net: Don't register pernet_operations if only one of id or size is specified.

We can allocate per-netns memory for struct pernet_operations by specifying
id and size.

register_pernet_operations() assigns an id to pernet_operations and later
ops_init() allocates the specified size of memory as net->gen->ptr[id].

If id is missing, no memory is allocated.  If size is not specified,
pernet_operations just wastes an entry of net->gen->ptr[] for every netns.

net_generic is available only when both id and size are specified, so let's
ensure that.

While we are at it, we add const to both fields.
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -451,8 +451,8 @@ struct pernet_operations {
 	/* Following method is called with RTNL held. */
 	void (*exit_batch_rtnl)(struct list_head *net_exit_list,
 				struct list_head *dev_kill_list);
-	unsigned int *id;
-	size_t size;
+	unsigned int * const id;
+	const size_t size;
 };
 
 /*

net/core/net_namespace.c

@@ -125,7 +125,7 @@ static int ops_init(const struct pernet_operations *ops, struct net *net)
 	int err = -ENOMEM;
 	void *data = NULL;
 
-	if (ops->id && ops->size) {
+	if (ops->id) {
 		data = kzalloc(ops->size, GFP_KERNEL);
 		if (!data)
 			goto out;
@@ -140,7 +140,7 @@ static int ops_init(const struct pernet_operations *ops, struct net *net)
 	if (!err)
 		return 0;
 
-	if (ops->id && ops->size) {
+	if (ops->id) {
 		ng = rcu_dereference_protected(net->gen,
 					       lockdep_is_held(&pernet_ops_rwsem));
 		ng->ptr[*ops->id] = NULL;
@@ -182,7 +182,8 @@ static void ops_free_list(const struct pernet_operations *ops,
 			  struct list_head *net_exit_list)
 {
 	struct net *net;
-	if (ops->size && ops->id) {
+
+	if (ops->id) {
 		list_for_each_entry(net, net_exit_list, exit_list)
 			kfree(net_generic(net, *ops->id));
 	}
@@ -1244,7 +1245,7 @@ static int __register_pernet_operations(struct list_head *list,
 	LIST_HEAD(net_exit_list);
 
 	list_add_tail(&ops->list, list);
-	if (ops->init || (ops->id && ops->size)) {
+	if (ops->init || ops->id) {
 		/* We held write locked pernet_ops_rwsem, and parallel
 		 * setup_net() and cleanup_net() are not possible.
 		 */
@@ -1310,6 +1311,9 @@ static int register_pernet_operations(struct list_head *list,
 {
 	int error;
 
+	if (WARN_ON(!!ops->id ^ !!ops->size))
+		return -EINVAL;
+
 	if (ops->id) {
 		error = ida_alloc_min(&net_generic_ids, MIN_PERNET_OPS_ID,
 				GFP_KERNEL);