netdev - Re: [PATCH iproute2-next 3/5] lib: add libbpf support

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Date:   Fri, 23 Oct 2020 17:21:20 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Hangbin Liu <haliu@...hat.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Ahern <dsahern@...il.com>,
        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>,
        Jiri Benc <jbenc@...hat.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH iproute2-next 3/5] lib: add libbpf support

On Thu, Oct 22, 2020 at 8:39 PM Hangbin Liu <haliu@...hat.com> wrote:
>
> This patch converts iproute2 to use libbpf for loading and attaching
> BPF programs when it is available, which is started by Toke's
> implementation[1]. With libbpf iproute2 could correctly process BTF
> information and support the new-style BTF-defined maps, while keeping
> compatibility with the old internal map definition syntax.
>
> The old iproute2 bpf code is kept and will be used if no suitable libbpf
> is available. When using libbpf, wrapper code in bpf_legacy.c ensures that
> iproute2 will still understand the old map definition format, including
> populating map-in-map and tail call maps before load.
>
> In bpf_libbpf.c, we init iproute2 ctx and elf info first to check the
> legacy bytes. When handling the legacy maps, for map-in-maps, we create
> them manually and re-use the fd as they are associated with id/inner_id.
> For pin maps, we only set the pin path and let libbp load to handle it.
> For tail calls, we find it first and update the element after prog load.

I never implemented tail call map initialization using the same
approach as declarative map-in-map support in libbpf, because no one
asked and/or showed a use case. But all the pieces are there, and if
there's interest, we should probably support that in libbpf as well.

>
> Other maps/progs will be loaded by libbpf directly.
>
> Note: ip/ipvrf.c is not convert to use libbpf as it only encodes a few
> instructions and load directly.
>
> [1] https://lore.kernel.org/bpf/20190820114706.18546-1-toke@redhat.com/
>
> Reviewed-by: Toke Høiland-Jørgensen <toke@...hat.com>
> Signed-off-by: Hangbin Liu <haliu@...hat.com>
> ---
>  include/bpf_util.h |  11 ++
>  lib/Makefile       |   4 +
>  lib/bpf_legacy.c   | 178 ++++++++++++++++++++++++
>  lib/bpf_libbpf.c   | 338 +++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 531 insertions(+)
>  create mode 100644 lib/bpf_libbpf.c
>

[...]

> +
> +static int load_bpf_object(struct bpf_cfg_in *cfg)
> +{
> +       struct bpf_program *p, *prog = NULL;
> +       struct bpf_object *obj;
> +       char root_path[PATH_MAX];
> +       struct bpf_map *map;
> +       int prog_fd, ret = 0;
> +
> +       ret = iproute2_get_root_path(root_path, PATH_MAX);
> +       if (ret)
> +               return ret;
> +
> +       DECLARE_LIBBPF_OPTS(bpf_object_open_opts, open_opts,
> +                       .relaxed_maps = true,
> +                       .pin_root_path = root_path,
> +       );
> +
> +       obj = bpf_object__open_file(cfg->object, &open_opts);
> +       if (IS_ERR_OR_NULL(obj))

libbpf defines libbpf_get_error() to check that the returned pointer
is not encoding error, you shouldn't need to define your IS_ERR
macros.

> +               return -ENOENT;
> +

[...]