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>

1 files changed, 6 insertions, 4 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 9550cc42de2d..45f750e85714 100644
--- a/net/ipv4/tcp_input.c
+++ b/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);
         }
 }