On 01/11/17 - 14:53:38, Eric Dumazet wrote:
> On Wed, 2017-11-01 at 14:32 -0700, Eric Dumazet wrote:
> > On Wed, Nov 1, 2017 at 2:10 PM, Christoph Paasch <cpaa...@apple.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);
Yes, that looks good to me. Thanks!
But we still need to clean up the skb in tcp_v4_send_reset and
tcp_v4_send_ack, as the fields (e.g., tcp_skb_cb->seq) have been set when
coming from tcp_v4_rcv.
Christoph
