Re: [PATCH v5 bpf-next 5/8] libbpf: add API to get XDP/XSK supported features

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Feb 27, 2023 at 2:05 PM Yonghong Song <yhs@meta.com> wrote:
>
>
>
> On 2/27/23 1:01 PM, Andrii Nakryiko wrote:
> > On Mon, Feb 27, 2023 at 12:39 PM Yonghong Song <yhs@meta.com> wrote:
> >>
> >>
> >>
> >> On 2/1/23 2:24 AM, Lorenzo Bianconi wrote:
> >>> Extend bpf_xdp_query routine in order to get XDP/XSK supported features
> >>> of netdev over route netlink interface.
> >>> Extend libbpf netlink implementation in order to support netlink_generic
> >>> protocol.
> >>>
> >>> Co-developed-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> >>> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> >>> Co-developed-by: Marek Majtyka <alardam@gmail.com>
> >>> Signed-off-by: Marek Majtyka <alardam@gmail.com>
> >>> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> >>> ---
> >>>    tools/lib/bpf/libbpf.h  |  3 +-
> >>>    tools/lib/bpf/netlink.c | 96 +++++++++++++++++++++++++++++++++++++++++
> >>>    tools/lib/bpf/nlattr.h  | 12 ++++++
> >>>    3 files changed, 110 insertions(+), 1 deletion(-)
> >>>
> >> [...]
> >>> +
> >>>    int bpf_xdp_query(int ifindex, int xdp_flags, struct bpf_xdp_query_opts *opts)
> >>>    {
> >>>        struct libbpf_nla_req req = {
> >>> @@ -366,6 +433,10 @@ int bpf_xdp_query(int ifindex, int xdp_flags, struct bpf_xdp_query_opts *opts)
> >>>                .ifinfo.ifi_family = AF_PACKET,
> >>>        };
> >>>        struct xdp_id_md xdp_id = {};
> >>> +     struct xdp_features_md md = {
> >>> +             .ifindex = ifindex,
> >>> +     };
> >>> +     __u16 id;
> >>>        int err;
> >>>
> >>>        if (!OPTS_VALID(opts, bpf_xdp_query_opts))
> >>> @@ -393,6 +464,31 @@ int bpf_xdp_query(int ifindex, int xdp_flags, struct bpf_xdp_query_opts *opts)
> >>>        OPTS_SET(opts, skb_prog_id, xdp_id.info.skb_prog_id);
> >>>        OPTS_SET(opts, attach_mode, xdp_id.info.attach_mode);
> >>>
> >>> +     if (!OPTS_HAS(opts, feature_flags))
> >>> +             return 0;
> >>> +
> >>> +     err = libbpf_netlink_resolve_genl_family_id("netdev", sizeof("netdev"), &id);
> >>> +     if (err < 0)
> >>> +             return libbpf_err(err);
> >>
> >> Hi, Lorenzo,
> >>
> >> Using latest libbpf repo (https://github.com/libbpf/libbpf, sync'ed from
> >> source), looks like the above change won't work if the program is
> >> running on an old kernel, e.g., 5.12 kernel.
> >>
> >> In this particular combination, in user space, bpf_xdp_query_opts does
> >> have 'feature_flags' member, so the control can reach
> >> libbpf_netlink_resolve_genl_family_id(). However, the family 'netdev'
> >> is only available in latest kernel (after this patch set). So
> >> the error will return in the above.
> >>
> >> This breaks backward compatibility since old working application won't
> >> work any more with a refresh of libbpf.
> >>
> >> I could not come up with an easy solution for this. One thing we could
> >> do is to treat 'libbpf_netlink_resolve_genl_family_id()' as a probe, so
> >> return 0 if probe fails.
> >>
> >>     err = libbpf_netlink_resolve_genl_family_id("netdev",
> >> sizeof("netdev"), &id);
> >>     if (err < 0)
> >>          return 0;
> >>
> >> Please let me know whether my suggestion makes sense or there could be a
> >> better solution.
> >>
> >
> > feature_flags is an output parameter and if the "netdev" family
> > doesn't exist then there are no feature flags to return, right?
> >
> > Is there a specific error code that's returned when such a family
> > doesn't exist? If yes, we should check for it and return 0 for
> > feature_flags. If not, we'll have to do a generic < 0 check as
> > Yonghong proposes.
>
> We can check -ENOENT.
>
>          err = libbpf_netlink_resolve_genl_family_id("netdev",
> sizeof("netdev"), &id);
> -       if (err < 0)
> +       if (err < 0) {
> +               if (err == -ENOENT)
> +                       return 0;
>                  return libbpf_err(err);
> +       }
>
> Let me propose a patch for this.

we might add more options beyond feature_flags, so these early returns
are a bit error-prone, let's convert this code to if () { } else?

>
> >
> >
> >>
> >>> +
> >>> +     memset(&req, 0, sizeof(req));
> >>> +     req.nh.nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);
> >>> +     req.nh.nlmsg_flags = NLM_F_REQUEST;
> >>> +     req.nh.nlmsg_type = id;
> >>> +     req.gnl.cmd = NETDEV_CMD_DEV_GET;
> >>> +     req.gnl.version = 2;
> >>> +
> >>> +     err = nlattr_add(&req, NETDEV_A_DEV_IFINDEX, &ifindex, sizeof(ifindex));
> >>> +     if (err < 0)
> >>> +             return err;
> >>> +
> >>> +     err = libbpf_netlink_send_recv(&req, NETLINK_GENERIC,
> >>> +                                    parse_xdp_features, NULL, &md);
> >>> +     if (err)
> >>> +             return libbpf_err(err);
> >>> +
> >>> +     opts->feature_flags = md.flags;
> >>> +
> >>>        return 0;
> >>>    }
> >>>
> >> [...]