From: Geliang Tang <tanggeliang@kylinos.cn>

Add tests for the SO_BINDTODEVICE socket option:
- Binds sockets to the loopback device ("lo")
- Verifies the binding via getsockopt
- Includes IPv4 and IPv6 test cases
- Reports results explicitly in the test script

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
---
 .../selftests/net/mptcp/mptcp_sockopt.c       | 25 +++++++++++++++++++
 .../selftests/net/mptcp/mptcp_sockopt.sh      |  6 +++--
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
index 816d71706f7d..df4f85f129f7 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
+++ b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
@@ -23,6 +23,7 @@
 
 #include <netdb.h>
 #include <netinet/in.h>
+#include <net/if.h>
 
 #include <linux/tcp.h>
 
@@ -198,10 +199,20 @@ static void do_setsockopt_reuseport(int fd)
 		perror("setsockopt(SO_REUSEPORT)");
 }
 
+static void do_setsockopt_bindtodevice(int fd)
+{
+	const char *interface = "lo";
+
+	if (setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE,
+		       interface, strlen(interface)))
+		perror("setsockopt(SO_BINDTODEVICE)");
+}
+
 static void do_setsockopts(int fd)
 {
 	do_setsockopt_reuseaddr(fd);
 	do_setsockopt_reuseport(fd);
+	do_setsockopt_bindtodevice(fd);
 }
 
 static int sock_listen_mptcp(const char * const listenaddr,
@@ -599,6 +610,18 @@ static void do_getsockopt_reuseport(int fd)
 	assert(reuse == 1);
 }
 
+static void do_getsockopt_bindtodevice(int fd)
+{
+	char ifname[IFNAMSIZ] = { 0 };
+	socklen_t len;
+
+	len = sizeof(ifname);
+	if (getsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, ifname, &len))
+		die_perror("getsockopt(SO_BINDTODEVICE)");
+
+	assert(!memcmp(ifname, "lo", len));
+}
+
 static void do_getsockopts(struct so_state *s, int fd, size_t r, size_t w)
 {
 	do_getsockopt_mptcp_info(s, fd, w);
@@ -613,6 +636,8 @@ static void do_getsockopts(struct so_state *s, int fd, size_t r, size_t w)
 	do_getsockopt_reuseaddr(fd);
 
 	do_getsockopt_reuseport(fd);
+
+	do_getsockopt_bindtodevice(fd);
 }
 
 static void connect_one_server(int fd, int pipefd)
diff --git a/tools/testing/selftests/net/mptcp/mptcp_sockopt.sh b/tools/testing/selftests/net/mptcp/mptcp_sockopt.sh
index 1c357ed757d6..44342ce6fd16 100755
--- a/tools/testing/selftests/net/mptcp/mptcp_sockopt.sh
+++ b/tools/testing/selftests/net/mptcp/mptcp_sockopt.sh
@@ -270,7 +270,8 @@ do_mptcp_sockopt_tests()
 	fi
 
 	for opt in "SOL_MPTCP" \
-		   "SO_REUSEADDR" "SO_REUSEPORT"; do
+		   "SO_REUSEADDR" "SO_REUSEPORT" \
+		   "SO_BINDTODEVICE"; do
 		print_title "$opt sockopt v4"
 		mptcp_lib_pr_ok
 		mptcp_lib_result_pass "$opt sockopt v4"
@@ -288,7 +289,8 @@ do_mptcp_sockopt_tests()
 	fi
 
 	for opt in "SOL_MPTCP" \
-		   "SO_REUSEADDR" "SO_REUSEPORT"; do
+		   "SO_REUSEADDR" "SO_REUSEPORT" \
+		   "SO_BINDTODEVICE"; do
 		print_title "$opt sockopt v6"
 		mptcp_lib_pr_ok
 		mptcp_lib_result_pass "$opt sockopt v6"
-- 
2.48.1