linux-kernel - [PATCH stable 4.19 4/4] tcp: adjust rto_base in retransmits_timed

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Date:   Sun, 26 Sep 2021 15:18:42 +0800
From:   Qiumiao Zhang <zhangqiumiao1@...wei.com>
To:     <linux-kernel@...r.kernel.org>
CC:     <stable@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sasha Levin <sasha.levin@...cle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        <netdev@...r.kernel.org>, <yanan@...wei.com>,
        <rose.chen@...wei.com>
Subject: [PATCH stable 4.19 4/4] tcp: adjust rto_base in retransmits_timed_out()

From: Eric Dumazet <edumazet@...gle.com>

commit 3256a2d6ab1f71f9a1bd2d7f6f18eb8108c48d17 upstream

The cited commit exposed an old retransmits_timed_out() bug
which assumed it could call tcp_model_timeout() with
TCP_RTO_MIN as rto_base for all states.

But flows in SYN_SENT or SYN_RECV state uses a different
RTO base (1 sec instead of 200 ms, unless BPF choses
another value)

This caused a reduction of SYN retransmits from 6 to 4 with
the default /proc/sys/net/ipv4/tcp_syn_retries value.

Fixes: a41e8a88b06e ("tcp: better handle TCP_USER_TIMEOUT in SYN_SENT state")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Yuchung Cheng <ycheng@...gle.com>
Cc: Marek Majkowski <marek@...udflare.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Qiumiao Zhang <zhangqiumiao1@...wei.com>
---
 net/ipv4/tcp_timer.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 9e9507f125a2..d071ed6b8b9a 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -197,8 +197,13 @@ static bool retransmits_timed_out(struct sock *sk,
 		return false;
 
 	start_ts = tcp_sk(sk)->retrans_stamp;
-	if (likely(timeout == 0))
-		timeout = tcp_model_timeout(sk, boundary, TCP_RTO_MIN);
+	if (likely(timeout == 0)) {
+		unsigned int rto_base = TCP_RTO_MIN;
+
+		if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV))
+			rto_base = tcp_timeout_init(sk);
+		timeout = tcp_model_timeout(sk, boundary, rto_base);
+	}
 
 	return (s32)(tcp_time_stamp(tcp_sk(sk)) - start_ts - timeout) >= 0;
 }
-- 
2.19.1