Merge tag 'namespace-6.18-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull namespace updates from Christian Brauner:
 "This contains a larger set of changes around the generic namespace
  infrastructure of the kernel.

  Each specific namespace type (net, cgroup, mnt, ...) embedds a struct
  ns_common which carries the reference count of the namespace and so
  on.

  We open-coded and cargo-culted so many quirks for each namespace type
  that it just wasn't scalable anymore. So given there's a bunch of new
  changes coming in that area I've started cleaning all of this up.

  The core change is to make it possible to correctly initialize every
  namespace uniformly and derive the correct initialization settings
  from the type of the namespace such as namespace operations, namespace
  type and so on. This leaves the new ns_common_init() function with a
  single parameter which is the specific namespace type which derives
  the correct parameters statically. This also means the compiler will
  yell as soon as someone does something remotely fishy.

  The ns_common_init() addition also allows us to remove ns_alloc_inum()
  and drops any special-casing of the initial network namespace in the
  network namespace initialization code that Linus complained about.

  Another part is reworking the reference counting. The reference
  counting was open-coded and copy-pasted for each namespace type even
  though they all followed the same rules. This also removes all open
  accesses to the reference count and makes it private and only uses a
  very small set of dedicated helpers to manipulate them just like we do
  for e.g., files.

  In addition this generalizes the mount namespace iteration
  infrastructure introduced a few cycles ago. As reminder, the vfs makes
  it possible to iterate sequentially and bidirectionally through all
  mount namespaces on the system or all mount namespaces that the caller
  holds privilege over. This allow userspace to iterate over all mounts
  in all mount namespaces using the listmount() and statmount() system
  call.

  Each mount namespace has a unique identifier for the lifetime of the
  systems that is exposed to userspace. The network namespace also has a
  unique identifier working exactly the same way. This extends the
  concept to all other namespace types.

  The new nstree type makes it possible to lookup namespaces purely by
  their identifier and to walk the namespace list sequentially and
  bidirectionally for all namespace types, allowing userspace to iterate
  through all namespaces. Looking up namespaces in the namespace tree
  works completely locklessly.

  This also means we can move the mount namespace onto the generic
  infrastructure and remove a bunch of code and members from struct
  mnt_namespace itself.

  There's a bunch of stuff coming on top of this in the future but for
  now this uses the generic namespace tree to extend a concept
  introduced first for pidfs a few cycles ago. For a while now we have
  supported pidfs file handles for pidfds. This has proven to be very
  useful.

  This extends the concept to cover namespaces as well. It is possible
  to encode and decode namespace file handles using the common
  name_to_handle_at() and open_by_handle_at() apis.

  As with pidfs file handles, namespace file handles are exhaustive,
  meaning it is not required to actually hold a reference to nsfs in
  able to decode aka open_by_handle_at() a namespace file handle.
  Instead the FD_NSFS_ROOT constant can be passed which will let the
  kernel grab a reference to the root of nsfs internally and thus decode
  the file handle.

  Namespaces file descriptors can already be derived from pidfds which
  means they aren't subject to overmount protection bugs. IOW, it's
  irrelevant if the caller would not have access to an appropriate
  /proc/<pid>/ns/ directory as they could always just derive the
  namespace based on a pidfd already.

  It has the same advantage as pidfds. It's possible to reliably and for
  the lifetime of the system refer to a namespace without pinning any
  resources and to compare them trivially.

  Permission checking is kept simple. If the caller is located in the
  namespace the file handle refers to they are able to open it otherwise
  they must hold privilege over the owning namespace of the relevant
  namespace.

  The namespace file handle layout is exposed as uapi and has a stable
  and extensible format. For now it simply contains the namespace
  identifier, the namespace type, and the inode number. The stable
  format means that userspace may construct its own namespace file
  handles without going through name_to_handle_at() as they are already
  allowed for pidfs and cgroup file handles"

* tag 'namespace-6.18-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs: (65 commits)
  ns: drop assert
  ns: move ns type into struct ns_common
  nstree: make struct ns_tree private
  ns: add ns_debug()
  ns: simplify ns_common_init() further
  cgroup: add missing ns_common include
  ns: use inode initializer for initial namespaces
  selftests/namespaces: verify initial namespace inode numbers
  ns: rename to __ns_ref
  nsfs: port to ns_ref_*() helpers
  net: port to ns_ref_*() helpers
  uts: port to ns_ref_*() helpers
  ipv4: use check_net()
  net: use check_net()
  net-sysfs: use check_net()
  user: port to ns_ref_*() helpers
  time: port to ns_ref_*() helpers
  pid: port to ns_ref_*() helpers
  ipc: port to ns_ref_*() helpers
  cgroup: port to ns_ref_*() helpers
  ...

