netdev - Re: [PATCH bpf-next v4 2/3] bpf_xdp_redirect_map: Perform map lookup in eBPF helper

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Date:   Fri, 14 Jun 2019 16:07:08 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Networking <netdev@...r.kernel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        David Miller <davem@...emloft.net>,
        Jonathan Lemon <jonathan.lemon@...il.com>
Subject: Re: [PATCH bpf-next v4 2/3] bpf_xdp_redirect_map: Perform map lookup
 in eBPF helper

On Thu, Jun 13, 2019 at 8:31 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> From: Toke Høiland-Jørgensen <toke@...hat.com>
>
> The bpf_redirect_map() helper used by XDP programs doesn't return any
> indication of whether it can successfully redirect to the map index it was
> given. Instead, BPF programs have to track this themselves, leading to
> programs using duplicate maps to track which entries are populated in the
> devmap.
>
> This patch fixes this by moving the map lookup into the bpf_redirect_map()
> helper, which makes it possible to return failure to the eBPF program. The
> lower bits of the flags argument is used as the return code, which means
> that existing users who pass a '0' flag argument will get XDP_ABORTED.

I see that we have absolutely no documentation for
bpf_xdp_redirect_map. Can you please add it to
include/uapi/linux/bpf.h? Don't forget to mention this handling of
lower bits of flags. Thanks!

>
> With this, a BPF program can check the return code from the helper call and
> react by, for instance, substituting a different redirect. This works for
> any type of map used for redirect.
>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>  include/linux/filter.h |    1 +
>  net/core/filter.c      |   27 +++++++++++++--------------
>  2 files changed, 14 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 43b45d6db36d..f31ae8b9035a 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -580,6 +580,7 @@ struct bpf_skb_data_end {
>  struct bpf_redirect_info {
>         u32 ifindex;
>         u32 flags;
> +       void *item;

This is so generic name that some short comment describing what that
item is would help a lot.

>         struct bpf_map *map;
>         struct bpf_map *map_to_flush;
>         u32 kern_flags;
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 7a996887c500..7d742ea61e2d 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3608,17 +3608,13 @@ static int xdp_do_redirect_map(struct net_device *dev, struct xdp_buff *xdp,
>                                struct bpf_redirect_info *ri)
>  {
>         u32 index = ri->ifindex;
> -       void *fwd = NULL;
> +       void *fwd = ri->item;
>         int err;
>
>         ri->ifindex = 0;
> +       ri->item = NULL;
>         WRITE_ONCE(ri->map, NULL);
>
> -       fwd = __xdp_map_lookup_elem(map, index);
> -       if (unlikely(!fwd)) {
> -               err = -EINVAL;
> -               goto err;
> -       }
>         if (ri->map_to_flush && unlikely(ri->map_to_flush != map))
>                 xdp_do_flush_map();
>
> @@ -3655,18 +3651,13 @@ static int xdp_do_generic_redirect_map(struct net_device *dev,
>  {
>         struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
>         u32 index = ri->ifindex;
> -       void *fwd = NULL;
> +       void *fwd = ri->item;
>         int err = 0;
>
>         ri->ifindex = 0;
> +       ri->item = NULL;
>         WRITE_ONCE(ri->map, NULL);
>
> -       fwd = __xdp_map_lookup_elem(map, index);
> -       if (unlikely(!fwd)) {
> -               err = -EINVAL;
> -               goto err;
> -       }
> -
>         if (map->map_type == BPF_MAP_TYPE_DEVMAP) {
>                 struct bpf_dtab_netdev *dst = fwd;
>
> @@ -3735,6 +3726,7 @@ BPF_CALL_2(bpf_xdp_redirect, u32, ifindex, u64, flags)
>
>         ri->ifindex = ifindex;
>         ri->flags = flags;
> +       ri->item = NULL;
>         WRITE_ONCE(ri->map, NULL);
>
>         return XDP_REDIRECT;
> @@ -3753,9 +3745,16 @@ BPF_CALL_3(bpf_xdp_redirect_map, struct bpf_map *, map, u32, ifindex,
>  {
>         struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
>
> -       if (unlikely(flags))
> +       /* Lower bits of the flags are used as return code on lookup failure */
> +       if (unlikely(flags > XDP_TX))
>                 return XDP_ABORTED;
>
> +       ri->item = __xdp_map_lookup_elem(map, ifindex);
> +       if (unlikely(!ri->item)) {
> +               WRITE_ONCE(ri->map, NULL);
> +               return flags;
> +       }
> +
>         ri->ifindex = ifindex;
>         ri->flags = flags;
>         WRITE_ONCE(ri->map, map);
>