static ssize_t o2nm_node_ipv4_address_store(struct config_item *item,
					    const char *page,
					    size_t count)
{
	struct o2nm_node *node = to_o2nm_node(item);
	struct o2nm_cluster *cluster;
	int ret, i;
	struct rb_node **p, *parent;
	unsigned int octets[4];
	__be32 ipv4_addr = 0;

	ret = sscanf(page, "%3u.%3u.%3u.%3u", &octets[3], &octets[2],
		     &octets[1], &octets[0]);
	if (ret != 4)
		return -EINVAL;

	for (i = 0; i < ARRAY_SIZE(octets); i++) {
		if (octets[i] > 255)
			return -ERANGE;
		be32_add_cpu(&ipv4_addr, octets[i] << (i * 8));
	}

	o2nm_lock_subsystem();
	cluster = to_o2nm_cluster_from_node(node);
	if (!cluster) {
		o2nm_unlock_subsystem();
		return -EINVAL;
	}

	ret = 0;
	write_lock(&cluster->cl_nodes_lock);
	if (o2nm_node_ip_tree_lookup(cluster, ipv4_addr, &p, &parent))
		ret = -EEXIST;
	else if (test_and_set_bit(O2NM_NODE_ATTR_ADDRESS,
			&node->nd_set_attributes))
		ret = -EBUSY;
	else {
		rb_link_node(&node->nd_ip_node, parent, p);
		rb_insert_color(&node->nd_ip_node, &cluster->cl_node_ip_tree);
	}
	write_unlock(&cluster->cl_nodes_lock);
	o2nm_unlock_subsystem();

	if (ret)
		return ret;

	memcpy(&node->nd_ipv4_address, &ipv4_addr, sizeof(ipv4_addr));

	return count;
}
A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.