tcp: consolidate paws check

Wow, it was quite tricky to merge that stream of negations
but I think I finally got it right:

check & replace_ts_recent:
(s32)(rcv_tsval - ts_recent) >= 0                  => 0
(s32)(ts_recent - rcv_tsval) <= 0                  => 0

discard:
(s32)(ts_recent - rcv_tsval)  > TCP_PAWS_WINDOW    => 1
(s32)(ts_recent - rcv_tsval) <= TCP_PAWS_WINDOW    => 0

I toggled the return values of tcp_paws_check around since
the old encoding added yet-another negation making tracking
of truth-values really complicated.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -997,11 +997,21 @@ static inline int tcp_fin_time(const struct sock *sk)
 	return fin_timeout;
 }
 
-static inline int tcp_paws_check(const struct tcp_options_received *rx_opt, int rst)
+static inline int tcp_paws_check(const struct tcp_options_received *rx_opt,
+				 int paws_win)
 {
-	if ((s32)(rx_opt->rcv_tsval - rx_opt->ts_recent) >= 0)
-		return 0;
-	if (get_seconds() >= rx_opt->ts_recent_stamp + TCP_PAWS_24DAYS)
+	if ((s32)(rx_opt->ts_recent - rx_opt->rcv_tsval) <= paws_win)
+		return 1;
+	if (unlikely(get_seconds() >= rx_opt->ts_recent_stamp + TCP_PAWS_24DAYS))
+		return 1;
+
+	return 0;
+}
+
+static inline int tcp_paws_reject(const struct tcp_options_received *rx_opt,
+				  int rst)
+{
+	if (tcp_paws_check(rx_opt, 0))
 		return 0;
 
 	/* RST segments are not recommended to carry timestamp,

net/ipv4/tcp_input.c

@@ -3883,8 +3883,7 @@ static inline void tcp_replace_ts_recent(struct tcp_sock *tp, u32 seq)
 		 * Not only, also it occurs for expired timestamps.
 		 */
 
-		if ((s32)(tp->rx_opt.rcv_tsval - tp->rx_opt.ts_recent) >= 0 ||
-		   get_seconds() >= tp->rx_opt.ts_recent_stamp + TCP_PAWS_24DAYS)
+		if (tcp_paws_check(&tp->rx_opt, 0))
 			tcp_store_ts_recent(tp);
 	}
 }
@@ -3936,9 +3935,9 @@ static inline int tcp_paws_discard(const struct sock *sk,
 				   const struct sk_buff *skb)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
-	return ((s32)(tp->rx_opt.ts_recent - tp->rx_opt.rcv_tsval) > TCP_PAWS_WINDOW &&
-		get_seconds() < tp->rx_opt.ts_recent_stamp + TCP_PAWS_24DAYS &&
-		!tcp_disordered_ack(sk, skb));
+
+	return !tcp_paws_check(&tp->rx_opt, TCP_PAWS_WINDOW) &&
+	       !tcp_disordered_ack(sk, skb);
 }
 
 /* Check segment sequence number for validity.
@@ -5513,7 +5512,7 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 
 	/* PAWS check. */
 	if (tp->rx_opt.ts_recent_stamp && tp->rx_opt.saw_tstamp &&
-	    tcp_paws_check(&tp->rx_opt, 0))
+	    tcp_paws_reject(&tp->rx_opt, 0))
 		goto discard_and_undo;
 
 	if (th->syn) {

net/ipv4/tcp_minisocks.c

@@ -107,7 +107,7 @@ tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
 		if (tmp_opt.saw_tstamp) {
 			tmp_opt.ts_recent	= tcptw->tw_ts_recent;
 			tmp_opt.ts_recent_stamp	= tcptw->tw_ts_recent_stamp;
-			paws_reject = tcp_paws_check(&tmp_opt, th->rst);
+			paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
 		}
 	}
 
@@ -511,7 +511,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 			 * from another data.
 			 */
 			tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->retrans);
-			paws_reject = tcp_paws_check(&tmp_opt, th->rst);
+			paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
 		}
 	}