1 files changed, 1429 insertions, 0 deletions

diff --git a/tools/testing/selftests/namespaces/file_handle_test.c b/tools/testing/selftests/namespaces/file_handle_test.c
new file mode 100644
index 000000000000..f1bc5773f552
--- /dev/null
+++ b/tools/testing/selftests/namespaces/file_handle_test.c
@@ -0,0 +1,1429 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <grp.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <linux/unistd.h>
+#include "../kselftest_harness.h"
+
+#ifndef FD_NSFS_ROOT
+#define FD_NSFS_ROOT -10003 /* Root of the nsfs filesystem */
+#endif
+
+TEST(nsfs_net_handle)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	struct stat st1, st2;
+
+	/* Drop to unprivileged uid/gid */
+	ASSERT_EQ(setresgid(65534, 65534, 65534), 0); /* nogroup */
+	ASSERT_EQ(setresuid(65534, 65534, 65534), 0); /* nobody */
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Open a namespace file descriptor */
+	ns_fd = open("/proc/self/ns/net", O_RDONLY);
+	ASSERT_GE(ns_fd, 0);
+
+	/* Get handle for the namespace */
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	ASSERT_GT(handle->handle_bytes, 0);
+
+	/* Try to open using FD_NSFS_ROOT as unprivileged user */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (fd < 0 && (errno == EINVAL || errno == EOPNOTSUPP)) {
+		SKIP(free(handle); close(ns_fd);
+		     return,
+			   "open_by_handle_at with FD_NSFS_ROOT not supported");
+	}
+	if (fd < 0 && errno == EPERM) {
+		SKIP(free(handle); close(ns_fd);
+		     return,
+			   "Permission denied for unprivileged user (expected)");
+	}
+	ASSERT_GE(fd, 0);
+
+	/* Verify we opened the correct namespace */
+	ASSERT_EQ(fstat(ns_fd, &st1), 0);
+	ASSERT_EQ(fstat(fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	close(fd);
+	close(ns_fd);
+	free(handle);
+}
+
+TEST(nsfs_uts_handle)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	struct stat st1, st2;
+
+	/* Drop to unprivileged uid/gid */
+	ASSERT_EQ(setresgid(65534, 65534, 65534), 0); /* nogroup */
+	ASSERT_EQ(setresuid(65534, 65534, 65534), 0); /* nobody */
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Open UTS namespace file descriptor */
+	ns_fd = open("/proc/self/ns/uts", O_RDONLY);
+	ASSERT_GE(ns_fd, 0);
+
+	/* Get handle for the namespace */
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	ASSERT_GT(handle->handle_bytes, 0);
+
+	/* Try to open using FD_NSFS_ROOT */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (fd < 0 && (errno == EINVAL || errno == EOPNOTSUPP)) {
+		SKIP(free(handle); close(ns_fd);
+		     return,
+			   "open_by_handle_at with FD_NSFS_ROOT not supported");
+	}
+	ASSERT_GE(fd, 0);
+
+	/* Verify we opened the correct namespace */
+	ASSERT_EQ(fstat(ns_fd, &st1), 0);
+	ASSERT_EQ(fstat(fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	close(fd);
+	close(ns_fd);
+	free(handle);
+}
+
+TEST(nsfs_ipc_handle)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	struct stat st1, st2;
+
+	/* Drop to unprivileged uid/gid */
+	ASSERT_EQ(setresgid(65534, 65534, 65534), 0); /* nogroup */
+	ASSERT_EQ(setresuid(65534, 65534, 65534), 0); /* nobody */
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Open IPC namespace file descriptor */
+	ns_fd = open("/proc/self/ns/ipc", O_RDONLY);
+	ASSERT_GE(ns_fd, 0);
+
+	/* Get handle for the namespace */
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	ASSERT_GT(handle->handle_bytes, 0);
+
+	/* Try to open using FD_NSFS_ROOT */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (fd < 0 && (errno == EINVAL || errno == EOPNOTSUPP)) {
+		SKIP(free(handle); close(ns_fd);
+		     return,
+			   "open_by_handle_at with FD_NSFS_ROOT not supported");
+	}
+	ASSERT_GE(fd, 0);
+
+	/* Verify we opened the correct namespace */
+	ASSERT_EQ(fstat(ns_fd, &st1), 0);
+	ASSERT_EQ(fstat(fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	close(fd);
+	close(ns_fd);
+	free(handle);
+}
+
+TEST(nsfs_pid_handle)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	struct stat st1, st2;
+
+	/* Drop to unprivileged uid/gid */
+	ASSERT_EQ(setresgid(65534, 65534, 65534), 0); /* nogroup */
+	ASSERT_EQ(setresuid(65534, 65534, 65534), 0); /* nobody */
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Open PID namespace file descriptor */
+	ns_fd = open("/proc/self/ns/pid", O_RDONLY);
+	ASSERT_GE(ns_fd, 0);
+
+	/* Get handle for the namespace */
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	ASSERT_GT(handle->handle_bytes, 0);
+
+	/* Try to open using FD_NSFS_ROOT */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (fd < 0 && (errno == EINVAL || errno == EOPNOTSUPP)) {
+		SKIP(free(handle); close(ns_fd);
+		     return,
+			   "open_by_handle_at with FD_NSFS_ROOT not supported");
