Merge tag 'namespace-6.19-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull namespace updates from Christian Brauner:
 "This contains substantial namespace infrastructure changes including a new
  system call, active reference counting, and extensive header cleanups.
  The branch depends on the shared kbuild branch for -fms-extensions support.

  Features:

   - listns() system call

     Add a new listns() system call that allows userspace to iterate
     through namespaces in the system. This provides a programmatic
     interface to discover and inspect namespaces, addressing
     longstanding limitations:

     Currently, there is no direct way for userspace to enumerate
     namespaces. Applications must resort to scanning /proc/*/ns/ across
     all processes, which is:
      - Inefficient - requires iterating over all processes
      - Incomplete - misses namespaces not attached to any running
        process but kept alive by file descriptors, bind mounts, or
        parent references
      - Permission-heavy - requires access to /proc for many processes
      - No ordering or ownership information
      - No filtering per namespace type

     The listns() system call solves these problems:

       ssize_t listns(const struct ns_id_req *req, u64 *ns_ids,
                      size_t nr_ns_ids, unsigned int flags);

       struct ns_id_req {
             __u32 size;
             __u32 spare;
             __u64 ns_id;
             struct /* listns */ {
                     __u32 ns_type;
                     __u32 spare2;
                     __u64 user_ns_id;
             };
       };

     Features include:
      - Pagination support for large namespace sets
      - Filtering by namespace type (MNT_NS, NET_NS, USER_NS, etc.)
      - Filtering by owning user namespace
      - Permission checks respecting namespace isolation

   - Active Reference Counting

     Introduce an active reference count that tracks namespace
     visibility to userspace. A namespace is visible in the following
     cases:
      - The namespace is in use by a task
      - The namespace is persisted through a VFS object (namespace file
        descriptor or bind-mount)
      - The namespace is a hierarchical type and is the parent of child
        namespaces

     The active reference count does not regulate lifetime (that's still
     done by the normal reference count) - it only regulates visibility
     to namespace file handles and listns().

     This prevents resurrection of namespaces that are pinned only for
     internal kernel reasons (e.g., user namespaces held by
     file->f_cred, lazy TLB references on idle CPUs, etc.) which should
     not be accessible via (1)-(3).

   - Unified Namespace Tree

     Introduce a unified tree structure for all namespaces with:
      - Fixed IDs assigned to initial namespaces
      - Lookup based solely on inode number
      - Maintained list of owned namespaces per user namespace
      - Simplified rbtree comparison helpers

   Cleanups

    - Header Reorganization:
      - Move namespace types into separate header (ns_common_types.h)
      - Decouple nstree from ns_common header
      - Move nstree types into separate header
      - Switch to new ns_tree_{node,root} structures with helper functions
      - Use guards for ns_tree_lock

   - Initial Namespace Reference Count Optimization
      - Make all reference counts on initial namespaces a nop to avoid
        pointless cacheline ping-pong for namespaces that can never go
        away
      - Drop custom reference count initialization for initial namespaces
      - Add NS_COMMON_INIT() macro and use it for all namespaces
      - pid: rely on common reference count behavior

   - Miscellaneous Cleanups
      - Rename exit_task_namespaces() to exit_nsproxy_namespaces()
      - Rename is_initial_namespace() and make argument const
      - Use boolean to indicate anonymous mount namespace
      - Simplify owner list iteration in nstree
      - nsfs: raise SB_I_NODEV, SB_I_NOEXEC, and DCACHE_DONTCACHE explicitly
      - nsfs: use inode_just_drop()
      - pidfs: raise DCACHE_DONTCACHE explicitly
      - pidfs: simplify PIDFD_GET__NAMESPACE ioctls
      - libfs: allow to specify s_d_flags
      - cgroup: add cgroup namespace to tree after owner is set
      - nsproxy: fix free_nsproxy() and simplify create_new_namespaces()

  Fixes:

   - setns(pidfd, ...) race condition

     Fix a subtle race when using pidfds with setns(). When the target
     task exits after prepare_nsset() but before commit_nsset(), the
     namespace's active reference count might have been dropped. If
     setns() then installs the namespaces, it would bump the active
     reference count from zero without taking the required reference on
     the owner namespace, leading to underflow when later decremented.

     The fix resurrects the ownership chain if necessary - if the caller
     succeeded in grabbing passive references, the setns() should
     succeed even if the target task exits or gets reaped.

