Test that initial namespaces can be reopened via file handle. Initial
namespaces should always have a ref count of one from boot.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/.gitignore      |  1 +
 tools/testing/selftests/namespaces/Makefile        |  5 +-
 .../selftests/namespaces/ns_active_ref_test.c      | 74 ++++++++++++++++++++++
 3 files changed, 79 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/namespaces/.gitignore b/tools/testing/selftests/namespaces/.gitignore
index ccfb40837a73..100cc5bfef04 100644
--- a/tools/testing/selftests/namespaces/.gitignore
+++ b/tools/testing/selftests/namespaces/.gitignore
@@ -1,3 +1,4 @@
 nsid_test
 file_handle_test
 init_ino_test
+ns_active_ref_test
diff --git a/tools/testing/selftests/namespaces/Makefile b/tools/testing/selftests/namespaces/Makefile
index 5fe4b3dc07d3..5cea938cdde8 100644
--- a/tools/testing/selftests/namespaces/Makefile
+++ b/tools/testing/selftests/namespaces/Makefile
@@ -1,7 +1,10 @@
 # SPDX-License-Identifier: GPL-2.0-only
 CFLAGS += -Wall -O0 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
+LDLIBS += -lcap
 
-TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test
+TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test ns_active_ref_test
 
 include ../lib.mk
 
+$(OUTPUT)/ns_active_ref_test: ../filesystems/utils.c
+
diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
new file mode 100644
index 000000000000..21514a537b26
--- /dev/null
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <linux/nsfs.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+
+#ifndef FD_NSFS_ROOT
+#define FD_NSFS_ROOT -10003 /* Root of the nsfs filesystem */
+#endif
+
+/*
+ * Test that initial namespaces can be reopened via file handle.
+ * Initial namespaces should have active ref count of 1 from boot.
+ */
+TEST(init_ns_always_active)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd1, fd2;
+	struct stat st1, st2;
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Open initial network namespace */
+	fd1 = open("/proc/1/ns/net", O_RDONLY);
+	ASSERT_GE(fd1, 0);
+
+	/* Get file handle for initial namespace */
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(fd1, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(fd1);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+
+	/* Close the namespace fd */
+	close(fd1);
+
+	/* Try to reopen via file handle - should succeed since init ns is always active */
+	fd2 = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (fd2 < 0 && (errno == EINVAL || errno == EOPNOTSUPP)) {
+		SKIP(free(handle);
+		     return, "open_by_handle_at with FD_NSFS_ROOT not supported");
+	}
+	ASSERT_GE(fd2, 0);
+
+	/* Verify we opened the same namespace */
+	fd1 = open("/proc/1/ns/net", O_RDONLY);
+	ASSERT_GE(fd1, 0);
+	ASSERT_EQ(fstat(fd1, &st1), 0);
+	ASSERT_EQ(fstat(fd2, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+
+	close(fd1);
+	close(fd2);
+	free(handle);
+}
+
+TEST_HARNESS_MAIN

-- 
2.47.3