+	}
+	ASSERT_GE(fd, 0);
+
+	/* Verify we opened the correct namespace */
+	ASSERT_EQ(fstat(ns_fd, &st1), 0);
+	ASSERT_EQ(fstat(fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	close(fd);
+	close(ns_fd);
+	free(handle);
+}
+
+TEST(nsfs_mnt_handle)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	struct stat st1, st2;
+
+	/* Drop to unprivileged uid/gid */
+	ASSERT_EQ(setresgid(65534, 65534, 65534), 0); /* nogroup */
+	ASSERT_EQ(setresuid(65534, 65534, 65534), 0); /* nobody */
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Open mount namespace file descriptor */
+	ns_fd = open("/proc/self/ns/mnt", O_RDONLY);
+	ASSERT_GE(ns_fd, 0);
+
+	/* Get handle for the namespace */
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	ASSERT_GT(handle->handle_bytes, 0);
+
+	/* Try to open using FD_NSFS_ROOT */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (fd < 0 && (errno == EINVAL || errno == EOPNOTSUPP)) {
+		SKIP(free(handle); close(ns_fd);
+		     return,
+			   "open_by_handle_at with FD_NSFS_ROOT not supported");
+	}
+	ASSERT_GE(fd, 0);
+
+	/* Verify we opened the correct namespace */
+	ASSERT_EQ(fstat(ns_fd, &st1), 0);
+	ASSERT_EQ(fstat(fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	close(fd);
+	close(ns_fd);
+	free(handle);
+}
+
+TEST(nsfs_user_handle)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	struct stat st1, st2;
+
+	/* Drop to unprivileged uid/gid */
+	ASSERT_EQ(setresgid(65534, 65534, 65534), 0); /* nogroup */
+	ASSERT_EQ(setresuid(65534, 65534, 65534), 0); /* nobody */
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Open user namespace file descriptor */
+	ns_fd = open("/proc/self/ns/user", O_RDONLY);
+	ASSERT_GE(ns_fd, 0);
+
+	/* Get handle for the namespace */
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	ASSERT_GT(handle->handle_bytes, 0);
+
+	/* Try to open using FD_NSFS_ROOT */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (fd < 0 && (errno == EINVAL || errno == EOPNOTSUPP)) {
+		SKIP(free(handle); close(ns_fd);
+		     return,
+			   "open_by_handle_at with FD_NSFS_ROOT not supported");
+	}
+	ASSERT_GE(fd, 0);
+
+	/* Verify we opened the correct namespace */
+	ASSERT_EQ(fstat(ns_fd, &st1), 0);
+	ASSERT_EQ(fstat(fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	close(fd);
+	close(ns_fd);
+	free(handle);
+}
+
+TEST(nsfs_cgroup_handle)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	struct stat st1, st2;
+
+	/* Drop to unprivileged uid/gid */
+	ASSERT_EQ(setresgid(65534, 65534, 65534), 0); /* nogroup */
+	ASSERT_EQ(setresuid(65534, 65534, 65534), 0); /* nobody */
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Open cgroup namespace file descriptor */
+	ns_fd = open("/proc/self/ns/cgroup", O_RDONLY);
+	if (ns_fd < 0) {
+		SKIP(free(handle); return, "cgroup namespace not available");
+	}
+
+	/* Get handle for the namespace */
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	ASSERT_GT(handle->handle_bytes, 0);
+
+	/* Try to open using FD_NSFS_ROOT */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (fd < 0 && (errno == EINVAL || errno == EOPNOTSUPP)) {
+		SKIP(free(handle); close(ns_fd);
+		     return,
+			   "open_by_handle_at with FD_NSFS_ROOT not supported");
+	}
+	ASSERT_GE(fd, 0);
+
+	/* Verify we opened the correct namespace */
+	ASSERT_EQ(fstat(ns_fd, &st1), 0);
+	ASSERT_EQ(fstat(fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	close(fd);
+	close(ns_fd);
+	free(handle);
+}
+
+TEST(nsfs_time_handle)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	struct stat st1, st2;
+
+	/* Drop to unprivileged uid/gid */
+	ASSERT_EQ(setresgid(65534, 65534, 65534), 0); /* nogroup */
+	ASSERT_EQ(setresuid(65534, 65534, 65534), 0); /* nobody */
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Open time namespace file descriptor */
+	ns_fd = open("/proc/self/ns/time", O_RDONLY);
+	if (ns_fd < 0) {
+		SKIP(free(handle); return, "time namespace not available");
+	}
+
+	/* Get handle for the namespace */
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	ASSERT_GT(handle->handle_bytes, 0);
+
+	/* Try to open using FD_NSFS_ROOT */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (fd < 0 && (errno == EINVAL || errno == EOPNOTSUPP)) {
+		SKIP(free(handle); close(ns_fd);
+		     return,
+			   "open_by_handle_at with FD_NSFS_ROOT not supported");
+	}
+	ASSERT_GE(fd, 0);
+
+	/* Verify we opened the correct namespace */
+	ASSERT_EQ(fstat(ns_fd, &st1), 0);
+	ASSERT_EQ(fstat(fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	close(fd);
+	close(ns_fd);
+	free(handle);
+}
+
+TEST(nsfs_user_net_namespace_isolation)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	pid_t pid;
