netdev - Re: [PATCH net-next] tcp: SYN packets are now simply consumed

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Date:	Fri, 22 Apr 2016 13:32:53 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] tcp: SYN packets are now simply consumed

On Thu, Apr 21, 2016 at 10:13 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> We now have proper per-listener but also per network namespace counters
> for SYN packets that might be dropped.
>
> We replace the kfree_skb() by consume_skb() to be drop monitor [1]
> friendly, and remove an obsolete comment.
> FastOpen SYN packets can carry payload in them just fine.
Thanks. I have long wanted to change that comment

>
> [1] perf record -a -g -e skb:kfree_skb sleep 1; perf report
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  net/ipv4/tcp_input.c |   19 +------------------
>  1 file changed, 1 insertion(+), 18 deletions(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 90e0d9256b74..ab674cd99827 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5813,24 +5813,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
>                         if (icsk->icsk_af_ops->conn_request(sk, skb) < 0)
>                                 return 1;
>
> -                       /* Now we have several options: In theory there is
> -                        * nothing else in the frame. KA9Q has an option to
> -                        * send data with the syn, BSD accepts data with the
> -                        * syn up to the [to be] advertised window and
> -                        * Solaris 2.1 gives you a protocol error. For now
> -                        * we just ignore it, that fits the spec precisely
> -                        * and avoids incompatibilities. It would be nice in
> -                        * future to drop through and process the data.
> -                        *
> -                        * Now that TTCP is starting to be used we ought to
> -                        * queue this data.
> -                        * But, this leaves one open to an easy denial of
> -                        * service attack, and SYN cookies can't defend
> -                        * against this problem. So, we drop the data
> -                        * in the interest of security over speed unless
> -                        * it's still in use.
> -                        */
> -                       kfree_skb(skb);
> +                       consume_skb(skb);
>                         return 0;
>                 }
>                 goto discard;
>
>