linux-kernel - [PATCH 5.15 335/779] tcp: make retransmitted SKB fit into the send window

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Message-Id: <20220815180351.597849976@linuxfoundation.org>
Date:   Mon, 15 Aug 2022 19:59:39 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Yonglong Li <liyonglong@...natelecom.cn>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.15 335/779] tcp: make retransmitted SKB fit into the send window

From: Yonglong Li <liyonglong@...natelecom.cn>

[ Upstream commit 536a6c8e05f95e3d1118c40ae8b3022ee2d05d52 ]

current code of __tcp_retransmit_skb only check TCP_SKB_CB(skb)->seq
in send window, and TCP_SKB_CB(skb)->seq_end maybe out of send window.
If receiver has shrunk his window, and skb is out of new window,  it
should retransmit a smaller portion of the payload.

test packetdrill script:
    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
   +0 fcntl(3, F_GETFL) = 0x2 (flags O_RDWR)
   +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0

   +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
   +0 > S 0:0(0)  win 65535 <mss 1460,sackOK,TS val 100 ecr 0,nop,wscale 8>
 +.05 < S. 0:0(0) ack 1 win 6000 <mss 1000,nop,nop,sackOK>
   +0 > . 1:1(0) ack 1

   +0 write(3, ..., 10000) = 10000

   +0 > . 1:2001(2000) ack 1 win 65535
   +0 > . 2001:4001(2000) ack 1 win 65535
   +0 > . 4001:6001(2000) ack 1 win 65535

 +.05 < . 1:1(0) ack 4001 win 1001

and tcpdump show:
192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 1:2001, ack 1, win 65535, length 2000
192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 2001:4001, ack 1, win 65535, length 2000
192.168.226.67.55 > 192.0.2.1.8080: Flags [P.], seq 4001:5001, ack 1, win 65535, length 1000
192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 5001:6001, ack 1, win 65535, length 1000
192.0.2.1.8080 > 192.168.226.67.55: Flags [.], ack 4001, win 1001, length 0
192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 5001:6001, ack 1, win 65535, length 1000
192.168.226.67.55 > 192.0.2.1.8080: Flags [P.], seq 4001:5001, ack 1, win 65535, length 1000

when cient retract window to 1001, send window is [4001,5002],
but TLP send 5001-6001 packet which is out of send window.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Yonglong Li <liyonglong@...natelecom.cn>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Link: https://lore.kernel.org/r/1657532838-20200-1-git-send-email-liyonglong@chinatelecom.cn
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 net/ipv4/tcp_output.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 9c9a0f7a3dee..24cadcdb9890 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3145,7 +3145,7 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 	struct tcp_sock *tp = tcp_sk(sk);
 	unsigned int cur_mss;
 	int diff, len, err;
-
+	int avail_wnd;
 
 	/* Inconclusive MTU probe */
 	if (icsk->icsk_mtup.probe_size)
@@ -3167,17 +3167,25 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 		return -EHOSTUNREACH; /* Routing failure or similar. */
 
 	cur_mss = tcp_current_mss(sk);
+	avail_wnd = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
 
 	/* If receiver has shrunk his window, and skb is out of
 	 * new window, do not retransmit it. The exception is the
 	 * case, when window is shrunk to zero. In this case
-	 * our retransmit serves as a zero window probe.
+	 * our retransmit of one segment serves as a zero window probe.
 	 */
-	if (!before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp)) &&
-	    TCP_SKB_CB(skb)->seq != tp->snd_una)
-		return -EAGAIN;
+	if (avail_wnd <= 0) {
+		if (TCP_SKB_CB(skb)->seq != tp->snd_una)
+			return -EAGAIN;
+		avail_wnd = cur_mss;
+	}
 
 	len = cur_mss * segs;
+	if (len > avail_wnd) {
+		len = rounddown(avail_wnd, cur_mss);
+		if (!len)
+			len = avail_wnd;
+	}
 	if (skb->len > len) {
 		if (tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb, len,
 				 cur_mss, GFP_ATOMIC))
@@ -3191,8 +3199,9 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 		diff -= tcp_skb_pcount(skb);
 		if (diff)
 			tcp_adjust_pcount(sk, skb, diff);
-		if (skb->len < cur_mss)
-			tcp_retrans_try_collapse(sk, skb, cur_mss);
+		avail_wnd = min_t(int, avail_wnd, cur_mss);
+		if (skb->len < avail_wnd)
+			tcp_retrans_try_collapse(sk, skb, avail_wnd);
 	}
 
 	/* RFC3168, section 6.1.1.1. ECN fallback */
-- 
2.35.1