netdev - Re: [PATCH bpf-next v3 01/16] bpf, netns: Handle multiple link attachments

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Date:   Wed, 8 Jul 2020 20:44:52 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        kernel-team <kernel-team@...udflare.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH bpf-next v3 01/16] bpf, netns: Handle multiple link attachments

On Thu, Jul 2, 2020 at 2:24 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> Extend the BPF netns link callbacks to rebuild (grow/shrink) or update the
> prog_array at given position when link gets attached/updated/released.
>
> This let's us lift the limit of having just one link attached for the new
> attach type introduced by subsequent patch.
>
> No functional changes intended.
>
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> ---
>
> Notes:
>     v3:
>     - New in v3 to support multi-prog attachments. (Alexei)
>
>  include/linux/bpf.h        |  4 ++
>  kernel/bpf/core.c          | 22 ++++++++++
>  kernel/bpf/net_namespace.c | 88 +++++++++++++++++++++++++++++++++++---
>  3 files changed, 107 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 3d2ade703a35..26bc70533db0 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -928,6 +928,10 @@ int bpf_prog_array_copy_to_user(struct bpf_prog_array *progs,
>
>  void bpf_prog_array_delete_safe(struct bpf_prog_array *progs,
>                                 struct bpf_prog *old_prog);
> +void bpf_prog_array_delete_safe_at(struct bpf_prog_array *array,
> +                                  unsigned int index);
> +void bpf_prog_array_update_at(struct bpf_prog_array *array, unsigned int index,
> +                             struct bpf_prog *prog);
>  int bpf_prog_array_copy_info(struct bpf_prog_array *array,
>                              u32 *prog_ids, u32 request_cnt,
>                              u32 *prog_cnt);
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 9df4cc9a2907..d4b3b9ee6bf1 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -1958,6 +1958,28 @@ void bpf_prog_array_delete_safe(struct bpf_prog_array *array,
>                 }
>  }
>
> +void bpf_prog_array_delete_safe_at(struct bpf_prog_array *array,
> +                                  unsigned int index)
> +{
> +       bpf_prog_array_update_at(array, index, &dummy_bpf_prog.prog);
> +}
> +
> +void bpf_prog_array_update_at(struct bpf_prog_array *array, unsigned int index,
> +                             struct bpf_prog *prog)

it's a good idea to mention it in a comment for both delete_safe_at
and update_at that slots with dummy entries are ignored.

Also, given that index can be out of bounds, should these functions
actually return error if the slot is not found?

> +{
> +       struct bpf_prog_array_item *item;
> +
> +       for (item = array->items; item->prog; item++) {
> +               if (item->prog == &dummy_bpf_prog.prog)
> +                       continue;
> +               if (!index) {
> +                       WRITE_ONCE(item->prog, prog);
> +                       break;
> +               }
> +               index--;
> +       }
> +}
> +
>  int bpf_prog_array_copy(struct bpf_prog_array *old_array,
>                         struct bpf_prog *exclude_prog,
>                         struct bpf_prog *include_prog,
> diff --git a/kernel/bpf/net_namespace.c b/kernel/bpf/net_namespace.c
> index 247543380fa6..6011122c35b6 100644
> --- a/kernel/bpf/net_namespace.c
> +++ b/kernel/bpf/net_namespace.c
> @@ -36,11 +36,51 @@ static void netns_bpf_run_array_detach(struct net *net,
>         bpf_prog_array_free(run_array);
>  }
>
> +static unsigned int link_index(struct net *net,
> +                              enum netns_bpf_attach_type type,
> +                              struct bpf_netns_link *link)
> +{
> +       struct bpf_netns_link *pos;
> +       unsigned int i = 0;
> +
> +       list_for_each_entry(pos, &net->bpf.links[type], node) {
> +               if (pos == link)
> +                       return i;
> +               i++;
> +       }
> +       return UINT_MAX;

Why not return a negative error, if the slot is not found? Feels a bit
unusual as far as error reporting goes.

> +}
> +

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