selftests/namespaces: verify initial namespace inode numbers

Make sure that all works correctly.

Signed-off-by: Christian Brauner <brauner@kernel.org>

tools/testing/selftests/namespaces/.gitignore

@@ -1,2 +1,3 @@
 nsid_test
 file_handle_test
+init_ino_test

tools/testing/selftests/namespaces/Makefile

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0-only
 CFLAGS += -Wall -O0 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
 
-TEST_GEN_PROGS := nsid_test file_handle_test
+TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test
 
 include ../lib.mk
 

tools/testing/selftests/namespaces/init_ino_test.c

@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+// Copyright (c) 2025 Christian Brauner <brauner@kernel.org>
+
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+#include <linux/nsfs.h>
+
+#include "../kselftest_harness.h"
+
+struct ns_info {
+	const char *name;
+	const char *proc_path;
+	unsigned int expected_ino;
+};
+
+static struct ns_info namespaces[] = {
+	{ "ipc", "/proc/1/ns/ipc", IPC_NS_INIT_INO },
+	{ "uts", "/proc/1/ns/uts", UTS_NS_INIT_INO },
+	{ "user", "/proc/1/ns/user", USER_NS_INIT_INO },
+	{ "pid", "/proc/1/ns/pid", PID_NS_INIT_INO },
+	{ "cgroup", "/proc/1/ns/cgroup", CGROUP_NS_INIT_INO },
+	{ "time", "/proc/1/ns/time", TIME_NS_INIT_INO },
+	{ "net", "/proc/1/ns/net", NET_NS_INIT_INO },
+	{ "mnt", "/proc/1/ns/mnt", MNT_NS_INIT_INO },
+};
+
+TEST(init_namespace_inodes)
+{
+	struct stat st;
+
+	for (int i = 0; i < sizeof(namespaces) / sizeof(namespaces[0]); i++) {
+		int ret = stat(namespaces[i].proc_path, &st);
+
+		/* Some namespaces might not be available (e.g., time namespace on older kernels) */
+		if (ret < 0) {
+			if (errno == ENOENT) {
+				ksft_test_result_skip("%s namespace not available\n",
+						      namespaces[i].name);
+				continue;
+			}
+			ASSERT_GE(ret, 0)
+			TH_LOG("Failed to stat %s: %s",
+			       namespaces[i].proc_path, strerror(errno));
+		}
+
+		ASSERT_EQ(st.st_ino, namespaces[i].expected_ino)
+			TH_LOG("Namespace %s has inode 0x%lx, expected 0x%x",
+			       namespaces[i].name, st.st_ino, namespaces[i].expected_ino);
+
+		ksft_print_msg("Namespace %s: inode 0x%lx matches expected 0x%x\n",
+			       namespaces[i].name, st.st_ino, namespaces[i].expected_ino);
+	}
+}
+
+TEST_HARNESS_MAIN