+	int status;
+	int pipefd[2];
+	char result;
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Create pipe for communication */
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	/* Get handle for current network namespace */
+	ns_fd = open("/proc/self/ns/net", O_RDONLY);
+	ASSERT_GE(ns_fd, 0);
+
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd); close(pipefd[0]);
+		     close(pipefd[1]);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	close(ns_fd);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* First create new user namespace to drop privileges */
+		ret = unshare(CLONE_NEWUSER);
+		if (ret < 0) {
+			write(pipefd[1], "U",
+			      1); /* Unable to create user namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Write uid/gid mappings to maintain some capabilities */
+		int uid_map_fd = open("/proc/self/uid_map", O_WRONLY);
+		int gid_map_fd = open("/proc/self/gid_map", O_WRONLY);
+		int setgroups_fd = open("/proc/self/setgroups", O_WRONLY);
+
+		if (uid_map_fd < 0 || gid_map_fd < 0 || setgroups_fd < 0) {
+			write(pipefd[1], "M", 1); /* Unable to set mappings */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Disable setgroups to allow gid mapping */
+		write(setgroups_fd, "deny", 4);
+		close(setgroups_fd);
+
+		/* Map current uid/gid to root in the new namespace */
+		char mapping[64];
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getuid());
+		write(uid_map_fd, mapping, strlen(mapping));
+		close(uid_map_fd);
+
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getgid());
+		write(gid_map_fd, mapping, strlen(mapping));
+		close(gid_map_fd);
+
+		/* Now create new network namespace */
+		ret = unshare(CLONE_NEWNET);
+		if (ret < 0) {
+			write(pipefd[1], "N",
+			      1); /* Unable to create network namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Try to open parent's network namespace handle from new user+net namespace */
+		fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+
+		if (fd >= 0) {
+			/* Should NOT succeed - we're in a different user namespace */
+			write(pipefd[1], "S", 1); /* Unexpected success */
+			close(fd);
+		} else if (errno == ESTALE) {
+			/* Expected: Stale file handle */
+			write(pipefd[1], "P", 1);
+		} else {
+			/* Other error */
+			write(pipefd[1], "F", 1);
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+	ASSERT_EQ(read(pipefd[0], &result, 1), 1);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	if (result == 'U') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new user namespace");
+	}
+	if (result == 'M') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot set uid/gid mappings");
+	}
+	if (result == 'N') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new network namespace");
+	}
+
+	/* Should fail with permission denied since we're in a different user namespace */
+	ASSERT_EQ(result, 'P');
+
+	close(pipefd[0]);
+	free(handle);
+}
+
+TEST(nsfs_user_uts_namespace_isolation)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	pid_t pid;
+	int status;
+	int pipefd[2];
+	char result;
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Create pipe for communication */
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	/* Get handle for current UTS namespace */
+	ns_fd = open("/proc/self/ns/uts", O_RDONLY);
+	ASSERT_GE(ns_fd, 0);
+
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd); close(pipefd[0]);
+		     close(pipefd[1]);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	close(ns_fd);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* First create new user namespace to drop privileges */
+		ret = unshare(CLONE_NEWUSER);
+		if (ret < 0) {
+			write(pipefd[1], "U",
+			      1); /* Unable to create user namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Write uid/gid mappings to maintain some capabilities */
+		int uid_map_fd = open("/proc/self/uid_map", O_WRONLY);
+		int gid_map_fd = open("/proc/self/gid_map", O_WRONLY);
+		int setgroups_fd = open("/proc/self/setgroups", O_WRONLY);
+
+		if (uid_map_fd < 0 || gid_map_fd < 0 || setgroups_fd < 0) {
+			write(pipefd[1], "M", 1); /* Unable to set mappings */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Disable setgroups to allow gid mapping */
+		write(setgroups_fd, "deny", 4);
+		close(setgroups_fd);
+
+		/* Map current uid/gid to root in the new namespace */
+		char mapping[64];
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getuid());
+		write(uid_map_fd, mapping, strlen(mapping));
+		close(uid_map_fd);
+
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getgid());
+		write(gid_map_fd, mapping, strlen(mapping));
+		close(gid_map_fd);
+
