netdev - Re: [PATCH v5 bpf-next 07/12] bpf: tcp: Add bpf_skops_hdr_opt_len() and bpf_skops_write_hdr

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Date:   Thu, 20 Aug 2020 15:54:40 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Mat Martineau <mathew.j.martineau@...ux.intel.com>
Cc:     Martin KaFai Lau <kafai@...com>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        kernel-team <kernel-team@...com>,
        Lawrence Brakmo <brakmo@...com>,
        Neal Cardwell <ncardwell@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH v5 bpf-next 07/12] bpf: tcp: Add bpf_skops_hdr_opt_len()
 and bpf_skops_write_hdr_opt()

On Thu, Aug 20, 2020 at 3:39 PM Mat Martineau
<mathew.j.martineau@...ux.intel.com> wrote:
>
>
> On Thu, 20 Aug 2020, Martin KaFai Lau wrote:
>
> > The bpf prog needs to parse the SYN header to learn what options have
> > been sent by the peer's bpf-prog before writing its options into SYNACK.
> > This patch adds a "syn_skb" arg to tcp_make_synack() and send_synack().
> > This syn_skb will eventually be made available (as read-only) to the
> > bpf prog.  This will be the only SYN packet available to the bpf
> > prog during syncookie.  For other regular cases, the bpf prog can
> > also use the saved_syn.

> > diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> > index 5084333b5ab6..631a5ee0dd4e 100644
> > --- a/net/ipv4/tcp_ipv4.c
> > +++ b/net/ipv4/tcp_ipv4.c
>
> ...
>
> > @@ -826,6 +886,15 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
> >                       opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
> >       }
> >
> > +     if (unlikely(BPF_SOCK_OPS_TEST_FLAG(tp,
> > +                                         BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG))) {
> > +             unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
> > +
> > +             bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts, &remaining);
> > +
> > +             size = MAX_TCP_OPTION_SPACE - remaining;
> > +     }
> > +
> >       return size;
> > }
> >
>
> Since bpf_skops_hdr_opt_len() is called after the SACK code tries to use
> up all remaining option space for SACK blocks, it's less likely that there
> will be sufficient space remaining. Did you consider moving this hunk
> before the SACK option space is allocated to give the bpf prog higher
> priority?

We have tcp_is_sack(tp) returning a bool. We do not want to return a
flaky value.

If you need to limit the number of sack ranges to one, please handle
this separately.