netdev - Re: [PATCH bpf-next v3 5/8] bpftool: support dumping metadata

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Date:   Wed, 2 Sep 2020 22:00:10 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     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>
Subject: Re: [PATCH bpf-next v3 5/8] bpftool: support dumping metadata

On Fri, Aug 28, 2020 at 12:37 PM Stanislav Fomichev <sdf@...gle.com> wrote:
>
> From: YiFei Zhu <zhuyifei@...gle.com>
>
> Added a flag "--metadata" to `bpftool prog list` to dump the metadata
> contents. For some formatting some BTF code is put directly in the
> metadata dumping. Sanity checks on the map and the kind of the btf_type
> to make sure we are actually dumping what we are expecting.
>
> A helper jsonw_reset is added to json writer so we can reuse the same
> json writer without having extraneous commas.
>
> Sample output:
>
>   $ bpftool prog --metadata
>   6: cgroup_skb  name prog  tag bcf7977d3b93787c  gpl
>   [...]
>         btf_id 4
>         metadata:
>                 metadata_a = "foo"
>                 metadata_b = 1
>
>   $ bpftool prog --metadata --json --pretty
>   [{
>           "id": 6,
>   [...]
>           "btf_id": 4,
>           "metadata": {
>               "metadata_a": "foo",
>               "metadata_b": 1
>           }
>       }
>   ]
>
> Cc: YiFei Zhu <zhuyifei1999@...il.com>
> Signed-off-by: YiFei Zhu <zhuyifei@...gle.com>
> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> ---
>  tools/bpf/bpftool/json_writer.c |   6 ++
>  tools/bpf/bpftool/json_writer.h |   3 +
>  tools/bpf/bpftool/main.c        |  10 +++
>  tools/bpf/bpftool/main.h        |   1 +
>  tools/bpf/bpftool/prog.c        | 130 ++++++++++++++++++++++++++++++++
>  5 files changed, 150 insertions(+)
>

[...]

> +
> +       if (bpf_map_lookup_elem(map_fd, &key, value)) {
> +               p_err("metadata map lookup failed: %s", strerror(errno));
> +               goto out_free;
> +       }
> +
> +       err = btf__get_from_id(map_info.btf_id, &btf);

what if the map has no btf_id associated (e.g., because of an old
kernel?); why fail in this case?

> +       if (err || !btf) {
> +               p_err("metadata BTF get failed: %s", strerror(-err));
> +               goto out_free;
> +       }
> +
> +       t_datasec = btf__type_by_id(btf, map_info.btf_value_type_id);
> +       if (BTF_INFO_KIND(t_datasec->info) != BTF_KIND_DATASEC) {

btf_is_datasec(t_datasec)

> +               p_err("bad metadata BTF");
> +               goto out_free;
> +       }
> +
> +       vlen = BTF_INFO_VLEN(t_datasec->info);

btf_vlen(t_datasec)

> +       vsi = (struct btf_var_secinfo *)(t_datasec + 1);

btf_var_secinfos(t_datasec)

> +
> +       /* We don't proceed to check the kinds of the elements of the DATASEC.
> +        * The verifier enforce then to be BTF_KIND_VAR.

typo: then -> them

> +        */
> +
> +       if (json_output) {
> +               struct btf_dumper d = {
> +                       .btf = btf,
> +                       .jw = json_wtr,
> +                       .is_plain_text = false,
> +               };
> +
> +               jsonw_name(json_wtr, "metadata");
> +
> +               jsonw_start_object(json_wtr);
> +               for (i = 0; i < vlen; i++) {

nit: doing ++vsi here


> +                       t_var = btf__type_by_id(btf, vsi[i].type);

and vsi->type here and below would look a bit cleaner

> +
> +                       jsonw_name(json_wtr, btf__name_by_offset(btf, t_var->name_off));
> +                       err = btf_dumper_type(&d, t_var->type, value + vsi[i].offset);
> +                       if (err) {
> +                               p_err("btf dump failed");
> +                               break;
> +                       }
> +               }
> +               jsonw_end_object(json_wtr);

[...]