   - Return EFAULT on put_user() error instead of success

   - Make sure references are dropped outside of RCU lock (some
     namespaces like mount namespace sleep when putting the last
     reference)

   - Don't skip active reference count initialization for network
     namespace

   - Add asserts for active refcount underflow

   - Add asserts for initial namespace reference counts (both passive
     and active)

   - ipc: enable is_ns_init_id() assertions

   - Fix kernel-doc comments for internal nstree functions

   - Selftests
      - 15 active reference count tests
      - 9 listns() functionality tests
      - 7 listns() permission tests
      - 12 inactive namespace resurrection tests
      - 3 threaded active reference count tests
      - commit_creds() active reference tests
      - Pagination and stress tests
      - EFAULT handling test
      - nsid tests fixes"

* tag 'namespace-6.19-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs: (103 commits)
  pidfs: simplify PIDFD_GET_<type>_NAMESPACE ioctls
  nstree: fix kernel-doc comments for internal functions
  nsproxy: fix free_nsproxy() and simplify create_new_namespaces()
  selftests/namespaces: fix nsid tests
  ns: drop custom reference count initialization for initial namespaces
  pid: rely on common reference count behavior
  ns: add asserts for initial namespace active reference counts
  ns: add asserts for initial namespace reference counts
  ns: make all reference counts on initial namespace a nop
  ipc: enable is_ns_init_id() assertions
  fs: use boolean to indicate anonymous mount namespace
  ns: rename is_initial_namespace()
  ns: make is_initial_namespace() argument const
  nstree: use guards for ns_tree_lock
  nstree: simplify owner list iteration
  nstree: switch to new structures
  nstree: add helper to operate on struct ns_tree_{node,root}
  nstree: move nstree types into separate header
  nstree: decouple from ns_common header
  ns: move namespace types into separate header
  ...

