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

[Stanislav Fomichev <sdf@google.com>]

On Wed, Sep 2, 2020 at 10:00 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Fri, Aug 28, 2020 at 12:37 PM Stanislav Fomichev <sdf@google.com> wrote:
> >
> > From: YiFei Zhu <zhuyifei@google.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@gmail.com>
> > Signed-off-by: YiFei Zhu <zhuyifei@google.com>
> > Signed-off-by: Stanislav Fomichev <sdf@google.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?
Thank you for the review, coming back at it a bit late :-(

This functionality is guarded by --metadata bpftool flag (off by default).
In case of no btf_id, it might be helpful to show why we don't have
the metadata rather than just quietly failing.
WDYT?

> > +       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
Agreed with all the above, except this one.
It feels like it's safer to do [i] in case somebody adds a 'continue'
clause later and we miss that '++vsi'.
Let me know if you feel strongly about it.

> > +                       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);
>
> [...]