netdev - Re: [PATCH v2 bpf-next 06/17] libbpf: xsk: use bpf

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Message-ID: <CAEf4Bza-pGTS+vmE5SvuMtEptGxS5wSbW2d0K34nvt9StG3C8A@mail.gmail.com>
Date:   Mon, 15 Mar 2021 22:34:11 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        ciara.loftus@...el.com, john fastabend <john.fastabend@...il.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH v2 bpf-next 06/17] libbpf: xsk: use bpf_link

On Thu, Mar 11, 2021 at 7:42 AM Maciej Fijalkowski
<maciej.fijalkowski@...el.com> wrote:
>
> Currently, if there are multiple xdpsock instances running on a single
> interface and in case one of the instances is terminated, the rest of
> them are left in an inoperable state due to the fact of unloaded XDP
> prog from interface.
>
> Consider the scenario below:
>
> // load xdp prog and xskmap and add entry to xskmap at idx 10
> $ sudo ./xdpsock -i ens801f0 -t -q 10
>
> // add entry to xskmap at idx 11
> $ sudo ./xdpsock -i ens801f0 -t -q 11
>
> terminate one of the processes and another one is unable to work due to
> the fact that the XDP prog was unloaded from interface.
>
> To address that, step away from setting bpf prog in favour of bpf_link.
> This means that refcounting of BPF resources will be done automatically
> by bpf_link itself.
>
> When setting up BPF resources during xsk socket creation, check whether
> bpf_link for a given ifindex already exists via set of calls to
> bpf_link_get_next_id -> bpf_link_get_fd_by_id -> bpf_obj_get_info_by_fd
> and comparing the ifindexes from bpf_link and xsk socket.
>
> If there's no bpf_link yet, create one for a given XDP prog. If bpf_link
> is already at a given ifindex and underlying program is not AF-XDP one,
> bail out or update the bpf_link's prog given the presence of
> XDP_FLAGS_UPDATE_IF_NOEXIST.
>
> If there's netlink-based XDP prog running on a interface, bail out and
> ask user to do removal by himself.
>
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> ---
>  tools/lib/bpf/xsk.c | 139 ++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 120 insertions(+), 19 deletions(-)
>

[...]

> +static int xsk_link_lookup(struct xsk_ctx *ctx, __u32 *prog_id)
> +{
> +       struct bpf_link_info link_info;
> +       __u32 link_len;
> +       __u32 id = 0;
> +       int err;
> +       int fd;
> +
> +       while (true) {
> +               err = bpf_link_get_next_id(id, &id);
> +               if (err) {
> +                       if (errno == ENOENT) {
> +                               err = 0;
> +                               break;
> +                       }
> +                       pr_warn("can't get next link: %s\n", strerror(errno));
> +                       break;
> +               }
> +
> +               fd = bpf_link_get_fd_by_id(id);
> +               if (fd < 0) {
> +                       if (errno == ENOENT)
> +                               continue;
> +                       pr_warn("can't get link by id (%u): %s\n", id, strerror(errno));
> +                       err = -errno;
> +                       break;
> +               }
> +
> +               link_len = sizeof(struct bpf_link_info);
> +               memset(&link_info, 0, link_len);
> +               err = bpf_obj_get_info_by_fd(fd, &link_info, &link_len);
> +               if (err) {
> +                       pr_warn("can't get link info: %s\n", strerror(errno));
> +                       close(fd);
> +                       break;
> +               }
> +               if (link_info.xdp.ifindex == ctx->ifindex) {

how do you know you are looking at XDP bpf_link? link_info.xdp.ifindex
might as well be attach_type for tracing bpf_linke, netns_ino for
netns bpf_link, and so on. Do check link_info.type before check other
per-link type properties.

> +                       ctx->link_fd = fd;
> +                       *prog_id = link_info.prog_id;
> +                       break;
> +               }
> +               close(fd);
> +       }
> +
> +       return err;
> +}
> +
>  static int __xsk_setup_xdp_prog(struct xsk_socket *_xdp,
>                                 int *xsks_map_fd)
>  {
> @@ -675,8 +777,7 @@ static int __xsk_setup_xdp_prog(struct xsk_socket *_xdp,
>         __u32 prog_id = 0;
>         int err;
>
> -       err = bpf_get_link_xdp_id(ctx->ifindex, &prog_id,
> -                                 xsk->config.xdp_flags);
> +       err = xsk_link_lookup(ctx, &prog_id);
>         if (err)
>                 return err;
>
> @@ -686,9 +787,12 @@ static int __xsk_setup_xdp_prog(struct xsk_socket *_xdp,
>                         return err;
>
>                 err = xsk_load_xdp_prog(xsk);
> -               if (err) {
> +               if (err)
>                         goto err_load_xdp_prog;
> -               }
> +
> +               err = xsk_create_bpf_link(xsk);
> +               if (err)
> +                       goto err_create_bpf_link;

what about the backwards compatibility with kernels that don't yet
support bpf_link?

>         } else {
>                 ctx->prog_fd = bpf_prog_get_fd_by_id(prog_id);
>                 if (ctx->prog_fd < 0)

[...]