[RFC PATCH] tcp: advertise MSS requested by user

[Tom Quetchenbach <virtualphtn@gmail.com>]

I'm trying to use the TCP_MAXSEG option to setsockopt() to set the MSS
for both sides of a bidirectional connection.

man tcp says: "If this option is set before connection establishment, it
also changes the MSS value announced to the other end in the initial
packet."

However, the kernel only uses the MTU/route cache to set the advertised
MSS. That means if I set the MSS to, say, 500 before calling connect(),
I will send at most 500-byte packets, but I will still receive 1500-byte
packets in reply.

This patch reduces the advertised value to that requested by the user as
long as setsockopt() is called before connect() or accept(). This seems
like the behavior that one would expect as well as that which is
documented.

I've tried to make sure that things that depend on the advertised MSS
get set correctly, but I'd appreciate it if somebody more familiar with
the code than I am could check that I'm not breaking something.

Currently there's no way for the user to change the MSS of an established
connection--calling setsockopt() after the connection is established has
no effect. Is this something that should be changed?

Thanks,
-Tom

Signed-off-by: Tom Quetchenbach <virtualphtn@gmail.com>
---
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index a82df63..7a43e40 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1370,6 +1370,11 @@ struct sock *tcp_v4_syn_recv_sock(struct sock *sk, struct sk_buff *skb,
 	tcp_mtup_init(newsk);
 	tcp_sync_mss(newsk, dst_mtu(dst));
 	newtp->advmss = dst_metric(dst, RTAX_ADVMSS);
+
+	if (tcp_sk(sk)->rx_opt.user_mss &&
+				tcp_sk(sk)->rx_opt.user_mss < newtp->advmss)
+		newtp->advmss = tcp_sk(sk)->rx_opt.user_mss;
+
 	tcp_initialize_rcv_mss(newsk);
 
 #ifdef CONFIG_TCP_MD5SIG
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 1fa683c..2ccc864 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2239,6 +2239,7 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
 	struct sk_buff *skb;
 	struct tcp_md5sig_key *md5;
 	__u8 *md5_hash_location;
+	int mss;
 
 	skb = sock_wmalloc(sk, MAX_TCP_HEADER + 15, 1, GFP_ATOMIC);
 	if (skb == NULL)
@@ -2249,13 +2250,17 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
 
 	skb->dst = dst_clone(dst);
 
+	mss = dst_metric(dst, RTAX_ADVMSS);
+	if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss)
+		mss = tp->rx_opt.user_mss;
+
 	if (req->rcv_wnd == 0) { /* ignored for retransmitted syns */
 		__u8 rcv_wscale;
 		/* Set this up on the first call only */
 		req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW);
 		/* tcp_full_space because it is guaranteed to be the first packet */
 		tcp_select_initial_window(tcp_full_space(sk),
-			dst_metric(dst, RTAX_ADVMSS) - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
+			mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
 			&req->rcv_wnd,
 			&req->window_clamp,
 			ireq->wscale_ok,
@@ -2265,8 +2270,7 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
 
 	memset(&opts, 0, sizeof(opts));
 	TCP_SKB_CB(skb)->when = tcp_time_stamp;
-	tcp_header_size = tcp_synack_options(sk, req,
-					     dst_metric(dst, RTAX_ADVMSS),
+	tcp_header_size = tcp_synack_options(sk, req, mss,
 					     skb, &opts, &md5) +
 			  sizeof(struct tcphdr);
 
@@ -2340,6 +2344,11 @@ static void tcp_connect_init(struct sock *sk)
 	if (!tp->window_clamp)
 		tp->window_clamp = dst_metric(dst, RTAX_WINDOW);
 	tp->advmss = dst_metric(dst, RTAX_ADVMSS);
+
+	if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->advmss){
+		tp->advmss = tp->rx_opt.user_mss;
+	}
+
 	tcp_initialize_rcv_mss(sk);
 
 	tcp_select_initial_window(tcp_full_space(sk),