bpf: Adds support for setting window clamp

Adds a new bpf_setsockopt for TCP sockets, TCP_BPF_WINDOW_CLAMP,
which sets the maximum receiver window size. It will be useful for
limiting receiver window based on RTT.
Signed-off-by: Prankur gupta <prankgup@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201202213152.435886-2-prankgup@fb.com

include/net/tcp.h

@@ -406,6 +406,7 @@ void tcp_syn_ack_timeout(const struct request_sock *req);
 int tcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int nonblock,
 		int flags, int *addr_len);
 int tcp_set_rcvlowat(struct sock *sk, int val);
+int tcp_set_window_clamp(struct sock *sk, int val);
 void tcp_data_ready(struct sock *sk);
 #ifdef CONFIG_MMU
 int tcp_mmap(struct file *file, struct socket *sock,

net/core/filter.c

@@ -4910,6 +4910,9 @@ static int _bpf_setsockopt(struct sock *sk, int level, int optname,
 				tp->notsent_lowat = val;
 				sk->sk_write_space(sk);
 				break;
+			case TCP_WINDOW_CLAMP:
+				ret = tcp_set_window_clamp(sk, val);
+				break;
 			default:
 				ret = -EINVAL;
 			}

net/ipv4/tcp.c

@@ -3022,6 +3022,21 @@ int tcp_sock_set_keepcnt(struct sock *sk, int val)
 }
 EXPORT_SYMBOL(tcp_sock_set_keepcnt);
 
+int tcp_set_window_clamp(struct sock *sk, int val)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (!val) {
+		if (sk->sk_state != TCP_CLOSE)
+			return -EINVAL;
+		tp->window_clamp = 0;
+	} else {
+		tp->window_clamp = val < SOCK_MIN_RCVBUF / 2 ?
+			SOCK_MIN_RCVBUF / 2 : val;
+	}
+	return 0;
+}
+
 /*
  *	Socket option code for TCP.
  */
@@ -3235,15 +3250,7 @@ static int do_tcp_setsockopt(struct sock *sk, int level, int optname,
 		break;
 
 	case TCP_WINDOW_CLAMP:
-		if (!val) {
-			if (sk->sk_state != TCP_CLOSE) {
-				err = -EINVAL;
-				break;
-			}
-			tp->window_clamp = 0;
-		} else
-			tp->window_clamp = val < SOCK_MIN_RCVBUF / 2 ?
-						SOCK_MIN_RCVBUF / 2 : val;
+		err = tcp_set_window_clamp(sk, val);
 		break;
 
 	case TCP_QUICKACK: