Test that socket file descriptors keep network namespaces active. Create
a network namespace, create a socket in it, then exit the namespace. The
namespace should remain active while the socket FD is held.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../testing/selftests/namespaces/siocgskns_test.c  | 126 +++++++++++++++++++++
 1 file changed, 126 insertions(+)

diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
index 0c9098624cd4..0ad5e39b7e16 100644
--- a/tools/testing/selftests/namespaces/siocgskns_test.c
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -69,4 +69,130 @@ TEST(siocgskns_basic)
 	close(current_netns_fd);
 }
 
+/*
+ * Test that socket file descriptors keep network namespaces active.
+ * Create a network namespace, create a socket in it, then exit the namespace.
+ * The namespace should remain active while the socket FD is held.
+ */
+TEST(siocgskns_keeps_netns_active)
+{
+	int sock_fd, netns_fd, test_fd;
+	int ipc_sockets[2];
+	pid_t pid;
+	int status;
+	struct stat st;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child: create new netns and socket */
+		close(ipc_sockets[0]);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			TH_LOG("unshare(CLONE_NEWNET) failed: %s", strerror(errno));
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Create a socket in the new network namespace */
+		sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
+		if (sock_fd < 0) {
+			TH_LOG("socket() failed: %s", strerror(errno));
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Send socket FD to parent via SCM_RIGHTS */
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1] = {'X'};
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(sock_fd);
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		close(sock_fd);
+		close(ipc_sockets[1]);
+		exit(0);
+	}
+
+	/* Parent: receive socket FD */
+	close(ipc_sockets[1]);
+
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(n, 1);
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	ASSERT_NE(cmsg, NULL);
+	ASSERT_EQ(cmsg->cmsg_type, SCM_RIGHTS);
+
+	memcpy(&sock_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	if (WEXITSTATUS(status) != 0)
+		SKIP(close(sock_fd); return, "Child failed to create namespace");
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	ASSERT_EQ(fstat(netns_fd, &st), 0);
+
+	/*
+	 * Namespace should still be active because socket FD keeps it alive.
+	 * Try to access it via /proc/self/fd/<fd>.
+	 */
+	char path[64];
+	snprintf(path, sizeof(path), "/proc/self/fd/%d", netns_fd);
+	test_fd = open(path, O_RDONLY);
+	ASSERT_GE(test_fd, 0);
+	close(test_fd);
+	close(netns_fd);
+
+	/* Close socket - namespace should become inactive */
+	close(sock_fd);
+
+	/* Try SIOCGSKNS again - should fail since socket is closed */
+	ASSERT_LT(ioctl(sock_fd, SIOCGSKNS), 0);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3