[PATCHv3] IPv4 TCP fails to send window scale option when window scale is zero

[Gilad Ben-Yossef <gilad@codefidence.com>]

From: Ori Finkelman <ori@comsleep.com>

Acknowledge TCP window scale support by inserting the proper option in SYN/ACK 
and SYN headers even if our window scale is zero.

This fixes the following observed behavior:

1. Client sends a SYN with TCP window scaling option and non zero window scale 
value to a Linux box.
2. Linux box notes large receive window from client.
3. Linux decides on a zero value of window scale for its part.
4. Due to compare against requested window scale size option, Linux does not to
 send windows scale TCP option header on SYN/ACK at all.

With the following result:

Client box thinks TCP window scaling is not supported, since SYN/ACK had no 
TCP window scale option, while Linux thinks that TCP window scaling is 
supported (and scale might be non zero), since SYN had  TCP window scale 
option and we have a mismatched idea between the client and server 
regarding window sizes.

Probably it also fixes up the following bug (not observed in practice):

1. Linux box opens TCP connection to some server.
2. Linux decides on zero value of window scale.
3. Due to compare against computed window scale size option, Linux does 
not to set windows scale TCP  option header on SYN. 

With the expected result that the server OS does not use window scale option 
due to not receiving such an option in the SYN headers, leading to suboptimal 
performance.

Signed-off-by: Gilad Ben-Yossef <gilad@codefidence.com>
Signed-off-by: Ori Finkelman <ori@comsleep.com>

---

Original bug reported and patch written by Ori Finkelman from Comsleep Ltd. 
I've fixed  the SYN header case based on feedback from Eric Dumazet and Ilpo 
Jarvinen, as part of trying to get the patch mainlined.

The SYN/ACK behavior was observed with a Windows box as the client and latest 
Debian kernel but for the best of my understanding this can happen with latest 
kernel versions and other client OS (probably also Linux) as well.

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 5200aab..fcd278a 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -361,6 +361,7 @@ static inline int tcp_urg_mode(const struct tcp_sock *tp)
 #define OPTION_SACK_ADVERTISE	(1 << 0)
 #define OPTION_TS		(1 << 1)
 #define OPTION_MD5		(1 << 2)
+#define OPTION_WSCALE		(1 << 3)
 
 struct tcp_out_options {
 	u8 options;		/* bit field of OPTION_* */
@@ -427,7 +428,7 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
 			       TCPOLEN_SACK_PERM);
 	}
 
-	if (unlikely(opts->ws)) {
+	if (unlikely(OPTION_WSCALE & opts->options)) {
 		*ptr++ = htonl((TCPOPT_NOP << 24) |
 			       (TCPOPT_WINDOW << 16) |
 			       (TCPOLEN_WINDOW << 8) |
@@ -494,8 +495,8 @@ static unsigned tcp_syn_options(struct sock *sk, struct sk_buff *skb,
 	}
 	if (likely(sysctl_tcp_window_scaling)) {
 		opts->ws = tp->rx_opt.rcv_wscale;
-		if (likely(opts->ws))
-			size += TCPOLEN_WSCALE_ALIGNED;
+		opts->options |= OPTION_WSCALE;
+		size += TCPOLEN_WSCALE_ALIGNED;
 	}
 	if (likely(sysctl_tcp_sack)) {
 		opts->options |= OPTION_SACK_ADVERTISE;
@@ -537,8 +538,8 @@ static unsigned tcp_synack_options(struct sock *sk,
 
 	if (likely(ireq->wscale_ok)) {
 		opts->ws = ireq->rcv_wscale;
-		if (likely(opts->ws))
-			size += TCPOLEN_WSCALE_ALIGNED;
+		opts->options |= OPTION_WSCALE;
+		size += TCPOLEN_WSCALE_ALIGNED;
 	}
 	if (likely(doing_ts)) {
 		opts->options |= OPTION_TS;