+		/* Now create new UTS namespace */
+		ret = unshare(CLONE_NEWUTS);
+		if (ret < 0) {
+			write(pipefd[1], "N",
+			      1); /* Unable to create UTS namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Try to open parent's UTS namespace handle from new user+uts namespace */
+		fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+
+		if (fd >= 0) {
+			/* Should NOT succeed - we're in a different user namespace */
+			write(pipefd[1], "S", 1); /* Unexpected success */
+			close(fd);
+		} else if (errno == ESTALE) {
+			/* Expected: Stale file handle */
+			write(pipefd[1], "P", 1);
+		} else {
+			/* Other error */
+			write(pipefd[1], "F", 1);
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+	ASSERT_EQ(read(pipefd[0], &result, 1), 1);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	if (result == 'U') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new user namespace");
+	}
+	if (result == 'M') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot set uid/gid mappings");
+	}
+	if (result == 'N') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new UTS namespace");
+	}
+
+	/* Should fail with ESTALE since we're in a different user namespace */
+	ASSERT_EQ(result, 'P');
+
+	close(pipefd[0]);
+	free(handle);
+}
+
+TEST(nsfs_user_ipc_namespace_isolation)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	pid_t pid;
+	int status;
+	int pipefd[2];
+	char result;
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Create pipe for communication */
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	/* Get handle for current IPC namespace */
+	ns_fd = open("/proc/self/ns/ipc", O_RDONLY);
+	ASSERT_GE(ns_fd, 0);
+
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd); close(pipefd[0]);
+		     close(pipefd[1]);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	close(ns_fd);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* First create new user namespace to drop privileges */
+		ret = unshare(CLONE_NEWUSER);
+		if (ret < 0) {
+			write(pipefd[1], "U",
+			      1); /* Unable to create user namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Write uid/gid mappings to maintain some capabilities */
+		int uid_map_fd = open("/proc/self/uid_map", O_WRONLY);
+		int gid_map_fd = open("/proc/self/gid_map", O_WRONLY);
+		int setgroups_fd = open("/proc/self/setgroups", O_WRONLY);
+
+		if (uid_map_fd < 0 || gid_map_fd < 0 || setgroups_fd < 0) {
+			write(pipefd[1], "M", 1); /* Unable to set mappings */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Disable setgroups to allow gid mapping */
+		write(setgroups_fd, "deny", 4);
+		close(setgroups_fd);
+
+		/* Map current uid/gid to root in the new namespace */
+		char mapping[64];
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getuid());
+		write(uid_map_fd, mapping, strlen(mapping));
+		close(uid_map_fd);
+
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getgid());
+		write(gid_map_fd, mapping, strlen(mapping));
+		close(gid_map_fd);
+
+		/* Now create new IPC namespace */
+		ret = unshare(CLONE_NEWIPC);
+		if (ret < 0) {
+			write(pipefd[1], "N",
+			      1); /* Unable to create IPC namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Try to open parent's IPC namespace handle from new user+ipc namespace */
+		fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+
+		if (fd >= 0) {
+			/* Should NOT succeed - we're in a different user namespace */
+			write(pipefd[1], "S", 1); /* Unexpected success */
+			close(fd);
+		} else if (errno == ESTALE) {
+			/* Expected: Stale file handle */
+			write(pipefd[1], "P", 1);
+		} else {
+			/* Other error */
+			write(pipefd[1], "F", 1);
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+	ASSERT_EQ(read(pipefd[0], &result, 1), 1);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	if (result == 'U') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new user namespace");
+	}
+	if (result == 'M') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot set uid/gid mappings");
+	}
+	if (result == 'N') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new IPC namespace");
+	}
+
+	/* Should fail with ESTALE since we're in a different user namespace */
+	ASSERT_EQ(result, 'P');
+
+	close(pipefd[0]);
+	free(handle);
+}
+
+TEST(nsfs_user_mnt_namespace_isolation)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	pid_t pid;
+	int status;
+	int pipefd[2];
+	char result;
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Create pipe for communication */
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	/* Get handle for current mount namespace */
+	ns_fd = open("/proc/self/ns/mnt", O_RDONLY);
+	ASSERT_GE(ns_fd, 0);
+
