tcp: report ECN_SEEN in tcp_info

Allows ss command (iproute2) to display "ecnseen" if at least one packet
with ECT(0) or ECT(1) or ECN was received by this socket.

"ecn" means ECN was negotiated at session establishment (TCP level)

"ecnseen" means we received at least one packet with ECT fields set (IP
level)

ss -i
...
ESTAB      0      0   192.168.20.110:22  192.168.20.144:38016
ino:5950 sk:f178e400
	 mem:(r0,w0,f0,t0) ts sack ecn ecnseen bic wscale:7,8 rto:210
rtt:12.5/7.5 cwnd:10 send 9.3Mbps rcv_space:14480
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/tcp.h

@@ -111,7 +111,8 @@ enum {
 #define TCPI_OPT_TIMESTAMPS	1
 #define TCPI_OPT_SACK		2
 #define TCPI_OPT_WSCALE		4
-#define TCPI_OPT_ECN		8
+#define TCPI_OPT_ECN		8 /* ECN was negociated at TCP session init */
+#define TCPI_OPT_ECN_SEEN	16 /* we received at least one packet with ECT */
 
 enum tcp_ca_state {
 	TCP_CA_Open = 0,

net/ipv4/tcp.c

@@ -2455,8 +2455,10 @@ void tcp_get_info(struct sock *sk, struct tcp_info *info)
 		info->tcpi_rcv_wscale = tp->rx_opt.rcv_wscale;
 	}
 
-	if (tp->ecn_flags&TCP_ECN_OK)
+	if (tp->ecn_flags & TCP_ECN_OK)
 		info->tcpi_options |= TCPI_OPT_ECN;
+	if (tp->ecn_flags & TCP_ECN_SEEN)
+		info->tcpi_options |= TCPI_OPT_ECN_SEEN;
 
 	info->tcpi_rto = jiffies_to_usecs(icsk->icsk_rto);
 	info->tcpi_ato = jiffies_to_usecs(icsk->icsk_ack.ato);