netdev - Re: [PATCH v4 bpf-next 2/4] libbpf: support libbpf-provided extern variables

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Date:   Mon, 16 Dec 2019 10:19:40 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v4 bpf-next 2/4] libbpf: support libbpf-provided extern variables

On Mon, Dec 16, 2019 at 4:43 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On Fri, Dec 13, 2019 at 05:47:08PM -0800, Andrii Nakryiko wrote:
> [...]
> > Config file itself is searched in /boot/config-$(uname -r) location with
> > fallback to /proc/config.gz, unless config path is specified explicitly
> > through bpf_object_open_opts' kernel_config_path option. Both gzipped and
> > plain text formats are supported. Libbpf adds explicit dependency on zlib
> > because of this, but this shouldn't be a problem, given libelf already depends
> > on zlib.
>
> Hm, given this seems to break the build and is not an essential feature,
> can't we use the feature detection from tooling infra which you invoke
> anyway to compile out bpf_object__read_kernel_config() internals and return
> an error there? Build could warn perf-style what won't be available for
> the user in that case.
>
> https://patchwork.ozlabs.org/patch/1210213/

libz is a dependency of libelf, so this doesn't really add any new
dependencies. Everywhere where libbpf could be built, both libelf and
libz should be present already. Unfortunately now that libz is
directly used by libbpf, though, it needs to be specified explicitly
in compiler invocation, which I missed for samples/bpf, sorry about
that.

>
> Also, does libbpf.pc.template need updating wrt zlib?

Yeah, wasn't aware of it, will post a follow-up patch adding -lz there. Thanks!