netdev - [PATCH net-next 1/2] tcp: use tp->total_rto to track number of linear timeouts in SYN

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Date: Tue, 14 Nov 2023 17:23:40 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Neal Cardwell <ncardwell@...gle.com>, Yuchung Cheng <ycheng@...gle.com>, 
	David Morley <morleyd@...gle.com>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next 1/2] tcp: use tp->total_rto to track number of linear
 timeouts in SYN_SENT state

In commit ccce324dabfe ("tcp: make the first N SYN RTO backoffs linear")
David used icsk->icsk_backoff field to track the number of linear timeouts.

Since then, tp->total_rto has been added.

This commit uses tp->total_rto instead of icsk->icsk_backoff
so that tcp_ld_RTO_revert() no longer can trigger an overflow
in inet_csk_rto_backoff(). Other than the potential UBSAN
report, there was no issue because receiving an ICMP message
currently aborts the connect().

In the following patch, we want to adhere to RFC 6069
and RFC 1122 4.2.3.9.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: David Morley <morleyd@...gle.com>
Cc: Neal Cardwell <ncardwell@...gle.com>
Cc: Yuchung Cheng <ycheng@...gle.com>
---
 net/ipv4/tcp_timer.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 1f9f6c1c196b2de35b0bc2f734484f09ba90541a..d1ad20ce1c8c7c013b8b0f26d71c0b0bc4800354 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -626,7 +626,6 @@ void tcp_retransmit_timer(struct sock *sk)
 	 * implemented ftp to mars will work nicely. We will have to fix
 	 * the 120 second clamps though!
 	 */
-	icsk->icsk_backoff++;
 
 out_reset_timer:
 	/* If stream is thin, use linear timeouts. Since 'icsk_backoff' is
@@ -647,11 +646,12 @@ void tcp_retransmit_timer(struct sock *sk)
 				       tcp_rto_min(sk),
 				       TCP_RTO_MAX);
 	} else if (sk->sk_state != TCP_SYN_SENT ||
-		   icsk->icsk_backoff >
+		   tp->total_rto >
 		   READ_ONCE(net->ipv4.sysctl_tcp_syn_linear_timeouts)) {
 		/* Use normal (exponential) backoff unless linear timeouts are
 		 * activated.
 		 */
+		icsk->icsk_backoff++;
 		icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
 	}
 	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
-- 
2.42.0.869.gea05f2083d-goog