[PATCH mptcp-next] selftests: mptcp: sockopt: test passive Fast Open early write push

[PATCH mptcp-next] selftests: mptcp: sockopt: test passive Fast Open early write push

[T S Rameshkumar]

Add automated test coverage for passive Fast Open early write data
push in mptcp_sockopt.c and mptcp_sockopt.sh.

The server enables TCP_FASTOPEN on an MPTCP listening socket, accepts
an incoming connection with SYN payload data, and immediately writes
reply bytes before the 3-way handshake fully completes. The client
connects with TCP_FASTOPEN_CONNECT, sends request data, and validates
reception of the server's reply over both IPv4 and IPv6.

Signed-off-by: T S Rameshkumar <rameshkumar.t@phytecembedded.in>
---
 .../selftests/net/mptcp/mptcp_sockopt.c       | 59 ++++++++++++++++++-
 .../selftests/net/mptcp/mptcp_sockopt.sh      | 34 +++++++++++
 2 files changed, 91 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
index b6e58d936..04af7cebc 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
+++ b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
@@ -28,6 +28,7 @@
 #include <linux/compiler.h>
 
 static int pf = AF_INET;
+static bool do_tfo;
 
 #ifndef IPPROTO_MPTCP
 #define IPPROTO_MPTCP 262
@@ -247,6 +248,14 @@ static int sock_connect_mptcp(const char * const remoteaddr,
 		if (sock < 0)
 			continue;
 
+		if (do_tfo) {
+			int one = 1;
+
+			if (setsockopt(sock, IPPROTO_TCP, TCP_FASTOPEN_CONNECT,
+				       &one, sizeof(one)) < 0)
+				die_perror("setsockopt TCP_FASTOPEN_CONNECT");
+		}
+
 		if (connect(sock, a->ai_addr, a->ai_addrlen) == 0)
 			break; /* success */
 
@@ -264,7 +273,7 @@ static void parse_opts(int argc, char **argv)
 {
 	int c;
 
-	while ((c = getopt(argc, argv, "h6")) != -1) {
+	while ((c = getopt(argc, argv, "h6f")) != -1) {
 		switch (c) {
 		case 'h':
 			die_usage(0);
@@ -272,6 +281,9 @@ static void parse_opts(int argc, char **argv)
 		case '6':
 			pf = AF_INET6;
 			break;
+		case 'f':
+			do_tfo = true;
+			break;
 		default:
 			die_usage(1);
 			break;
@@ -719,13 +731,36 @@ static int server(int pipefd)
 		break;
 	}
 
+	if (do_tfo) {
+		int qlen = 5;
+
+		if (setsockopt(fd, IPPROTO_TCP, TCP_FASTOPEN, &qlen, sizeof(qlen)) < 0)
+			die_perror("setsockopt TCP_FASTOPEN");
+	}
+
 	r = write(pipefd, "conn", 4);
 	assert(r == 4);
 
 	alarm(15);
 	r = xaccept(fd);
 
-	process_one_client(r, pipefd);
+	if (do_tfo) {
+		char buf[64] = {0};
+		ssize_t n;
+
+		n = read(r, buf, sizeof(buf) - 1);
+		if (n <= 0)
+			die_perror("read tfo syn data");
+
+		n = write(r, "PONG_REPLY", 10);
+		if (n != 10)
+			die_perror("write tfo reply");
+
+		n = read(r, buf, 1);
+		close(r);
+	} else {
+		process_one_client(r, pipefd);
+	}
 
 	close(fd);
 	return 0;
@@ -775,6 +810,26 @@ static int client(int pipefd)
 
 	alarm(15);
 
