[PATCH net] tcp: initialize send sequence numbers before deferring TFO

[Qingfang Deng <qingfang.deng@linux.dev>]

TCP_FASTOPEN_CONNECT can return from connect() before tcp_connect_init()
when a cached cookie or cookieless Fast Open allows deferring the SYN.
At this point write_seq contains the initial sequence number, while
snd_una and snd_nxt have not been initialized to that sequence number.

Consequently, inet_diag and /proc/net/tcp{,6} report the initial
sequence number as the send queue length despite no SYN or payload being
queued.

Initialize snd_una and snd_nxt to write_seq before marking the
connection as deferred. Both sequence-based queue lengths then remain
zero until the first write starts the connection and runs
tcp_connect_init().

Fixes: 19f6d3f3c842 ("net/tcp-fastopen: Add new API support")
Signed-off-by: Qingfang Deng <qingfang.deng@linux.dev>
---
Reproducer:

#define _GNU_SOURCE
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <stdio.h>
#include <sys/socket.h>
#include <unistd.h>

int main(void)
{
	struct sockaddr_in dst = {
		.sin_family = AF_INET,
		.sin_port = htons(9),
		.sin_addr.s_addr = htonl(INADDR_LOOPBACK),
	};
	struct tcp_info info = {0};
	socklen_t len = sizeof(info);
	int fd = socket(AF_INET, SOCK_STREAM, 0), one = 1;

	if (fd < 0 ||
	    setsockopt(fd, IPPROTO_TCP, TCP_FASTOPEN_CONNECT, &one, sizeof(one)) < 0 ||
	    setsockopt(fd, IPPROTO_TCP, TCP_FASTOPEN_NO_COOKIE, &one, sizeof(one)) < 0 ||
	    connect(fd, (struct sockaddr *)&dst, sizeof(dst)) < 0 ||
	    getsockopt(fd, IPPROTO_TCP, TCP_INFO, &info, &len) < 0) {
		perror("TFO setup");
		return 1;
	}
	if (info.tcpi_state != TCP_SYN_SENT) {
		fprintf(stderr, "Expected SYN_SENT, got state %u\n", info.tcpi_state);
		return 1;
	}
	printf("PID %ld: deferred SYN_SENT; no SYN or payload sent\n", (long)getpid());
	fflush(stdout);
	for (;;)
		pause();
}

`ss -ntp state syn-sent` will show a bogus Send-Q number, which is the
initial sequence number interpreted as signed 32-bit:

Recv-Q          Send-Q                        Local Address:Port                      Peer Address:Port          Process
0               -1405690727                       127.0.0.1:57290                        127.0.0.1:9              users:(("a.out",pid=63363,fd=3))

Diagnosis and implementation were assisted by GPT Codex.
---
 net/ipv4/tcp_fastopen.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv4/tcp_fastopen.c b/net/ipv4/tcp_fastopen.c
index 6dfe40322fb5..ff81e663ac61 100644
--- a/net/ipv4/tcp_fastopen.c
+++ b/net/ipv4/tcp_fastopen.c
@@ -545,6 +545,8 @@ bool tcp_fastopen_defer_connect(struct sock *sk, int *err)
 
 	if (tp->fastopen_connect && !tp->fastopen_req) {
 		if (tcp_fastopen_cookie_check(sk, &mss, &cookie)) {
+			WRITE_ONCE(tp->snd_una, tp->write_seq);
+			WRITE_ONCE(tp->snd_nxt, tp->write_seq);
 			inet_set_bit(DEFER_CONNECT, sk);
 			return true;
 		}
-- 
2.43.0