bpf: Adds support for setting initial cwnd

Adds a new bpf_setsockopt for TCP sockets, TCP_BPF_IW, which sets the
initial congestion window. This can be used when the hosts are far
apart (large RTTs) and it is safe to start with a large inital cwnd.

Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/bpf.h

@@ -783,4 +783,6 @@ enum {
 					 */
 };
 
+#define TCP_BPF_IW		1001	/* Set TCP initial congestion window */
+
 #endif /* _UAPI__LINUX_BPF_H__ */

net/core/filter.c

@@ -2732,7 +2732,23 @@ BPF_CALL_5(bpf_setsockopt, struct bpf_sock_ops_kern *, bpf_sock,
 				tcp_reinit_congestion_control(sk,
 					inet_csk(sk)->icsk_ca_ops);
 		} else {
-			ret = -EINVAL;
+			struct tcp_sock *tp = tcp_sk(sk);
+
+			if (optlen != sizeof(int))
+				return -EINVAL;
+
+			val = *((int *)optval);
+			/* Only some options are supported */
+			switch (optname) {
+			case TCP_BPF_IW:
+				if (val <= 0 || tp->data_segs_out > 0)
+					ret = -EINVAL;
+				else
+					tp->snd_cwnd = val;
+				break;
+			default:
+				ret = -EINVAL;
+			}
 		}
 #else
 		ret = -EINVAL;