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

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Message-ID: <CAEf4BzY0sHqEXaY8no0VgwEbNoPEaQz0h53Gav=T1DCsjsjo8A@mail.gmail.com>
Date:   Mon, 27 Feb 2023 13:01:43 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...a.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, kuba@...nel.org,
        Martin KaFai Lau <martin.lau@...nel.org>
Subject: Re: [PATCH v5 bpf-next 5/8] libbpf: add API to get XDP/XSK supported features

On Mon, Feb 27, 2023 at 12:39 PM Yonghong Song <yhs@...a.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@...il.com>
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> > Co-developed-by: Marek Majtyka <alardam@...il.com>
> > Signed-off-by: Marek Majtyka <alardam@...il.com>
> > Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.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.


>
> > +
> > +     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;
> >   }
> >
> [...]