netdev - Re: [PATCH bpf-next v3 3/8] libbpf: Add BPF_PROG_BIND_MAP syscall and use it on .metadata section

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Date:   Tue, 8 Sep 2020 11:10:40 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        YiFei Zhu <zhuyifei@...gle.com>,
        YiFei Zhu <zhuyifei1999@...il.com>,
        Andrey Ignatov <rdna@...com>
Subject: Re: [PATCH bpf-next v3 3/8] libbpf: Add BPF_PROG_BIND_MAP syscall and
 use it on .metadata section

On Mon, Sep 7, 2020 at 1:49 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>
> >> May be we should talk about problem statement and goals.
> >> Do we actually need metadata per program or metadata per single .o
> >> or metadata per final .o with multiple .o linked together?
> >> What is this metadata?
> >
> > Yep, that's a very valid question. I've also CC'ed Andrey.
>
> For the libxdp use case, I need metadata per program. But I'm already
> sticking that in a single section and disambiguating by struct name
> (just prefixing the function name with a _ ), so I think it's fine to
> have this kind of "concatenated metadata" per elf file and parse out the
> per-program information from that. This is similar to the BTF-encoded
> "metadata" we can do today.
>
> >> If it's just unreferenced by program read only data then no special names or
> >> prefixes are needed. We can introduce BPF_PROG_BIND_MAP to bind any map to any
> >> program and it would be up to tooling to decide the meaning of the data in the
> >> map. For example, bpftool can choose to print all variables from all read only
> >> maps that match "bpf_metadata_" prefix, but it will be bpftool convention only
> >> and not hard coded in libbpf.
> >
> > Agree as well. It feels a bit odd for libbpf to handle ".metadata"
> > specially, given libbpf itself doesn't care about its contents at all.
> >
> > So thanks for bringing this up, I think this is an important
> > discussion to have.
>
> I'm fine with having this be part of .rodata. One drawback, though, is
> that if any metadata is defined, it becomes a bit more complicated to
> use bpf_map__set_initial_value() because that now also has to include
> the metadata. Any way we can improve upon that?

I know that skeleton is not an answer for you, so you'll have to find
DATASEC and corresponding variable offset and size (libbpf provides
APIs for all those operations, but you'll need to combine them
together). Then mmap() map and then you can do partial updates. There
is no other way to update only portions of an ARRAY map, except
through memory-mapping.

>
> -Toke
>