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

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Date:   Fri, 23 Jul 2021 17:25:25 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     David Ahern <dsahern@...il.com>
Cc:     Hangbin Liu <haliu@...hat.com>, Martynas Pumputis <m@...bda.lt>,
        Networking <netdev@...r.kernel.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH iproute2] libbpf: fix attach of prog with multiple sections

On Fri, Jul 23, 2021 at 5:12 PM David Ahern <dsahern@...il.com> wrote:
>
> On 7/22/21 10:51 PM, Andrii Nakryiko wrote:
> > On Thu, Jul 22, 2021 at 9:41 PM Hangbin Liu <haliu@...hat.com> wrote:
> >>
> >> On Wed, Jul 21, 2021 at 04:47:14PM +0200, 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.
> >>
> >> How about add a "prog" arg to load specified program name and mark
> >> "sec" as not recommended? To keep backwards compatibility we just load the
> >> first program in the section.
> >
> > Why not error out if there is more than one program with the same
> > section name? if there is just one (and thus section name is still
> > unique) -- then proceed. It seems much less confusing, IMO.
> >
>
> Let' see if I understand this correctly: libbpf 1.0 is not going to
> allow SEC("xdp_foo") or SEC("xdp_bar") kind of section names - which is
> the hint for libbpf to know program type. Instead only SEC("xdp") is
> allowed.

Right.

>
> Further, a single object file is not going to be allowed to have
> multiple SEC("xdp") instances for each program name.

On the contrary. Libbpf already allows (and will keep allowing)
multiple BPF programs with SEC("xdp") in a single object file. Which
is why section_name is not a unique program identifier.

>
> Correct? If so, it seems like this is limiting each object file to a
> single XDP program or a single object file can have 1 XDP program and 1
> tc program.