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

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Message-ID: <a68f1e05-e9c7-595e-23e9-6f02a3a209de@lambda.lt>
Date:   Wed, 21 Jul 2021 17:27:29 +0200
From:   Martynas Pumputis <m@...bda.lt>
To:     David Ahern <dsahern@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     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 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?


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