpf_cfg_in *cfg)
> >>>>           }
> >>>>
> >>>>           bpf_object__for_each_program(p, obj) {
> >>>> +               bool prog_to_attach = !prog && cfg->section &&
> >>>> +                       !strcmp(get_bpf_program__section_name(p),
> >>>> cfg->section);
> >>>
> >>> This is still problematic, because one section can have multiple BPF
> >>> programs. I.e., it's possible two define two or more XDP BPF programs
> >>> all with SEC("xdp") and libbpf works just fine with that. I suggest
> >>> moving users to specify the program name (i.e., C function name
> >>> representing the BPF program). All the xdp_mycustom_suffix namings are
> >>> a hack and will be rejected by libbpf 1.0, so it would be great to get
> >>> a head start on fixing this early on.
> >>
> >> Thanks for bringing this up. Currently, there is no way to specify a
> >> function name with "tc exec bpf" (only a section name via the "sec"
> >> arg). So probably, we should just add another arg to specify the
> >> function name.
> >>
> >> It would be interesting to hear thoughts from iproute2 maintainers
> >> before fixing this.
> >
> > maintaining backwards compatibility is a core principle for iproute2. If
> > we know of a libbpf change is going to cause a breakage then it is best
> > to fix it before any iproute2 release is affected.
> >
>
> Just to avoid any confusion (if there is any), the required change we
> are discussing doesn't have anything to do with my fix.
>
> To set the context, the motivation for unifying section names is
> documented and discussed in "Stricter and more uniform BPF program
> section name (SEC()) handling" of [1].
>
> Andrii: is bpftool able to load programs with multiple sections which
> are named the same today?
>

I'm not familiar with those parts of bpftool, I've never used
bpftool's command to load BPF programs. Please go check the code.

>
> [1]:
> https://docs.google.com/document/d/1UyjTZuPFWiPFyKk1tV5an11_iaRuec6U-ZESZ54nNTY/edit#