[PATCH net-next 4/6] selftests/vsock: add a helper to assign the g2h device to a netns

[Bobby Eshleman <bobbyeshleman@gmail.com>]

From: Bobby Eshleman <bobbyeshleman@meta.com>

No shell tool can issue IOCTL_VM_SOCKETS_ASSIGN_G2H_NETNS, so anything
that wants to move the guest's vsock device from a script needs a small
program to do it.

It exits with the ioctl's errno so a caller can differentiate the
reasons for failure.

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
---
 tools/testing/selftests/vsock/.gitignore           |  1 +
 tools/testing/selftests/vsock/Makefile             |  3 +-
 .../selftests/vsock/vsock_assign_g2h_netns.c       | 45 ++++++++++++++++++++++
 3 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/vsock/.gitignore b/tools/testing/selftests/vsock/.gitignore
index 9c5bf379480f..ffca0d9c34b1 100644
--- a/tools/testing/selftests/vsock/.gitignore
+++ b/tools/testing/selftests/vsock/.gitignore
@@ -1,2 +1,3 @@
 vmtest.log
+vsock_assign_g2h_netns
 vsock_test
diff --git a/tools/testing/selftests/vsock/Makefile b/tools/testing/selftests/vsock/Makefile
index c407c0afd938..d7170a15150f 100644
--- a/tools/testing/selftests/vsock/Makefile
+++ b/tools/testing/selftests/vsock/Makefile
@@ -11,7 +11,6 @@ $(OUTPUT)/vsock_test: $(VSOCK_TEST_DIR)/vsock_test
 $(VSOCK_TEST_DIR)/vsock_test: $(VSOCK_TEST_SRCS)
 	$(MAKE) -C $(VSOCK_TEST_DIR) vsock_test
 TEST_PROGS += vmtest.sh
-TEST_GEN_FILES := vsock_test
+TEST_GEN_FILES := vsock_test vsock_assign_g2h_netns
 
 include ../lib.mk
-
diff --git a/tools/testing/selftests/vsock/vsock_assign_g2h_netns.c b/tools/testing/selftests/vsock/vsock_assign_g2h_netns.c
new file mode 100644
index 000000000000..6f15629af607
--- /dev/null
+++ b/tools/testing/selftests/vsock/vsock_assign_g2h_netns.c
@@ -0,0 +1,45 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Assign the guest->host vsock transport, i.e. the guest's virtio-vsock
+ * device, to the network namespace of the invoking process.
+ *
+ * Exits with the ioctl's errno, so that callers can tell why it was refused.
+ *
+ * Copyright (c) 2026 Meta Platforms, Inc. and affiliates
+ */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <unistd.h>
+
+#include <linux/vm_sockets.h>
+
+#ifndef IOCTL_VM_SOCKETS_ASSIGN_G2H_NETNS
+#define IOCTL_VM_SOCKETS_ASSIGN_G2H_NETNS	_IO(7, 0xba)
+#endif
+
+int main(void)
+{
+	int fd, ret;
+
+	fd = open("/dev/vsock", O_RDONLY);
+	if (fd < 0) {
+		fprintf(stderr, "open /dev/vsock: %s\n", strerror(errno));
+		return -1;
+	}
+
+	ret = ioctl(fd, IOCTL_VM_SOCKETS_ASSIGN_G2H_NETNS);
+	if (ret < 0) {
+		ret = errno;
+		fprintf(stderr,
+			"IOCTL_VM_SOCKETS_ASSIGN_G2H_NETNS: %s (errno %d)\n",
+			strerror(errno), errno);
+	}
+
+	close(fd);
+
+	return ret;
+}

-- 
2.53.0-Meta