selftests/namespaces: add listns() wrapper

Add a wrapper for the listns() system call.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-38-2e6f823ebdc0@kernel.org
Tested-by: syzbot@syzkaller.appspotmail.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

tools/testing/selftests/namespaces/wrappers.h

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/nsfs.h>
+#include <linux/types.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#ifndef __SELFTESTS_NAMESPACES_WRAPPERS_H__
+#define __SELFTESTS_NAMESPACES_WRAPPERS_H__
+
+#ifndef __NR_listns
+	#if defined __alpha__
+		#define __NR_listns 580
+	#elif defined _MIPS_SIM
+		#if _MIPS_SIM == _MIPS_SIM_ABI32	/* o32 */
+			#define __NR_listns 4470
+		#endif
+		#if _MIPS_SIM == _MIPS_SIM_NABI32	/* n32 */
+			#define __NR_listns 6470
+		#endif
+		#if _MIPS_SIM == _MIPS_SIM_ABI64	/* n64 */
+			#define __NR_listns 5470
+		#endif
+	#else
+		#define __NR_listns 470
+	#endif
+#endif
+
+static inline int sys_listns(const struct ns_id_req *req, __u64 *ns_ids,
+			     size_t nr_ns_ids, unsigned int flags)
+{
+	return syscall(__NR_listns, req, ns_ids, nr_ns_ids, flags);
+}
+
+#endif /* __SELFTESTS_NAMESPACES_WRAPPERS_H__ */