queue-5.15/tcp-tcp_tx_timestamp-must-look-at-the-rtx-queue.patch

From 3ecf52ac5fee37cc3aa799b1d806f8ac8ad6dd93 Mon Sep 17 00:00:00 2001
From: Sasha Levin <sashal@kernel.org>
Date: Tue, 27 Jan 2026 12:38:27 +0000
Subject: tcp: tcp_tx_timestamp() must look at the rtx queue

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit 838eb9687691d29915797a885b861fd09353386e ]

tcp_tx_timestamp() is only called at the end of tcp_sendmsg_locked()
before the final tcp_push().

By the time it is called, it is possible all the copied data
has been sent already (transmit queue is empty).

If this is the case, use the last skb in the rtx queue.

Fixes: 75c119afe14f ("tcp: implement rb-tree based retransmit queue")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://patch.msgid.link/20260127123828.4098577-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/ipv4/tcp.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index b3d373372e841..36981a3e9013f 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -471,6 +471,9 @@ static void tcp_tx_timestamp(struct sock *sk, u16 tsflags)
 {
 	struct sk_buff *skb = tcp_write_queue_tail(sk);
 
+	if (unlikely(!skb))
+		skb = skb_rb_last(&sk->tcp_rtx_queue);
+
 	if (tsflags && skb) {
 		struct skb_shared_info *shinfo = skb_shinfo(skb);
 		struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);
-- 
2.51.0