tcp: add tcp_mss_clamp() helper

Small cleanup factorizing code doing the TCP_MAXSEG clamping.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/tcp.h

@@ -445,4 +445,13 @@ static inline void tcp_saved_syn_free(struct tcp_sock *tp)
 
 struct sk_buff *tcp_get_timestamping_opt_stats(const struct sock *sk);
 
+static inline u16 tcp_mss_clamp(const struct tcp_sock *tp, u16 mss)
+{
+	/* We use READ_ONCE() here because socket might not be locked.
+	 * This happens for listeners.
+	 */
+	u16 user_mss = READ_ONCE(tp->rx_opt.user_mss);
+
+	return (user_mss && user_mss < mss) ? user_mss : mss;
+}
 #endif	/* _LINUX_TCP_H */

net/ipv4/tcp_ipv4.c

@@ -1324,10 +1324,7 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 	tcp_ca_openreq_child(newsk, dst);
 
 	tcp_sync_mss(newsk, dst_mtu(dst));
-	newtp->advmss = dst_metric_advmss(dst);
-	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;
+	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), dst_metric_advmss(dst));
 
 	tcp_initialize_rcv_mss(newsk);
 

net/ipv4/tcp_minisocks.c

@@ -360,15 +360,12 @@ void tcp_openreq_init_rwin(struct request_sock *req,
 {
 	struct inet_request_sock *ireq = inet_rsk(req);
 	const struct tcp_sock *tp = tcp_sk(sk_listener);
-	u16 user_mss = READ_ONCE(tp->rx_opt.user_mss);
 	int full_space = tcp_full_space(sk_listener);
-	int mss = dst_metric_advmss(dst);
 	u32 window_clamp;
 	__u8 rcv_wscale;
+	int mss;
 
-	if (user_mss && user_mss < mss)
-		mss = user_mss;
-
+	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
 	window_clamp = READ_ONCE(tp->window_clamp);
 	/* Set this up on the first call only */
 	req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);

net/ipv4/tcp_output.c

@@ -3062,7 +3062,6 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
 	struct sk_buff *skb;
 	int tcp_header_size;
 	struct tcphdr *th;
-	u16 user_mss;
 	int mss;
 
 	skb = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
@@ -3092,10 +3091,7 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
 	}
 	skb_dst_set(skb, dst);
 
-	mss = dst_metric_advmss(dst);
-	user_mss = READ_ONCE(tp->rx_opt.user_mss);
-	if (user_mss && user_mss < mss)
-		mss = user_mss;
+	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
 
 	memset(&opts, 0, sizeof(opts));
 #ifdef CONFIG_SYN_COOKIES
@@ -3201,9 +3197,7 @@ static void tcp_connect_init(struct sock *sk)
 
 	if (!tp->window_clamp)
 		tp->window_clamp = dst_metric(dst, RTAX_WINDOW);
-	tp->advmss = dst_metric_advmss(dst);
-	if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->advmss)
-		tp->advmss = tp->rx_opt.user_mss;
+	tp->advmss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
 
 	tcp_initialize_rcv_mss(sk);
 
@@ -3280,8 +3274,8 @@ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
 	 * user-MSS. Reserve maximum option space for middleboxes that add
 	 * private TCP options. The cost is reduced data space in SYN :(
 	 */
-	if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->rx_opt.mss_clamp)
-		tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
+	tp->rx_opt.mss_clamp = tcp_mss_clamp(tp, tp->rx_opt.mss_clamp);
+
 	space = __tcp_mtu_to_mss(sk, inet_csk(sk)->icsk_pmtu_cookie) -
 		MAX_TCP_OPTION_SPACE;
 

net/ipv6/tcp_ipv6.c

@@ -1147,10 +1147,7 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 	tcp_ca_openreq_child(newsk, dst);
 
 	tcp_sync_mss(newsk, dst_mtu(dst));
-	newtp->advmss = dst_metric_advmss(dst);
-	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;
+	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), dst_metric_advmss(dst));
 
 	tcp_initialize_rcv_mss(newsk);