Test hierarchical active reference propagation.
When a child namespace is active, its owning user namespace should also
be active automatically due to hierarchical active reference propagation.
This ensures parents are always reachable when children are active.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/namespaces/ns_active_ref_test.c      | 242 +++++++++++++++++++++
 1 file changed, 242 insertions(+)

diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
index dbb1eb8a04b2..6377f5d72ed9 100644
--- a/tools/testing/selftests/namespaces/ns_active_ref_test.c
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -20,6 +20,10 @@
 #define FD_NSFS_ROOT -10003 /* Root of the nsfs filesystem */
 #endif
 
+#ifndef FILEID_NSFS
+#define FILEID_NSFS 0xf1
+#endif
+
 /*
  * Test that initial namespaces can be reopened via file handle.
  * Initial namespaces should have active ref count of 1 from boot.
@@ -641,4 +645,242 @@ TEST(ns_fd_keeps_active)
 	}
 }
 
+/*
+ * Test hierarchical active reference propagation.
+ * When a child namespace is active, its owning user namespace should also
+ * be active automatically due to hierarchical active reference propagation.
+ * This ensures parents are always reachable when children are active.
+ */
+TEST(ns_parent_always_reachable)
+{
+	struct file_handle *parent_handle, *child_handle;
+	int ret;
+	int child_nsfd;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 parent_id, child_id;
+	char parent_buf[sizeof(*parent_handle) + MAX_HANDLE_SZ];
+	char child_buf[sizeof(*child_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		TH_LOG("Child: creating parent user namespace and setting up mappings");
+
+		/* Create parent user namespace with mappings */
+		ret = setup_userns();
+		if (ret < 0) {
+			TH_LOG("Child: setup_userns() for parent failed: %s", strerror(errno));
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		TH_LOG("Child: parent user namespace created, now uid=%d gid=%d", getuid(), getgid());
+
+		/* Get namespace ID for parent user namespace */
+		int parent_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (parent_fd < 0) {
+			TH_LOG("Child: failed to open parent /proc/self/ns/user: %s", strerror(errno));
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		TH_LOG("Child: opened parent userns fd %d", parent_fd);
+
+		if (ioctl(parent_fd, NS_GET_ID, &parent_id) < 0) {
+			TH_LOG("Child: NS_GET_ID for parent failed: %s", strerror(errno));
+			close(parent_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(parent_fd);
+
+		TH_LOG("Child: got parent namespace ID %llu", (unsigned long long)parent_id);
+
+		/* Create child user namespace within parent */
+		TH_LOG("Child: creating nested child user namespace");
+		ret = setup_userns();
+		if (ret < 0) {
+			TH_LOG("Child: setup_userns() for child failed: %s", strerror(errno));
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		TH_LOG("Child: nested child user namespace created, uid=%d gid=%d", getuid(), getgid());
+
+		/* Get namespace ID for child user namespace */
+		int child_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (child_fd < 0) {
+			TH_LOG("Child: failed to open child /proc/self/ns/user: %s", strerror(errno));
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		TH_LOG("Child: opened child userns fd %d", child_fd);
+
+		if (ioctl(child_fd, NS_GET_ID, &child_id) < 0) {
+			TH_LOG("Child: NS_GET_ID for child failed: %s", strerror(errno));
+			close(child_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(child_fd);
+
+		TH_LOG("Child: got child namespace ID %llu", (unsigned long long)child_id);
+
+		/* Send both namespace IDs to parent */
+		TH_LOG("Child: sending both namespace IDs to parent");
+		write(pipefd[1], &parent_id, sizeof(parent_id));
+		write(pipefd[1], &child_id, sizeof(child_id));
+		close(pipefd[1]);
+
+		TH_LOG("Child: exiting - parent userns should become inactive");
+		/* Exit - parent user namespace should become inactive */
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	TH_LOG("Parent: reading both namespace IDs from child");
+
+	/* Read both namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &parent_id, sizeof(parent_id));
+	if (ret != sizeof(parent_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read parent namespace ID from child");
+	}
+
+	ret = read(pipefd[0], &child_id, sizeof(child_id));
+	close(pipefd[0]);
+	if (ret != sizeof(child_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read child namespace ID from child");
