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

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Message-ID: <CANn89iLv+0+A79-pNu8HZ2kBbyNt2ini2Zjbm++s9TT7mtOamw@mail.gmail.com>
Date:   Wed, 1 Nov 2017 14:32:53 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Christoph Paasch <cpaasch@...le.com>
Cc:     David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] tcp: Always cleanup skb before sending

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);