[MPTCP] [PATCH 05/18] tcp6: Prepare for maximum TCP option-space

[Christoph Paasch <cpaasch at apple.com>]

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With a generic TCP-option framework we don't know in advance how much
space will be used. So, estimate the worst and allocate the maximum.
Later on, we pull skb->tail back in again.

Signed-off-by: Christoph Paasch <cpaasch(a)apple.com>
---
 net/ipv6/tcp_ipv6.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 64d94afa427f..17f7e0cd8755 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -784,17 +784,11 @@ static void tcp_v6_send_response(const struct sock *sk, struct sk_buff *skb, u32
 	struct flowi6 fl6;
 	struct net *net = sk ? sock_net(sk) : dev_net(skb_dst(skb)->dev);
 	struct sock *ctl_sk = net->ipv6.tcp_sk;
-	unsigned int tot_len = sizeof(struct tcphdr);
+	unsigned int tot_len = sizeof(struct tcphdr) + MAX_TCP_OPTION_SPACE;
+	unsigned int reduce = 0; /* By how much to pull skb->tail back in? */
 	struct dst_entry *dst;
 	__be32 *topt;
 
-	if (tsecr)
-		tot_len += TCPOLEN_TSTAMP_ALIGNED;
-#ifdef CONFIG_TCP_MD5SIG
-	if (key)
-		tot_len += TCPOLEN_MD5SIG_ALIGNED;
-#endif
-
 	buff = alloc_skb(MAX_HEADER + sizeof(struct ipv6hdr) + tot_len,
 			 GFP_ATOMIC);
 	if (!buff)
@@ -823,6 +817,8 @@ static void tcp_v6_send_response(const struct sock *sk, struct sk_buff *skb, u32
 				(TCPOPT_TIMESTAMP << 8) | TCPOLEN_TIMESTAMP);
 		*topt++ = htonl(tsval);
 		*topt++ = htonl(tsecr);
+
+		reduce += TCPOLEN_TSTAMP_ALIGNED;
 	}
 
 #ifdef CONFIG_TCP_MD5SIG
@@ -832,9 +828,13 @@ static void tcp_v6_send_response(const struct sock *sk, struct sk_buff *skb, u32
 		tcp_v6_md5_hash_hdr((__u8 *)topt, key,
 				    &ipv6_hdr(skb)->saddr,
 				    &ipv6_hdr(skb)->daddr, t1);
+
+		reduce += TCPOLEN_MD5SIG_ALIGNED;
 	}
 #endif
 
+	buff->tail -= reduce;
+
 	memset(&fl6, 0, sizeof(fl6));
 	fl6.daddr = ipv6_hdr(skb)->saddr;
 	fl6.saddr = ipv6_hdr(skb)->daddr;
-- 
2.14.1