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

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Date:   Thu, 17 Mar 2022 16:11:20 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     davem@...emloft.net, edumazet@...gle.com, ycheng@...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 04:59:08PM -0700, Jakub Kicinski 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>
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
Acked-by: Martin KaFai Lau <kafai@...com>