tcp: fix potential underestimation on rcv_rtt

When ms timestamp is used, current logic uses 1us in
tcp_rcv_rtt_update() when the real rcv_rtt is within 1 - 999us.
This could cause rcv_rtt underestimation.
Fix it by always using a min value of 1ms if ms timestamp is used.

Fixes: 645f4c6f2ebd ("tcp: switch rcv_rtt_est and rcvq_space to high resolution timestamps")
Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -508,9 +508,6 @@ static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
 	u32 new_sample = tp->rcv_rtt_est.rtt_us;
 	long m = sample;
 
-	if (m == 0)
-		m = 1;
-
 	if (new_sample != 0) {
 		/* If we sample in larger samples in the non-timestamp
 		 * case, we could grossly overestimate the RTT especially
@@ -547,6 +544,8 @@ static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)
 	if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))
 		return;
 	delta_us = tcp_stamp_us_delta(tp->tcp_mstamp, tp->rcv_rtt_est.time);
+	if (!delta_us)
+		delta_us = 1;
 	tcp_rcv_rtt_update(tp, delta_us, 1);
 
 new_measure:
@@ -563,8 +562,11 @@ static inline void tcp_rcv_rtt_measure_ts(struct sock *sk,
 	    (TCP_SKB_CB(skb)->end_seq -
 	     TCP_SKB_CB(skb)->seq >= inet_csk(sk)->icsk_ack.rcv_mss)) {
 		u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr;
-		u32 delta_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
+		u32 delta_us;
 
+		if (!delta)
+			delta = 1;
+		delta_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
 		tcp_rcv_rtt_update(tp, delta_us, 0);
 	}
 }