On Mon, Feb 27, 2023 at 12:39 PM Yonghong Song <yhs@xxxxxxxx> 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@xxxxxxxxx>
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> > Co-developed-by: Marek Majtyka <alardam@xxxxxxxxx>
> > Signed-off-by: Marek Majtyka <alardam@xxxxxxxxx>
> > Signed-off-by: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
> > ---
> > 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;
> > }
> >
> [...]