+	if (do_tfo) {
+		const char *addr = (pf == AF_INET) ? "127.0.0.1" : "::1";
+		struct timeval tv = {.tv_sec = 5, .tv_usec = 0};
+		char buf[64] = {0};
+		ssize_t n;
+
+		fd = sock_connect_mptcp(addr, "15432", IPPROTO_MPTCP);
+		n = write(fd, "PING_DATA", 9);
+		if (n != 9)
+			die_perror("write PING_DATA");
+
+		setsockopt(fd, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));
+		n = read(fd, buf, sizeof(buf) - 1);
+		if (n <= 0 || strcmp(buf, "PONG_REPLY") != 0)
+			xerror("Fastopen early write failed: received %zd bytes '%s'", n, buf);
+
+		close(fd);
+		return 0;
+	}
+
 	switch (pf) {
 	case AF_INET:
 		fd = sock_connect_mptcp("127.0.0.1", "15432", IPPROTO_MPTCP);
diff --git a/tools/testing/selftests/net/mptcp/mptcp_sockopt.sh b/tools/testing/selftests/net/mptcp/mptcp_sockopt.sh
index e850a8742..b3b523ce8 100755
--- a/tools/testing/selftests/net/mptcp/mptcp_sockopt.sh
+++ b/tools/testing/selftests/net/mptcp/mptcp_sockopt.sh
@@ -351,6 +351,39 @@ do_tcpinq_tests()
 	return $?
 }
 
+do_fastopen_tests()
+{
+	local lret=0
+
+	ip netns exec "$ns_sbox" sysctl -q net.ipv4.tcp_fastopen=3
+
+	ip netns exec "$ns_sbox" ./mptcp_sockopt -f
+	lret=$?
+
+	print_title "Fast Open early write v4"
+	if [ $lret -ne 0 ]; then
+		mptcp_lib_pr_fail
+		mptcp_lib_result_fail "Fast Open early write v4"
+		ret=$lret
+		return
+	fi
+	mptcp_lib_pr_ok
+	mptcp_lib_result_pass "Fast Open early write v4"
+
+	ip netns exec "$ns_sbox" ./mptcp_sockopt -6 -f
+	lret=$?
+
+	print_title "Fast Open early write v6"
+	if [ $lret -ne 0 ]; then
+		mptcp_lib_pr_fail
+		mptcp_lib_result_fail "Fast Open early write v6"
+		ret=$lret
+		return
+	fi
+	mptcp_lib_pr_ok
+	mptcp_lib_result_pass "Fast Open early write v6"
+}
+
 sin=$(mktemp)
 sout=$(mktemp)
 cin=$(mktemp)
@@ -366,6 +399,7 @@ run_tests $ns1 $ns2 dead:beef:1::1
 
 do_mptcp_sockopt_tests
 do_tcpinq_tests
+do_fastopen_tests
 
 mptcp_lib_result_print_all_tap
 exit $ret
-- 
2.34.1

Re: [PATCH mptcp-next] selftests: mptcp: sockopt: test passive Fast Open early write push

[Matthieu Baerts]

Hi T S,

On 08/10/2026 13:10, T S Rameshkumar wrote:
> Add automated test coverage for passive Fast Open early write data
> push in mptcp_sockopt.c and mptcp_sockopt.sh.

Please next time join the test with the fix, otherwise this patch alone
will not work.

Also, if you use 'mptcp*' prefix in the subject, please don't Cc the
netdev mailing list. (and remove it from any replies here)

> The server enables TCP_FASTOPEN on an MPTCP listening socket, accepts
> an incoming connection with SYN payload data, and immediately writes
> reply bytes before the 3-way handshake fully completes. The client
> connects with TCP_FASTOPEN_CONNECT, sends request data, and validates
> reception of the server's reply over both IPv4 and IPv6.

A more efficient way of validating that is to add a packetdrill test
instead of increasing the complexity of the "sockopt" test to validate a
connection issue.

Can you add your new test in the MPTCP fork [1], and validate it with:

  # cd gtests/net
  # ./packetdrill/run_all.py -lv mptcp/fastopen/<test>.pkt

[1] https://github.com/multipath-tcp/packetdrill/

> Signed-off-by: T S Rameshkumar <rameshkumar.t@phytecembedded.in>

If LLM is used, you need to add the Assisted-by tag:

  https://docs.kernel.org/process/coding-assistants.html

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.