netdev - Re: [PATCH net-next 1/1] net/ipv4: disable SMC TCP option with SYN Cookies

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Date:   Tue, 20 Mar 2018 09:21:32 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Ursula Braun <ubraun@...ux.vnet.ibm.com>, davem@...emloft.net
Cc:     netdev@...r.kernel.org, linux-s390@...r.kernel.org,
        schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        raspl@...ux.vnet.ibm.com
Subject: Re: [PATCH net-next 1/1] net/ipv4: disable SMC TCP option with SYN
 Cookies



On 03/20/2018 08:53 AM, Ursula Braun wrote:
> From: Hans Wippel <hwippel@...ux.vnet.ibm.com>
> 
> Currently, the SMC experimental TCP option in a SYN packet is lost on
> the server side when SYN Cookies are active. However, the corresponding
> SYNACK sent back to the client contains the SMC option. This causes an
> inconsistent view of the SMC capabilities on the client and server.
> 
> This patch disables the SMC option in the SYNACK when SYN Cookies are
> active to avoid this issue.
> 
> Signed-off-by: Hans Wippel <hwippel@...ux.vnet.ibm.com>
> Signed-off-by: Ursula Braun <ubraun@...ux.vnet.ibm.com>
> ---
>  net/ipv4/tcp_output.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 383cac0ff0ec..22894514feae 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -3199,6 +3199,8 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
>  		/* Under synflood, we do not attach skb to a socket,
>  		 * to avoid false sharing.
>  		 */
> +		if (IS_ENABLED(CONFIG_SMC))
> +			ireq->smc_ok = 0;
>  		break;
>  	case TCP_SYNACK_FASTOPEN:
>  		/* sk is a const pointer, because we want to express multiple
> 

I disagree with net-next qualification.

This fixes a bug, so please send it for net tree, and including an appropriate Fixes: tag.

Thanks !