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

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Message-ID: <cce56767-efbe-e572-6290-111c6c845578@gmail.com>
Date:   Mon, 26 Jul 2021 07:58:28 -0600
From:   David Ahern <dsahern@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...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 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.