+	}
+
+	TH_LOG("Parent: received parent_id=%llu, child_id=%llu",
+	       (unsigned long long)parent_id, (unsigned long long)child_id);
+
+	/* Construct file handles from namespace IDs */
+	parent_handle = (struct file_handle *)parent_buf;
+	parent_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	parent_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *parent_fh = (struct nsfs_file_handle *)parent_handle->f_handle;
+	parent_fh->ns_id = parent_id;
+	parent_fh->ns_type = 0;
+	parent_fh->ns_inum = 0;
+
+	child_handle = (struct file_handle *)child_buf;
+	child_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	child_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *child_fh = (struct nsfs_file_handle *)child_handle->f_handle;
+	child_fh->ns_id = child_id;
+	child_fh->ns_type = 0;
+	child_fh->ns_inum = 0;
+
+	TH_LOG("Parent: opening child namespace BEFORE child exits");
+
+	/* Open child namespace while child is still alive to keep it active */
+	child_nsfd = open_by_handle_at(FD_NSFS_ROOT, child_handle, O_RDONLY);
+	if (child_nsfd < 0) {
+		TH_LOG("Failed to open child namespace: %s (errno=%d)", strerror(errno), errno);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open child namespace");
+	}
+
+	TH_LOG("Opened child namespace fd %d", child_nsfd);
+
+	/* Now wait for child to exit */
+	TH_LOG("Parent: waiting for child to exit");
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+
+	if (WEXITSTATUS(status) != 0) {
+		close(child_nsfd);
+		SKIP(return, "Child failed to create namespaces");
+	}
+
+	TH_LOG("Child process exited, parent holds fd to child namespace");
+
+	/*
+	 * With hierarchical active reference propagation:
+	 * Since the child namespace is active (parent process holds fd),
+	 * the parent user namespace should ALSO be active automatically.
+	 * This is because when we took an active reference on the child,
+	 * it propagated up to the owning user namespace.
+	 */
+	TH_LOG("Attempting to reopen parent namespace (should SUCCEED - hierarchical propagation)");
+	int parent_fd = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	if (parent_fd < 0) {
+		close(child_nsfd);
+		TH_LOG("ERROR: Parent namespace inactive despite active child: %s (errno=%d)",
+		       strerror(errno), errno);
+		TH_LOG("This indicates hierarchical active reference propagation is not working!");
+		ASSERT_TRUE(false);
+	}
+
+	TH_LOG("SUCCESS: Parent namespace is active (fd=%d) due to active child", parent_fd);
+
+	/* Verify we can also get parent via NS_GET_USERNS */
+	TH_LOG("Verifying NS_GET_USERNS also works");
+	int parent_fd2 = ioctl(child_nsfd, NS_GET_USERNS);
+	if (parent_fd2 < 0) {
+		close(parent_fd);
+		close(child_nsfd);
+		TH_LOG("NS_GET_USERNS failed: %s (errno=%d)", strerror(errno), errno);
+		SKIP(return, "NS_GET_USERNS not supported or failed");
+	}
+
+	TH_LOG("NS_GET_USERNS succeeded, got parent fd %d", parent_fd2);
+
+	/* Verify both methods give us the same namespace */
+	struct stat st1, st2;
+	ASSERT_EQ(fstat(parent_fd, &st1), 0);
+	ASSERT_EQ(fstat(parent_fd2, &st2), 0);
+	TH_LOG("Parent namespace inodes: parent_fd=%lu, parent_fd2=%lu", st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+
+	/*
+	 * Close child fd - parent should remain active because we still
+	 * hold direct references to it (parent_fd and parent_fd2).
+	 */
+	TH_LOG("Closing child fd - parent should remain active (direct refs held)");
+	close(child_nsfd);
+
+	/* Parent should still be openable */
+	TH_LOG("Verifying parent still active via file handle");
+	int parent_fd3 = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	if (parent_fd3 < 0) {
+		close(parent_fd);
+		close(parent_fd2);
+		TH_LOG("ERROR: Parent became inactive despite holding fds: %s (errno=%d)",
+		       strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+	close(parent_fd3);
+
+	TH_LOG("Closing all fds to parent namespace");
+	close(parent_fd);
+	close(parent_fd2);
+
+	/* Both should now be inactive */
+	TH_LOG("Attempting to reopen parent (should fail - inactive, no refs)");
+	parent_fd = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	if (parent_fd >= 0) {
+		close(parent_fd);
+		TH_LOG("ERROR: Parent namespace still active after closing all fds (fd=%d)", parent_fd);
+	} else {
+		TH_LOG("Parent inactive as expected: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+	}
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3