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd); close(pipefd[0]);
+		     close(pipefd[1]);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	close(ns_fd);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* First create new user namespace to drop privileges */
+		ret = unshare(CLONE_NEWUSER);
+		if (ret < 0) {
+			write(pipefd[1], "U",
+			      1); /* Unable to create user namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Write uid/gid mappings to maintain some capabilities */
+		int uid_map_fd = open("/proc/self/uid_map", O_WRONLY);
+		int gid_map_fd = open("/proc/self/gid_map", O_WRONLY);
+		int setgroups_fd = open("/proc/self/setgroups", O_WRONLY);
+
+		if (uid_map_fd < 0 || gid_map_fd < 0 || setgroups_fd < 0) {
+			write(pipefd[1], "M", 1); /* Unable to set mappings */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Disable setgroups to allow gid mapping */
+		write(setgroups_fd, "deny", 4);
+		close(setgroups_fd);
+
+		/* Map current uid/gid to root in the new namespace */
+		char mapping[64];
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getuid());
+		write(uid_map_fd, mapping, strlen(mapping));
+		close(uid_map_fd);
+
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getgid());
+		write(gid_map_fd, mapping, strlen(mapping));
+		close(gid_map_fd);
+
+		/* Now create new mount namespace */
+		ret = unshare(CLONE_NEWNS);
+		if (ret < 0) {
+			write(pipefd[1], "N",
+			      1); /* Unable to create mount namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Try to open parent's mount namespace handle from new user+mnt namespace */
+		fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+
+		if (fd >= 0) {
+			/* Should NOT succeed - we're in a different user namespace */
+			write(pipefd[1], "S", 1); /* Unexpected success */
+			close(fd);
+		} else if (errno == ESTALE) {
+			/* Expected: Stale file handle */
+			write(pipefd[1], "P", 1);
+		} else {
+			/* Other error */
+			write(pipefd[1], "F", 1);
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+	ASSERT_EQ(read(pipefd[0], &result, 1), 1);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	if (result == 'U') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new user namespace");
+	}
+	if (result == 'M') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot set uid/gid mappings");
+	}
+	if (result == 'N') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new mount namespace");
+	}
+
+	/* Should fail with ESTALE since we're in a different user namespace */
+	ASSERT_EQ(result, 'P');
+
+	close(pipefd[0]);
+	free(handle);
+}
+
+TEST(nsfs_user_cgroup_namespace_isolation)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	pid_t pid;
+	int status;
+	int pipefd[2];
+	char result;
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Create pipe for communication */
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	/* Get handle for current cgroup namespace */
+	ns_fd = open("/proc/self/ns/cgroup", O_RDONLY);
+	if (ns_fd < 0) {
+		SKIP(free(handle); close(pipefd[0]); close(pipefd[1]);
+		     return, "cgroup namespace not available");
+	}
+
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd); close(pipefd[0]);
+		     close(pipefd[1]);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	close(ns_fd);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* First create new user namespace to drop privileges */
+		ret = unshare(CLONE_NEWUSER);
+		if (ret < 0) {
+			write(pipefd[1], "U",
+			      1); /* Unable to create user namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Write uid/gid mappings to maintain some capabilities */
+		int uid_map_fd = open("/proc/self/uid_map", O_WRONLY);
+		int gid_map_fd = open("/proc/self/gid_map", O_WRONLY);
+		int setgroups_fd = open("/proc/self/setgroups", O_WRONLY);
+
+		if (uid_map_fd < 0 || gid_map_fd < 0 || setgroups_fd < 0) {
+			write(pipefd[1], "M", 1); /* Unable to set mappings */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Disable setgroups to allow gid mapping */
+		write(setgroups_fd, "deny", 4);
+		close(setgroups_fd);
+
+		/* Map current uid/gid to root in the new namespace */
+		char mapping[64];
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getuid());
+		write(uid_map_fd, mapping, strlen(mapping));
+		close(uid_map_fd);
+
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getgid());
+		write(gid_map_fd, mapping, strlen(mapping));
+		close(gid_map_fd);
+
+		/* Now create new cgroup namespace */
+		ret = unshare(CLONE_NEWCGROUP);
+		if (ret < 0) {
+			write(pipefd[1], "N",
+			      1); /* Unable to create cgroup namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Try to open parent's cgroup namespace handle from new user+cgroup namespace */
