pidfs: handle kernels without namespaces cleanly

The nsproxy structure contains nearly all of the namespaces associated
with a task. When a given namespace type is not supported by this kernel
the rules whether the corresponding pointer in struct nsproxy is NULL or
always init_<ns_type>_ns differ per namespace. Ideally, that wouldn't be
the case and for all namespace types we'd always set it to
init_<ns_type>_ns when the corresponding namespace type isn't supported.

Make sure we handle all namespaces where the pointer in struct nsproxy
can be NULL when the namespace type isn't supported.

Link: https://lore.kernel.org/r/20240722-work-pidfs-e6a83030f63e@brauner
Fixes: 5b08bd40 ("pidfs: allow retrieval of namespace file descriptors") # mainline only
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/pidfs.c

@@ -119,7 +119,7 @@ static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	struct task_struct *task __free(put_task) = NULL;
 	struct nsproxy *nsp __free(put_nsproxy) = NULL;
 	struct pid *pid = pidfd_pid(file);
-	struct ns_common *ns_common;
+	struct ns_common *ns_common = NULL;
 
 	if (arg)
 		return -EINVAL;
@@ -146,54 +146,73 @@ static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	switch (cmd) {
 	/* Namespaces that hang of nsproxy. */
 	case PIDFD_GET_CGROUP_NAMESPACE:
+		if (IS_ENABLED(CONFIG_CGROUPS)) {
 			get_cgroup_ns(nsp->cgroup_ns);
 			ns_common = to_ns_common(nsp->cgroup_ns);
+		}
 		break;
 	case PIDFD_GET_IPC_NAMESPACE:
+		if (IS_ENABLED(CONFIG_IPC_NS)) {
 			get_ipc_ns(nsp->ipc_ns);
 			ns_common = to_ns_common(nsp->ipc_ns);
+		}
 		break;
 	case PIDFD_GET_MNT_NAMESPACE:
 		get_mnt_ns(nsp->mnt_ns);
 		ns_common = to_ns_common(nsp->mnt_ns);
 		break;
 	case PIDFD_GET_NET_NAMESPACE:
+		if (IS_ENABLED(CONFIG_NET_NS)) {
 			ns_common = to_ns_common(nsp->net_ns);
 			get_net_ns(ns_common);
+		}
 		break;
 	case PIDFD_GET_PID_FOR_CHILDREN_NAMESPACE:
+		if (IS_ENABLED(CONFIG_PID_NS)) {
 			get_pid_ns(nsp->pid_ns_for_children);
 			ns_common = to_ns_common(nsp->pid_ns_for_children);
+		}
 		break;
 	case PIDFD_GET_TIME_NAMESPACE:
+		if (IS_ENABLED(CONFIG_TIME_NS)) {
 			get_time_ns(nsp->time_ns);
 			ns_common = to_ns_common(nsp->time_ns);
-		if (!nsp->time_ns)
-			return -EINVAL;
+		}
 		break;
 	case PIDFD_GET_TIME_FOR_CHILDREN_NAMESPACE:
+		if (IS_ENABLED(CONFIG_TIME_NS)) {
 			get_time_ns(nsp->time_ns_for_children);
 			ns_common = to_ns_common(nsp->time_ns_for_children);
+		}
 		break;
 	case PIDFD_GET_UTS_NAMESPACE:
+		if (IS_ENABLED(CONFIG_UTS_NS)) {
 			get_uts_ns(nsp->uts_ns);
 			ns_common = to_ns_common(nsp->uts_ns);
+		}
 		break;
 	/* Namespaces that don't hang of nsproxy. */
 	case PIDFD_GET_USER_NAMESPACE:
+		if (IS_ENABLED(CONFIG_USER_NS)) {
 			rcu_read_lock();
 			ns_common = to_ns_common(get_user_ns(task_cred_xxx(task, user_ns)));
 			rcu_read_unlock();
+		}
 		break;
 	case PIDFD_GET_PID_NAMESPACE:
+		if (IS_ENABLED(CONFIG_PID_NS)) {
 			rcu_read_lock();
-		ns_common = to_ns_common(get_pid_ns(task_active_pid_ns(task)));
+			ns_common = to_ns_common( get_pid_ns(task_active_pid_ns(task)));
 			rcu_read_unlock();
+		}
 		break;
 	default:
 		return -ENOIOCTLCMD;
 	}
 
+	if (!ns_common)
+		return -EOPNOTSUPP;
+
 	/* open_namespace() unconditionally consumes the reference */
 	return open_namespace(ns_common);
 }