Re: [Patch bpf-next v3 4/5] skmsg: use skb ext instead of TCP_SKB_CB

[Cong Wang <xiyou.wangcong@gmail.com>]

On Mon, Feb 15, 2021 at 11:20 AM John Fastabend
<john.fastabend@gmail.com> wrote:
>
> Cong Wang wrote:
> > From: Cong Wang <cong.wang@bytedance.com>
> >
> > Currently TCP_SKB_CB() is hard-coded in skmsg code, it certainly
> > does not work for any other non-TCP protocols. We can move them to
> > skb ext instead of playing with skb cb, which is harder to make
> > correct.
> >
> > Cc: John Fastabend <john.fastabend@gmail.com>
> > Cc: Daniel Borkmann <daniel@iogearbox.net>
> > Cc: Jakub Sitnicki <jakub@cloudflare.com>
> > Reviewed-by: Lorenz Bauer <lmb@cloudflare.com>
> > Signed-off-by: Cong Wang <cong.wang@bytedance.com>
> > ---
>
> I'm not seeing the advantage of doing this at the moment. We can
> continue to use cb[] here, which is simpler IMO and use the ext
> if needed for the other use cases. This is adding a per packet
> alloc cost that we don't have at the moment as I understand it.

Hmm? How can we continue using TCP_SKB_CB() for UDP or
AF_UNIX?

I am not sure I get your "at the moment" correctly, do you mean
I should move this patch to a later patchset, maybe the UDP
patchset? At least this patch is needed, no matter by which patchset,
so it should not be dropped.


>
> [...]
>
> > diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> > index e3bb712af257..d5c711ef6d4b 100644
> > --- a/include/linux/skmsg.h
> > +++ b/include/linux/skmsg.h
> > @@ -459,4 +459,44 @@ static inline bool sk_psock_strp_enabled(struct sk_psock *psock)
> >               return false;
> >       return !!psock->saved_data_ready;
> >  }
> > +
> > +struct skb_bpf_ext {
> > +     __u32 flags;
> > +     struct sock *sk_redir;
> > +};
> > +
> > +#if IS_ENABLED(CONFIG_NET_SOCK_MSG)
> > +static inline
> > +bool skb_bpf_ext_ingress(const struct sk_buff *skb)
> > +{
> > +     struct skb_bpf_ext *ext = skb_ext_find(skb, SKB_EXT_BPF);
> > +
> > +     return ext->flags & BPF_F_INGRESS;
> > +}
> > +
> > +static inline
> > +void skb_bpf_ext_set_ingress(const struct sk_buff *skb)
> > +{
> > +     struct skb_bpf_ext *ext = skb_ext_find(skb, SKB_EXT_BPF);
> > +
> > +     ext->flags |= BPF_F_INGRESS;
> > +}
> > +
> > +static inline
> > +struct sock *skb_bpf_ext_redirect_fetch(struct sk_buff *skb)
> > +{
> > +     struct skb_bpf_ext *ext = skb_ext_find(skb, SKB_EXT_BPF);
> > +
> > +     return ext->sk_redir;
> > +}
> > +
> > +static inline
> > +void skb_bpf_ext_redirect_clear(struct sk_buff *skb)
> > +{
> > +     struct skb_bpf_ext *ext = skb_ext_find(skb, SKB_EXT_BPF);
> > +
> > +     ext->flags = 0;
> > +     ext->sk_redir = NULL;
> > +}
> > +#endif /* CONFIG_NET_SOCK_MSG */
>
> So we will have some slight duplication for cb[] variant and ext
> variant above. I'm OK with that to avoid an allocation.

Not sure what you mean by "duplication", these are removed from
TCP_SKB_CB(), so there is clearly no duplication.

>
> [...]
>
> > @@ -1003,11 +1008,17 @@ static int sk_psock_verdict_recv(read_descriptor_t *desc, struct sk_buff *skb,
> >               goto out;
> >       }
> >       skb_set_owner_r(skb, sk);
> > +     if (!skb_ext_add(skb, SKB_EXT_BPF)) {
> > +             len = 0;
> > +             kfree_skb(skb);
> > +             goto out;
> > +     }
> > +
>
> per packet cost here. Perhaps you can argue small alloc will usually not be
> noticable in such a large stack, but once we convert over it will be very
> hard to go back. And I'm looking at optimizing this path now.

This is a price we need to pay to avoid CB, and skb_ext_add() has been
used on other fast paths too, for example, tcf_classify_ingress() and
mptcp_incoming_options(). So, it is definitely acceptable.

>
> >       prog = READ_ONCE(psock->progs.skb_verdict);
> >       if (likely(prog)) {
> > -             tcp_skb_bpf_redirect_clear(skb);
> > +             skb_bpf_ext_redirect_clear(skb);
> >               ret = sk_psock_bpf_run(psock, prog, skb);
> > -             ret = sk_psock_map_verd(ret, tcp_skb_bpf_redirect_fetch(skb));
> > +             ret = sk_psock_map_verd(ret, skb_bpf_ext_redirect_fetch(skb));
> >       }
> >       sk_psock_verdict_apply(psock, skb, ret);
>
> Thanks for the series Cong. Drop this patch and resubmit carry ACKs forward
> and then lets revisit this later.

I still believe it is best to stay in this patchset, as it does not change
any functionality and is a preparation too. And the next patchset will be
UDP/AF_UNIX changes as you suggested, it is very awkward to put this
patch into either UDP or AF_UNIX changes.

So, let's keep it in this patchset, and I am happy to address any concerns
and open to other ideas than using skb ext.

Thanks.