netdev - Re: [RFC bpf-next 0/5] Convert iproute2 to use libbpf (WIP)

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Date:   Tue, 4 Feb 2020 10:47:58 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     David Ahern <dsahern@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        David Miller <davem@...emloft.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [RFC bpf-next 0/5] Convert iproute2 to use libbpf (WIP)

On Tue, Feb 4, 2020 at 12:25 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>
> > On Mon, Feb 3, 2020 at 8:53 PM David Ahern <dsahern@...il.com> wrote:
> >>
> >> On 2/3/20 8:41 PM, Andrii Nakryiko wrote:
> >> > On Mon, Feb 3, 2020 at 5:46 PM David Ahern <dsahern@...il.com> wrote:
> >> >>
> >> >> On 2/3/20 5:56 PM, Andrii Nakryiko wrote:
> >> >>> Great! Just to disambiguate and make sure we are in agreement, my hope
> >> >>> here is that iproute2 can completely delegate to libbpf all the ELF
> >> >>>
> >> >>
> >> >> iproute2 needs to compile and continue working as is when libbpf is not
> >> >> available. e.g., add check in configure to define HAVE_LIBBPF and move
> >> >> the existing code and move under else branch.
> >> >
> >> > Wouldn't it be better to statically compile against libbpf in this
> >> > case and get rid a lot of BPF-related code and simplify the rest of
> >> > it? This can be easily done by using libbpf through submodule, the
> >> > same way as BCC and pahole do it.
> >> >
> >>
> >> iproute2 compiles today and runs on older distributions and older
> >> distributions with newer kernels. That needs to hold true after the move
> >> to libbpf.
> >
> > And by statically compiling against libbpf, checked out as a
> > submodule, that will still hold true, wouldn't it? Or there is some
> > complications I'm missing? Libbpf is designed to handle old kernels
> > with no problems.
>
> My plan was to use the same configure test I'm using for xdp-tools
> (where I in turn copied the structure of the configure script from
> iproute2):
>
> https://github.com/xdp-project/xdp-tools/blob/master/configure#L59
>
> This will look for a system libbpf install and compile against it if it
> is compatible, and otherwise fall back to a statically linking against a
> git submodule.

How will this work when build host has libbpf installed, but target
host doesn't? You'll get dynamic linker error when trying to run that
tool.

If the goal is to have a reliable tool working everywhere, and you
already support having libbpf as a submodule, why not always use
submodule's libbpf? What's the concern? Libbpf is a small library, I
don't think a binary size argument is enough reason to not do this. On
the other hand, by using libbpf from submodule, your tool is built
*and tested* with a well-known libbpf version that tool-producer
controls.

>
> We'll need to double-check that this will work on everything currently
> supported by iproute2, and fix libbpf if there are any issues with that.
> Not that I foresee any, but you never know :)
>
> -Toke
>