netdev - Re: [PATCH bpf-next v2 4/7] net: sched: add lightweight update path for cls

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Message-ID: <CAEf4BzaLdLgwnjajPu=ZtzH+HB=eKKCWMrs3P+uUmQKBuANPew@mail.gmail.com>
Date:   Mon, 7 Jun 2021 16:32:04 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf <bpf@...r.kernel.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Vlad Buslov <vladbu@...dia.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 4/7] net: sched: add lightweight update path
 for cls_bpf

On Thu, Jun 3, 2021 at 11:32 PM Kumar Kartikeya Dwivedi
<memxor@...il.com> wrote:
>
> This is used by BPF_LINK_UPDATE to replace the attach SCHED_CLS bpf prog
> effectively changing the classifier implementation for a given filter
> owned by a bpf_link.
>
> Note that READ_ONCE suffices in this case as the ordering for loads from
> the filter are implicitly provided by the data dependency on BPF prog
> pointer.
>
> On the writer side we can just use a relaxed WRITE_ONCE store to make
> sure one or the other value is visible to a reader in cls_bpf_classify.
> Lifetime is managed using RCU so bpf_prog_put path should wait until
> readers are done for old_prog.
>
> All other parties accessing the BPF prog are under RTNL protection, so
> need no changes.
>
> Reviewed-by: Toke Høiland-Jørgensen <toke@...hat.com>.
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> ---
>  net/sched/cls_bpf.c | 55 +++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 53 insertions(+), 2 deletions(-)
>
> diff --git a/net/sched/cls_bpf.c b/net/sched/cls_bpf.c
> index bf61ffbb7fd0..f23304685c48 100644
> --- a/net/sched/cls_bpf.c
> +++ b/net/sched/cls_bpf.c
> @@ -9,6 +9,7 @@
>   * (C) 2013 Daniel Borkmann <dborkman@...hat.com>
>   */
>
> +#include <linux/atomic.h>
>  #include <linux/module.h>
>  #include <linux/types.h>
>  #include <linux/skbuff.h>
> @@ -104,11 +105,11 @@ static int cls_bpf_classify(struct sk_buff *skb, const struct tcf_proto *tp,
>                         /* It is safe to push/pull even if skb_shared() */
>                         __skb_push(skb, skb->mac_len);
>                         bpf_compute_data_pointers(skb);
> -                       filter_res = BPF_PROG_RUN(prog->filter, skb);
> +                       filter_res = BPF_PROG_RUN(READ_ONCE(prog->filter), skb);
>                         __skb_pull(skb, skb->mac_len);
>                 } else {
>                         bpf_compute_data_pointers(skb);
> -                       filter_res = BPF_PROG_RUN(prog->filter, skb);
> +                       filter_res = BPF_PROG_RUN(READ_ONCE(prog->filter), skb);
>                 }
>
>                 if (prog->exts_integrated) {
> @@ -775,6 +776,55 @@ static int cls_bpf_link_detach(struct bpf_link *link)
>         return 0;
>  }
>
> +static int cls_bpf_link_update(struct bpf_link *link, struct bpf_prog *new_prog,
> +                              struct bpf_prog *old_prog)
> +{
> +       struct cls_bpf_link *cls_link;
> +       struct cls_bpf_prog cls_prog;
> +       struct cls_bpf_prog *prog;
> +       int ret;
> +
> +       rtnl_lock();
> +
> +       cls_link = container_of(link, struct cls_bpf_link, link);
> +       if (!cls_link->prog) {
> +               ret = -ENOLINK;
> +               goto out;
> +       }
> +
> +       prog = cls_link->prog;
> +
> +       /* BPF_F_REPLACEing? */
> +       if (old_prog && prog->filter != old_prog) {
> +               ret = -EINVAL;
> +               goto out;
> +       }
> +
> +       old_prog = prog->filter;
> +
> +       if (new_prog == old_prog) {
> +               ret = 0;

So the contract is that if update is successful, new_prog's refcount
taken by link_update() in kernel/bpf/syscall.c is transferred here. On
error, it will be bpf_prog_put() by link_update(). So here you don't
need extra refcnt, but it's also not an error, so you need to
bpf_prog_put(new_prog) explicitly to balance out refcnt. See how it's
done for XDP, for example.


> +               goto out;
> +       }
> +
> +       cls_prog = *prog;
> +       cls_prog.filter = new_prog;
> +
> +       ret = cls_bpf_offload(prog->tp, &cls_prog, prog, NULL);
> +       if (ret < 0)
> +               goto out;
> +
> +       WRITE_ONCE(prog->filter, new_prog);
> +
> +       bpf_prog_inc(new_prog);

and you don't need this, you already got the reference from link_update()

> +       /* release our reference */
> +       bpf_prog_put(old_prog);
> +
> +out:
> +       rtnl_unlock();
> +       return ret;
> +}
> +
>  static void __bpf_fill_link_info(struct cls_bpf_link *link,
>                                  struct bpf_link_info *info)
>  {
> @@ -859,6 +909,7 @@ static const struct bpf_link_ops cls_bpf_link_ops = {
>         .show_fdinfo = cls_bpf_link_show_fdinfo,
>  #endif
>         .fill_link_info = cls_bpf_link_fill_link_info,
> +       .update_prog = cls_bpf_link_update,
>  };
>
>  static inline char *cls_bpf_link_name(u32 prog_id, const char *name)
> --
> 2.31.1
>