Subject: [PATCH net-next v2 2/2] selftests/bpf: verify syncookie statistics in tcp_custom_syncookie

From: Jiayuan Chen <[email-masked]>
Date: Sat, 11 Apr 2026 09:32:05 +0800

Add read_tcpext_snmp() helper to network_helpers which reads a
TcpExt SNMP counter via nstat, and use it in the tcp_custom_syncookie
test to verify that LINUX_MIB_SYNCOOKIESRECV is incremented and
LINUX_MIB_SYNCOOKIESFAILED stays unchanged across a successful
BPF custom syncookie validation.

The delta is captured between start_server() and accept(), which
covers the full SYN/ACK/cookie-check path for one connection.

Signed-off-by: Jiayuan Chen <[email-masked]>
---
 tools/testing/selftests/bpf/network_helpers.c | 22 +++++++++++++++++++
 tools/testing/selftests/bpf/network_helpers.h |  1 +
 .../bpf/prog_tests/tcp_custom_syncookie.c     | 20 +++++++++++++++++
 3 files changed, 43 insertions(+)

diff --git a/tools/testing/selftests/bpf/network_helpers.c b/tools/testing/selftests/bpf/network_helpers.c
index b82f572641b7..3388dd5112b6 100644
--- a/tools/testing/selftests/bpf/network_helpers.c
+++ b/tools/testing/selftests/bpf/network_helpers.c
@@ -621,6 +621,28 @@ int get_socket_local_port(int sock_fd)
 	return -1;
 }
 
+int read_tcpext_snmp(const char *name, unsigned long *val)
+{
+	char cmd[128], buf[128];
+	int ret = 0;
+	FILE *f;
+
+	snprintf(cmd, sizeof(cmd),
+		 "nstat -az TcpExt%s | awk '/TcpExt/ {print $2}'", name);
+	f = popen(cmd, "r");
+	if (!f)
+		return -errno;
+
+	if (!fgets(buf, sizeof(buf), f)) {
+		ret = ferror(f) ? -errno : -ENODATA;
+		goto out;
+	}
+	*val = strtoul(buf, NULL, 10);
+out:
+	pclose(f);
+	return ret;
+}
+
 int get_hw_ring_size(char *ifname, struct ethtool_ringparam *ring_param)
 {
 	struct ifreq ifr = {0};
diff --git a/tools/testing/selftests/bpf/network_helpers.h b/tools/testing/selftests/bpf/network_helpers.h
index 79a010c88e11..c53cd781df6e 100644
--- a/tools/testing/selftests/bpf/network_helpers.h
+++ b/tools/testing/selftests/bpf/network_helpers.h
@@ -84,6 +84,7 @@ int make_sockaddr(int family, const char *addr_str, __u16 port,
 		  struct sockaddr_storage *addr, socklen_t *len);
 char *ping_command(int family);
 int get_socket_local_port(int sock_fd);
+int read_tcpext_snmp(const char *name, unsigned long *val);
 int get_hw_ring_size(char *ifname, struct ethtool_ringparam *ring_param);
 int set_hw_ring_size(char *ifname, struct ethtool_ringparam *ring_param);
 
diff --git a/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c b/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c
index eaf441dc7e79..6adfb4b892f8 100644
--- a/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c
+++ b/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c
@@ -91,12 +91,21 @@ static void transfer_message(int sender, int receiver)
 
 static void create_connection(struct test_tcp_custom_syncookie_case *test_case)
 {
+	unsigned long recv_before, recv_after;
+	unsigned long failed_before, failed_after;
 	int server, client, child;
 
 	server = start_server(test_case->family, test_case->type, test_case->addr, 0, 0);
 	if (!ASSERT_NEQ(server, -1, "start_server"))
 		return;
 
+	if (!ASSERT_OK(read_tcpext_snmp("SyncookiesRecv", &recv_before),
+		       "read SyncookiesRecv before"))
+		goto close_server;
+	if (!ASSERT_OK(read_tcpext_snmp("SyncookiesFailed", &failed_before),
+		       "read SyncookiesFailed before"))
+		goto close_server;
+
 	client = connect_to_fd(server, 0);
 	if (!ASSERT_NEQ(client, -1, "connect_to_fd"))
 		goto close_server;
@@ -105,9 +114,20 @@ static void create_connection(struct test_tcp_custom_syncookie_case *test_case)
 	if (!ASSERT_NEQ(child, -1, "accept"))
 		goto close_client;
 
+	if (!ASSERT_OK(read_tcpext_snmp("SyncookiesRecv", &recv_after),
+		       "read SyncookiesRecv after"))
+		goto close_child;
+	if (!ASSERT_OK(read_tcpext_snmp("SyncookiesFailed", &failed_after),
+		       "read SyncookiesFailed after"))
+		goto close_child;
+
+	ASSERT_EQ(recv_after - recv_before, 1, "SyncookiesRecv delta");
+	ASSERT_EQ(failed_after - failed_before, 0, "SyncookiesFailed delta");
+
 	transfer_message(client, child);
 	transfer_message(child, client);
 
+close_child:
 	close(child);
 close_client:
 	close(client);
-- 
2.43.0