linux-kernel - [PATCH net] tcp: reset tp->sacked

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Date:   Wed, 26 Oct 2022 23:15:58 +0800
From:   Lu Wei <luwei32@...wei.com>
To:     <edumazet@...gle.com>, <davem@...emloft.net>,
        <yoshfuji@...ux-ipv6.org>, <dsahern@...nel.org>, <kuba@...nel.org>,
        <pabeni@...hat.com>, <xemul@...allels.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: [PATCH net] tcp: reset tp->sacked_out when sack is enabled

The meaning of tp->sacked_out depends on whether sack is enabled
or not. If setsockopt is called to enable sack_ok via
tcp_repair_options_est(), tp->sacked_out should be cleared, or it
will trigger warning in tcp_verify_left_out as follows:

============================================
WARNING: CPU: 8 PID: 0 at net/ipv4/tcp_input.c:2132
tcp_timeout_mark_lost+0x154/0x160
tcp_enter_loss+0x2b/0x290
tcp_retransmit_timer+0x50b/0x640
tcp_write_timer_handler+0x1c8/0x340
tcp_write_timer+0xe5/0x140
call_timer_fn+0x3a/0x1b0
__run_timers.part.0+0x1bf/0x2d0
run_timer_softirq+0x43/0xb0
__do_softirq+0xfd/0x373
__irq_exit_rcu+0xf6/0x140

This warning occurs in several steps:
Step1. If sack is not enabled, when server receives dup-ack,
       it calls tcp_add_reno_sack() to increase tp->sacked_out.

Step2. Setsockopt() is called to enable sack

Step3. The retransmit timer expires, it calls tcp_timeout_mark_lost()
       to increase tp->lost_out but not clear tp->sacked_out because
       sack is enabled and tcp_is_reno() is false.

So tp->left_out is increased repeatly in Step1 and Step3 and it is
greater than tp->packets_out and trigger the warning. In function
tcp_timeout_mark_lost(), tp->sacked_out will be cleared if Step2 not
happen and the warning will not be triggered. So this patch clears
tp->sacked_out in tcp_repair_options_est().

Fixes: b139ba4e90dc ("tcp: Repair connection-time negotiated parameters")
Signed-off-by: Lu Wei <luwei32@...wei.com>
---
 net/ipv4/tcp.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index ef14efa1fb70..188d5c0e440f 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3282,6 +3282,9 @@ static int tcp_repair_options_est(struct sock *sk, sockptr_t optbuf,
 			if (opt.opt_val != 0)
 				return -EINVAL;
 
+			if (tcp_is_reno(tp))
+				tp->sacked_out = 0;
+
 			tp->rx_opt.sack_ok |= TCP_SACK_SEEN;
 			break;
 		case TCPOPT_TIMESTAMP:
-- 
2.31.1