+		fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+
+		if (fd >= 0) {
+			/* Should NOT succeed - we're in a different user namespace */
+			write(pipefd[1], "S", 1); /* Unexpected success */
+			close(fd);
+		} else if (errno == ESTALE) {
+			/* Expected: Stale file handle */
+			write(pipefd[1], "P", 1);
+		} else {
+			/* Other error */
+			write(pipefd[1], "F", 1);
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+	ASSERT_EQ(read(pipefd[0], &result, 1), 1);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	if (result == 'U') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new user namespace");
+	}
+	if (result == 'M') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot set uid/gid mappings");
+	}
+	if (result == 'N') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new cgroup namespace");
+	}
+
+	/* Should fail with ESTALE since we're in a different user namespace */
+	ASSERT_EQ(result, 'P');
+
+	close(pipefd[0]);
+	free(handle);
+}
+
+TEST(nsfs_user_pid_namespace_isolation)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	pid_t pid;
+	int status;
+	int pipefd[2];
+	char result;
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Create pipe for communication */
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	/* Get handle for current PID namespace */
+	ns_fd = open("/proc/self/ns/pid", O_RDONLY);
+	ASSERT_GE(ns_fd, 0);
+
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd); close(pipefd[0]);
+		     close(pipefd[1]);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	close(ns_fd);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* First create new user namespace to drop privileges */
+		ret = unshare(CLONE_NEWUSER);
+		if (ret < 0) {
+			write(pipefd[1], "U",
+			      1); /* Unable to create user namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Write uid/gid mappings to maintain some capabilities */
+		int uid_map_fd = open("/proc/self/uid_map", O_WRONLY);
+		int gid_map_fd = open("/proc/self/gid_map", O_WRONLY);
+		int setgroups_fd = open("/proc/self/setgroups", O_WRONLY);
+
+		if (uid_map_fd < 0 || gid_map_fd < 0 || setgroups_fd < 0) {
+			write(pipefd[1], "M", 1); /* Unable to set mappings */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Disable setgroups to allow gid mapping */
+		write(setgroups_fd, "deny", 4);
+		close(setgroups_fd);
+
+		/* Map current uid/gid to root in the new namespace */
+		char mapping[64];
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getuid());
+		write(uid_map_fd, mapping, strlen(mapping));
+		close(uid_map_fd);
+
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getgid());
+		write(gid_map_fd, mapping, strlen(mapping));
+		close(gid_map_fd);
+
+		/* Now create new PID namespace - requires fork to take effect */
+		ret = unshare(CLONE_NEWPID);
+		if (ret < 0) {
+			write(pipefd[1], "N",
+			      1); /* Unable to create PID namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Fork again for PID namespace to take effect */
+		pid_t child_pid = fork();
+		if (child_pid < 0) {
+			write(pipefd[1], "N",
+			      1); /* Unable to fork in PID namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		if (child_pid == 0) {
+			/* Grandchild in new PID namespace */
+			/* Try to open parent's PID namespace handle from new user+pid namespace */
+			fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+
+			if (fd >= 0) {
+				/* Should NOT succeed - we're in a different user namespace */
+				write(pipefd[1], "S",
+				      1); /* Unexpected success */
+				close(fd);
+			} else if (errno == ESTALE) {
+				/* Expected: Stale file handle */
+				write(pipefd[1], "P", 1);
+			} else {
+				/* Other error */
+				write(pipefd[1], "F", 1);
+			}
+
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Wait for grandchild */
+		waitpid(child_pid, NULL, 0);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+	ASSERT_EQ(read(pipefd[0], &result, 1), 1);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	if (result == 'U') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new user namespace");
+	}
+	if (result == 'M') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot set uid/gid mappings");
+	}
+	if (result == 'N') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new PID namespace");
+	}
+
+	/* Should fail with ESTALE since we're in a different user namespace */
+	ASSERT_EQ(result, 'P');
+
+	close(pipefd[0]);
+	free(handle);
+}
+
+TEST(nsfs_user_time_namespace_isolation)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+	pid_t pid;
+	int status;
+	int pipefd[2];
+	char result;
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Create pipe for communication */
