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

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Date:   Mon, 26 Jul 2021 08:13:39 -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 Mon, Jul 26, 2021 at 6:58 AM David Ahern <dsahern@...il.com> wrote:
>
> On 7/23/21 6:25 PM, Andrii Nakryiko wrote:
> >>>>>> 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.
> >
>
> Does that require BTF? My attempts at loading an object file with 2
> SEC("xdp") programs failed. This is using bpftool from top of tree and
> loadall.

You mean kernel BTF? Not if XDP programs themselves were built
requiring CO-RE. So if those programs use #include "vmlinux.h", or
there is BPF_CORE_READ() use somewhere in the code, or explicit
__attribute__((preserve_access_index)) is used on some of the used
structs, then yes, vmlinux BTF will be needed. But otherwise no. Do
you have verbose error logs? I think with bpftool you can get them
with -d argument.