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

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Date:   Mon, 21 Mar 2022 12:32:34 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     David Miller <davem@...emloft.net>,
        Yuchung Cheng <ycheng@...gle.com>,
        Wei Wang <weiwan@...gle.com>, netdev <netdev@...r.kernel.org>,
        Neil Spring <ntspring@...com>
Subject: Re: [PATCH net] tcp: ensure PMTU updates are processed during fastopen

On Mon, Mar 21, 2022 at 10:00 AM 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>
> Acked-by: Martin KaFai Lau <kafai@...com>
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---

Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Do you have a packetdrill test by any chance ?

Thanks!