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Message-ID: <CAKH8qBvCQ0q+R0RPX-nhSwXqybKHFYZ0ovu08W6Z6tiFSkzsGA@mail.gmail.com>
Date: Tue, 8 Sep 2020 15:49:38 -0700
From: Stanislav Fomichev <sdf@...gle.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.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 Tue, Sep 8, 2020 at 3:35 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Tue, Sep 8, 2020 at 1:53 PM Stanislav Fomichev <sdf@...gle.com> wrote:
> >
> > On Wed, Sep 2, 2020 at 10:00 PM Andrii Nakryiko
> > <andrii.nakryiko@...il.com> wrote:
> > >
> > > 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?
> > 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?
>
> we might do it similarly to PID info I added with bpf_iter: if it's
> supported -- emit it, if not -- skip and still succeed. So maybe we
> don't really need extra --metadata flag and should do all this always?
Sounds reasonable, especially if there is an existing precedent.
Let me explore that option.
> > > > + 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.
>
> I meant to add vsi++ inside the for clause, no way to miss it:
>
> for (i = 0; i < vlen, i++, vsi++) {
> continue/break/whatever you want, except extra i++ or vsi++
> }
>
> it's the safest way, imo
Ack, I can do that, thanks!
> > > > + 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);
> > >
> > > [...]
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