1 files changed, 626 insertions, 0 deletions

diff --git a/tools/testing/selftests/namespaces/stress_test.c b/tools/testing/selftests/namespaces/stress_test.c
new file mode 100644
index 000000000000..dd7df7d6cb27
--- /dev/null
+++ b/tools/testing/selftests/namespaces/stress_test.c
@@ -0,0 +1,626 @@
+// 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 <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <linux/nsfs.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+/*
+ * Stress tests for namespace active reference counting.
+ *
+ * These tests validate that the active reference counting system can handle
+ * high load scenarios including rapid namespace creation/destruction, large
+ * numbers of concurrent namespaces, and various edge cases under stress.
+ */
+
+/*
+ * Test rapid creation and destruction of user namespaces.
+ * Create and destroy namespaces in quick succession to stress the
+ * active reference tracking and ensure no leaks occur.
+ */
+TEST(rapid_namespace_creation_destruction)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[256], ns_ids_after[256];
+	ssize_t ret_before, ret_after;
+	int i;
+
+	/* Get baseline count of active user namespaces */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active user namespaces", ret_before);
+
+	/* Rapidly create and destroy 100 user namespaces */
+	for (i = 0; i < 100; i++) {
+		pid_t pid = fork();
+		ASSERT_GE(pid, 0);
+
+		if (pid == 0) {
+			/* Child: create user namespace and immediately exit */
+			if (setup_userns() < 0)
+				exit(1);
+			exit(0);
+		}
+
+		/* Parent: wait for child */
+		int status;
+		waitpid(pid, &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+		ASSERT_EQ(WEXITSTATUS(status), 0);
+	}
+
+	/* Verify we're back to baseline (no leaked namespaces) */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After 100 rapid create/destroy cycles: %zd active user namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test creating many concurrent namespaces.
+ * Verify that listns() correctly tracks all of them and that they all
+ * become inactive after processes exit.
+ */
+TEST(many_concurrent_namespaces)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_during[512], ns_ids_after[512];
+	ssize_t ret_before, ret_during, ret_after;
+	pid_t pids[50];
+	int num_children = 50;
+	int i;
+	int sv[2];
+
+	/* Get baseline */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active user namespaces", ret_before);
+
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	/* Create many children, each with their own user namespace */
+	for (i = 0; i < num_children; i++) {
+		pids[i] = fork();
+		ASSERT_GE(pids[i], 0);
+
+		if (pids[i] == 0) {
+			/* Child: create user namespace and wait for parent signal */
+			char c;
+
+			close(sv[0]);
+
+			if (setup_userns() < 0) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Signal parent we're ready */
+			if (write(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Wait for parent signal to exit */
+			if (read(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			close(sv[1]);
+			exit(0);
+		}
+	}
+
+	close(sv[1]);
+
+	/* Wait for all children to signal ready */
+	for (i = 0; i < num_children; i++) {
+		char c;
+		if (read(sv[0], &c, 1) != 1) {
+			/* If we fail to read, kill all children and exit */
+			close(sv[0]);
+			for (int j = 0; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	/* List namespaces while all children are running */
+	ret_during = sys_listns(&req, ns_ids_during, ARRAY_SIZE(ns_ids_during), 0);
+	ASSERT_GE(ret_during, 0);
+
+	TH_LOG("With %d children running: %zd active user namespaces", num_children, ret_during);
+
+	/* Should have at least num_children more namespaces than baseline */
+	ASSERT_GE(ret_during, ret_before + num_children);
+
+	/* Signal all children to exit */
+	for (i = 0; i < num_children; i++) {
+		char c = 'X';
+		if (write(sv[0], &c, 1) != 1) {
+			/* If we fail to write, kill remaining children */
+			close(sv[0]);
+			for (int j = i; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	close(sv[0]);
+
+	/* Wait for all children */
+	for (i = 0; i < num_children; i++) {
+		int status;
+		waitpid(pids[i], &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After all children exit: %zd active user namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test rapid namespace creation with different namespace types.
+ * Create multiple types of namespaces rapidly to stress the tracking system.
+ */
+TEST(rapid_mixed_namespace_creation)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,  /* All types */
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_after[512];
+	ssize_t ret_before, ret_after;
+	int i;
+
+	/* Get baseline count */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active namespaces (all types)", ret_before);
+
+	/* Rapidly create and destroy namespaces with multiple types */
+	for (i = 0; i < 50; i++) {
+		pid_t pid = fork();
+		ASSERT_GE(pid, 0);
+
+		if (pid == 0) {
+			/* Child: create multiple namespace types */
+			if (setup_userns() < 0)
+				exit(1);
+
+			/* Create additional namespace types */
+			if (unshare(CLONE_NEWNET) < 0)
+				exit(1);
+			if (unshare(CLONE_NEWUTS) < 0)
+				exit(1);
+			if (unshare(CLONE_NEWIPC) < 0)
+				exit(1);
+
+			exit(0);
+		}
+
+		/* Parent: wait for child */
+		int status;
+		waitpid(pid, &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After 50 rapid mixed namespace cycles: %zd active namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test nested namespace creation under stress.
+ * Create deeply nested namespace hierarchies and verify proper cleanup.
+ */
+TEST(nested_namespace_stress)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_after[512];
+	ssize_t ret_before, ret_after;
+	int i;
+
+	/* Get baseline */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active user namespaces", ret_before);
+
+	/* Create 20 processes, each with nested user namespaces */
+	for (i = 0; i < 20; i++) {
+		pid_t pid = fork();
+		ASSERT_GE(pid, 0);
+
+		if (pid == 0) {
+			int userns_fd;
+			uid_t orig_uid = getuid();
+			int depth;
+
