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

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Date:   Tue, 20 Mar 2018 16:53:40 +0100
From:   Ursula Braun <ubraun@...ux.vnet.ibm.com>
To:     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, ubraun@...ux.vnet.ibm.com
Subject: [PATCH net-next 1/1] net/ipv4: disable SMC TCP option with SYN Cookies

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
-- 
2.13.5