netdev - Re: [PATCH net] tcp: Always cleanup skb before sending

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Date:   Wed, 01 Nov 2017 14:53:38 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Christoph Paasch <cpaasch@...le.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] tcp: Always cleanup skb before sending

On Wed, 2017-11-01 at 14:32 -0700, Eric Dumazet wrote:
> On Wed, Nov 1, 2017 at 2:10 PM, Christoph Paasch <cpaasch@...le.com> wrote:
> > Since commit 971f10eca186 ("tcp: better TCP_SKB_CB layout to reduce cache
> > line misses") IPCB/IP6CB is no more at the beginning of the tcp_skb_cb.
> > This means that on the output path, we need to make sure that it has
> > been correctly initialized to 0, as is done in tcp_transmit_skb.
> >
> > However, when going through the other code-path in TCP that can send an
> > skb (e.g., through tcp_v6_send_synack), we end up in a situation where
> > IP6CB has some of its fields set to unexpected values. Depending on the
> > layout of tcp_skb_cb across the different kernel-versions this can be
> > lastopt, flags,...
> 
> Or not use tcp_init_nondata_skb() on non fast clones, since it adds
> unnecessary writes and clears.
> 
> tcp_make_synack() really has no business using tcp_init_nondata_skb()
> and could simply set th->seq = htonl(tcp_rsk(req)->snt_isn);

Something like :

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 69cfdead0cb49e4365158048a0d1a9bbdd55fa83..5502abc5307f0ce1de610d4b70f3a59c4d5383c5 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3399,13 +3399,8 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
        tcp_ecn_make_synack(req, th);
        th->source = htons(ireq->ir_num);
        th->dest = ireq->ir_rmt_port;
-       /* Setting of flags are superfluous here for callers (and ECE is
-        * not even correctly set)
-        */
-       tcp_init_nondata_skb(skb, tcp_rsk(req)->snt_isn,
-                            TCPHDR_SYN | TCPHDR_ACK);
-
-       th->seq = htonl(TCP_SKB_CB(skb)->seq);
+       skb->ip_summed = CHECKSUM_PARTIAL;
+       th->seq = htonl(tcp_rsk(req)->snt_isn);
        /* XXX data is queued and acked as is. No buffer/window check */
        th->ack_seq = htonl(tcp_rsk(req)->rcv_nxt);