+			/* Create nested user namespaces (up to 5 levels) */
+			for (depth = 0; depth < 5; depth++) {
+				userns_fd = get_userns_fd(0, (depth == 0) ? orig_uid : 0, 1);
+				if (userns_fd < 0)
+					exit(1);
+
+				if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+					close(userns_fd);
+					exit(1);
+				}
+				close(userns_fd);
+			}
+
+			exit(0);
+		}
+
+		/* Parent: wait for child */
+		int status;
+		waitpid(pid, &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After 20 nested namespace hierarchies: %zd active user namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test listns() pagination under stress.
+ * Create many namespaces and verify pagination works correctly.
+ */
+TEST(listns_pagination_stress)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	pid_t pids[30];
+	int num_children = 30;
+	int i;
+	int sv[2];
+	__u64 all_ns_ids[512];
+	int total_found = 0;
+
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	/* Create many children with user namespaces */
+	for (i = 0; i < num_children; i++) {
+		pids[i] = fork();
+		ASSERT_GE(pids[i], 0);
+
+		if (pids[i] == 0) {
+			char c;
+			close(sv[0]);
+
+			if (setup_userns() < 0) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Signal parent we're ready */
+			if (write(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Wait for parent signal to exit */
+			if (read(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			close(sv[1]);
+			exit(0);
+		}
+	}
+
+	close(sv[1]);
+
+	/* Wait for all children to signal ready */
+	for (i = 0; i < num_children; i++) {
+		char c;
+		if (read(sv[0], &c, 1) != 1) {
+			/* If we fail to read, kill all children and exit */
+			close(sv[0]);
+			for (int j = 0; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	/* Paginate through all namespaces using small batch sizes */
+	req.ns_id = 0;
+	while (1) {
+		__u64 batch[5];  /* Small batch size to force pagination */
+		ssize_t ret;
+
+		ret = sys_listns(&req, batch, ARRAY_SIZE(batch), 0);
+		if (ret < 0) {
+			if (errno == ENOSYS) {
+				close(sv[0]);
+				for (i = 0; i < num_children; i++)
+					kill(pids[i], SIGKILL);
+				for (i = 0; i < num_children; i++)
+					waitpid(pids[i], NULL, 0);
+				SKIP(return, "listns() not supported");
+			}
+			ASSERT_GE(ret, 0);
+		}
+
+		if (ret == 0)
+			break;
+
+		/* Store results */
+		for (i = 0; i < ret && total_found < 512; i++) {
+			all_ns_ids[total_found++] = batch[i];
+		}
+
+		/* Update cursor for next batch */
+		if (ret == ARRAY_SIZE(batch))
+			req.ns_id = batch[ret - 1];
+		else
+			break;
+	}
+
+	TH_LOG("Paginated through %d user namespaces", total_found);
+
+	/* Verify no duplicates in pagination */
+	for (i = 0; i < total_found; i++) {
+		for (int j = i + 1; j < total_found; j++) {
+			if (all_ns_ids[i] == all_ns_ids[j]) {
+				TH_LOG("Found duplicate ns_id: %llu at positions %d and %d",
+				       (unsigned long long)all_ns_ids[i], i, j);
+				ASSERT_TRUE(false);
+			}
+		}
+	}
+
+	/* Signal all children to exit */
+	for (i = 0; i < num_children; i++) {
+		char c = 'X';
+		if (write(sv[0], &c, 1) != 1) {
+			close(sv[0]);
+			for (int j = i; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	close(sv[0]);
+
+	/* Wait for all children */
+	for (i = 0; i < num_children; i++) {
+		int status;
+		waitpid(pids[i], &status, 0);
+	}
+}
+
+/*
+ * Test concurrent namespace operations.
+ * Multiple processes creating, querying, and destroying namespaces concurrently.
+ */
+TEST(concurrent_namespace_operations)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_after[512];
+	ssize_t ret_before, ret_after;
+	pid_t pids[20];
+	int num_workers = 20;
+	int i;
+
+	/* Get baseline */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active namespaces", ret_before);
+
+	/* Create worker processes that do concurrent operations */
+	for (i = 0; i < num_workers; i++) {
+		pids[i] = fork();
+		ASSERT_GE(pids[i], 0);
+
+		if (pids[i] == 0) {
+			/* Each worker: create namespaces, list them, repeat */
+			int iterations;
+
+			for (iterations = 0; iterations < 10; iterations++) {
+				int userns_fd;
+				__u64 temp_ns_ids[100];
+				ssize_t ret;
+
+				/* Create a user namespace */
+				userns_fd = get_userns_fd(0, getuid(), 1);
+				if (userns_fd < 0)
+					continue;
+
+				/* List namespaces */
+				ret = sys_listns(&req, temp_ns_ids, ARRAY_SIZE(temp_ns_ids), 0);
+				(void)ret;
+
+				close(userns_fd);
+
+				/* Small delay */
+				usleep(1000);
+			}
+
+			exit(0);
+		}
+	}
+
+	/* Wait for all workers */
+	for (i = 0; i < num_workers; i++) {
+		int status;
+		waitpid(pids[i], &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+		ASSERT_EQ(WEXITSTATUS(status), 0);
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After concurrent operations: %zd active namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test namespace churn - continuous creation and destruction.
+ * Simulates high-churn scenarios like container orchestration.
+ */
+TEST(namespace_churn)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER | CLONE_NEWNET | CLONE_NEWUTS,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_after[512];
+	ssize_t ret_before, ret_after;
+	int cycle;
+
+	/* Get baseline */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active namespaces", ret_before);
+
+	/* Simulate churn: batches of namespaces created and destroyed */
+	for (cycle = 0; cycle < 10; cycle++) {
+		pid_t batch_pids[10];
+		int i;
+
+		/* Create batch */
+		for (i = 0; i < 10; i++) {
+			batch_pids[i] = fork();
+			ASSERT_GE(batch_pids[i], 0);
+
+			if (batch_pids[i] == 0) {
+				/* Create multiple namespace types */
+				if (setup_userns() < 0)
+					exit(1);
+				if (unshare(CLONE_NEWNET) < 0)
+					exit(1);
+				if (unshare(CLONE_NEWUTS) < 0)
+					exit(1);
+
+				/* Keep namespaces alive briefly */
+				usleep(10000);
+				exit(0);
+			}
+		}
+
+		/* Wait for batch to complete */
+		for (i = 0; i < 10; i++) {
+			int status;
+			waitpid(batch_pids[i], &status, 0);
+		}
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After 10 churn cycles (100 namespace sets): %zd active namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+TEST_HARNESS_MAIN