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	/* Get handle for current time namespace */
+	ns_fd = open("/proc/self/ns/time", O_RDONLY);
+	if (ns_fd < 0) {
+		SKIP(free(handle); close(pipefd[0]); close(pipefd[1]);
+		     return, "time namespace not available");
+	}
+
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd); close(pipefd[0]);
+		     close(pipefd[1]);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	close(ns_fd);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* First create new user namespace to drop privileges */
+		ret = unshare(CLONE_NEWUSER);
+		if (ret < 0) {
+			write(pipefd[1], "U",
+			      1); /* Unable to create user namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Write uid/gid mappings to maintain some capabilities */
+		int uid_map_fd = open("/proc/self/uid_map", O_WRONLY);
+		int gid_map_fd = open("/proc/self/gid_map", O_WRONLY);
+		int setgroups_fd = open("/proc/self/setgroups", O_WRONLY);
+
+		if (uid_map_fd < 0 || gid_map_fd < 0 || setgroups_fd < 0) {
+			write(pipefd[1], "M", 1); /* Unable to set mappings */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Disable setgroups to allow gid mapping */
+		write(setgroups_fd, "deny", 4);
+		close(setgroups_fd);
+
+		/* Map current uid/gid to root in the new namespace */
+		char mapping[64];
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getuid());
+		write(uid_map_fd, mapping, strlen(mapping));
+		close(uid_map_fd);
+
+		snprintf(mapping, sizeof(mapping), "0 %d 1", getgid());
+		write(gid_map_fd, mapping, strlen(mapping));
+		close(gid_map_fd);
+
+		/* Now create new time namespace - requires fork to take effect */
+		ret = unshare(CLONE_NEWTIME);
+		if (ret < 0) {
+			write(pipefd[1], "N",
+			      1); /* Unable to create time namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Fork again for time namespace to take effect */
+		pid_t child_pid = fork();
+		if (child_pid < 0) {
+			write(pipefd[1], "N",
+			      1); /* Unable to fork in time namespace */
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		if (child_pid == 0) {
+			/* Grandchild in new time namespace */
+			/* Try to open parent's time namespace handle from new user+time namespace */
+			fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+
+			if (fd >= 0) {
+				/* Should NOT succeed - we're in a different user namespace */
+				write(pipefd[1], "S",
+				      1); /* Unexpected success */
+				close(fd);
+			} else if (errno == ESTALE) {
+				/* Expected: Stale file handle */
+				write(pipefd[1], "P", 1);
+			} else {
+				/* Other error */
+				write(pipefd[1], "F", 1);
+			}
+
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Wait for grandchild */
+		waitpid(child_pid, NULL, 0);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+	ASSERT_EQ(read(pipefd[0], &result, 1), 1);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	if (result == 'U') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new user namespace");
+	}
+	if (result == 'M') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot set uid/gid mappings");
+	}
+	if (result == 'N') {
+		SKIP(free(handle); close(pipefd[0]);
+		     return, "Cannot create new time namespace");
+	}
+
+	/* Should fail with ESTALE since we're in a different user namespace */
+	ASSERT_EQ(result, 'P');
+
+	close(pipefd[0]);
+	free(handle);
+}
+
+TEST(nsfs_open_flags)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int ns_fd;
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Open a namespace file descriptor */
+	ns_fd = open("/proc/self/ns/net", O_RDONLY);
+	ASSERT_GE(ns_fd, 0);
+
+	/* Get handle for the namespace */
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(ns_fd, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(ns_fd);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+	ASSERT_GT(handle->handle_bytes, 0);
+
+	/* Test invalid flags that should fail */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_WRONLY);
+	ASSERT_LT(fd, 0);
+	ASSERT_EQ(errno, EPERM);
+
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDWR);
+	ASSERT_LT(fd, 0);
+	ASSERT_EQ(errno, EPERM);
+
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_TRUNC);
+	ASSERT_LT(fd, 0);
+	ASSERT_EQ(errno, EPERM);
+
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_DIRECT);
+	ASSERT_LT(fd, 0);
+	ASSERT_EQ(errno, EINVAL);
+
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_TMPFILE);
+	ASSERT_LT(fd, 0);
+	ASSERT_EQ(errno, EINVAL);
+
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_DIRECTORY);
+	ASSERT_LT(fd, 0);
+	ASSERT_EQ(errno, ENOTDIR);
+
+	close(ns_fd);
+	free(handle);
+}
+
+TEST_HARNESS_MAIN