Re: [PATCH bpf-next 09/10] libbpf: simplify look up by name of internal maps - Toke Høiland-Jørgensen

From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: andrii.nakryiko@gmail.com, bpf@vger.kernel.org, ast@kernel.org,
	daniel@iogearbox.net
Cc: andrii@kernel.org, kernel-team@fb.com,
	Stanislav Fomichev <sdf@google.com>
Subject: Re: [PATCH bpf-next 09/10] libbpf: simplify look up by name of internal maps
Date: Fri, 08 Oct 2021 19:30:59 +0200	[thread overview]
Message-ID: <87pmsfl8z0.fsf@toke.dk> (raw)
In-Reply-To: <20211008000309.43274-10-andrii@kernel.org>

andrii.nakryiko@gmail.com writes:

> From: Andrii Nakryiko <andrii@kernel.org>
>
> Map name that's assigned to internal maps (.rodata, .data, .bss, etc)
> consist of a small prefix of bpf_object's name and ELF section name as
> a suffix. This makes it hard for users to "guess" the name to use for
> looking up by name with bpf_object__find_map_by_name() API.
>
> One proposal was to drop object name prefix from the map name and just
> use ".rodata", ".data", etc, names. One downside called out was that
> when multiple BPF applications are active on the host, it will be hard
> to distinguish between multiple instances of .rodata and know which BPF
> object (app) they belong to. Having few first characters, while quite
> limiting, still can give a bit of a clue, in general.
>
> Another downside of such approach is that it is not backwards compatible
> and, among direct use of bpf_object__find_map_by_name() API, will break
> any BPF skeleton generated using bpftool that was compiled with older
> libbpf version.
>
> Instead of causing all this pain, libbpf will still generate map name
> using a combination of object name and ELF section name, but it will
> allow looking such maps up by their natural names, which correspond to
> their respective ELF section names. This means non-truncated ELF section
> names longer than 15 characters are going to be expected and supported.
>
> With such set up, we get the best of both worlds: leave small bits of
> a clue about BPF application that instantiated such maps, as well as
> making it easy for user apps to lookup such maps at runtime. In this
> sense it closes corresponding libbpf 1.0 issue ([0]).

I like this approach. Only possible problem I can see is that it might
be confusing that a map can be looked up with one name, but that it
disappears once it's loaded into the kernel (and the BPF object is
closed).

Hmm, couldn't we just extend the kernel to accept longer names? Kinda
like with the netdev name aliases: support a secondary label that can be
longer, and have bpftool display both?

-Toke