netdev - Re: [RFC net] tcp: ensure PMTU updates are processed during fastopen

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Date:   Thu, 17 Mar 2022 09:40:53 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     davem@...emloft.net, edumazet@...gle.com, weiwan@...gle.com,
        netdev@...r.kernel.org, ntspring@...com
Subject: Re: [RFC net] tcp: ensure PMTU updates are processed during fastopen

On Wed, Mar 16, 2022 at 4:59 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> tp->rx_opt.mss_clamp is not populated, yet, during TFO send so we
> rise it to the local MSS. tp->mss_cache is not updated, however:
>
> tcp_v6_connect():
>   tp->rx_opt.mss_clamp = IPV6_MIN_MTU - headers;
>   tcp_connect():
>      tcp_connect_init():
>        tp->mss_cache = min(mtu, tp->rx_opt.mss_clamp)
>      tcp_send_syn_data():
>        tp->rx_opt.mss_clamp = tp->advmss
>
> After recent fixes to ICMPv6 PTB handling we started dropping
> PMTU updates higher than tp->mss_cache. Because of the stale
> tp->mss_cache value PMTU updates during TFO are always dropped.
>
> Thanks to Wei for helping zero in on the problem and the fix!
>
> Fixes: c7bb4b89033b ("ipv6: tcp: drop silly ICMPv6 packet too big messages")
> Reported-by: Andre Nash <alnash@...com>
> Reported-by: Neil Spring <ntspring@...com>
> Reviewed-by: Wei Wang <weiwan@...gle.com>
Acked-by: Yuchung Cheng <ycheng@...gle.com>

Thanks for the fix.

> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---
>  net/ipv4/tcp_output.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 5079832af5c1..257780f93305 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -3719,6 +3719,7 @@ static void tcp_connect_queue_skb(struct sock *sk, struct sk_buff *skb)
>   */
>  static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
>  {
> +       struct inet_connection_sock *icsk = inet_csk(sk);
>         struct tcp_sock *tp = tcp_sk(sk);
>         struct tcp_fastopen_request *fo = tp->fastopen_req;
>         int space, err = 0;
> @@ -3733,8 +3734,10 @@ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
>          * private TCP options. The cost is reduced data space in SYN :(
>          */
>         tp->rx_opt.mss_clamp = tcp_mss_clamp(tp, tp->rx_opt.mss_clamp);
> +       /* Sync mss_cache after updating the mss_clamp */
> +       tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
>
> -       space = __tcp_mtu_to_mss(sk, inet_csk(sk)->icsk_pmtu_cookie) -
> +       space = __tcp_mtu_to_mss(sk, icsk->icsk_pmtu_cookie) -
>                 MAX_TCP_OPTION_SPACE;
>
>         space = min_t(size_t, space, fo->size);
> --
> 2.34.1
>