/*
 * Tunnel interfaces can nest, also they may cause infinite recursion
 * calls when misconfigured. We'll prevent this by detecting loops.
 * High nesting level may cause stack exhaustion. We'll prevent this
 * by introducing upper limit.
 *
 * Return 0, if tunnel nesting count is equal or less than limit.
 */
int
if_tunnel_check_nesting(struct ifnet *ifp, struct mbuf *m, uint32_t cookie,
    int limit)
{
	struct m_tag *mtag;
	int count;

	count = 1;
	mtag = NULL;
	while ((mtag = m_tag_locate(m, cookie, 0, mtag)) != NULL) {
		if (*(struct ifnet **)(mtag + 1) == ifp) {
			log(LOG_NOTICE, "%s: loop detected\n", if_name(ifp));
			return (EIO);
		}
		count++;
	}
	if (count > limit) {
		log(LOG_NOTICE,
		    "%s: if_output recursively called too many times(%d)\n",
		    if_name(ifp), count);
		return (EIO);
	}
	mtag = m_tag_alloc(cookie, 0, sizeof(struct ifnet *), M_NOWAIT);
	if (mtag == NULL)
		return (ENOMEM);
	*(struct ifnet **)(mtag + 1) = ifp;
	m_tag_prepend(m, mtag);
	return (0);
}