text: /*
   * netfilter module to enforce network quotas
   *
   * Sam Johnston <samj@samj.net>
   */
  #include <linux/skbuff.h>
  #include <linux/slab.h>
  #include <linux/spinlock.h>
  
  #include <linux/netfilter/x_tables.h>
  #include <linux/netfilter/xt_quota.h>
  #include <linux/module.h>
  
  struct xt_quota_priv {
  	spinlock_t	lock;
  	uint64_t	quota;
  };
  
  MODULE_LICENSE("GPL");
  MODULE_AUTHOR("Sam Johnston <samj@samj.net>");
  MODULE_DESCRIPTION("Xtables: countdown quota match");
  MODULE_ALIAS("ipt_quota");
  MODULE_ALIAS("ip6t_quota");
  
  static bool
  quota_mt(const struct sk_buff *skb, struct xt_action_param *par)
  {
  	struct xt_quota_info *q = (void *)par->matchinfo;
  	struct xt_quota_priv *priv = q->master;
  	bool ret = q->flags & XT_QUOTA_INVERT;
  
  	spin_lock_bh(&priv->lock);
  	if (priv->quota >= skb->len) {
  		priv->quota -= skb->len;
  		ret = !ret;
  	} else {
  		/* we do not allow even small packets from now on */
  		priv->quota = 0;
  	}
  	spin_unlock_bh(&priv->lock);
  
  	return ret;
  }
  
  static int quota_mt_check(const struct xt_mtchk_param *par)
  {
  	struct xt_quota_info *q = par->matchinfo;
  
  	if (q->flags & ~XT_QUOTA_MASK)
  		return -EINVAL;
  
  	q->master = kmalloc(sizeof(*q->master), GFP_KERNEL);
  	if (q->master == NULL)
  		return -ENOMEM;
  
  	spin_lock_init(&q->master->lock);
  	q->master->quota = q->quota;
  	return 0;
  }
  
  static void quota_mt_destroy(const struct xt_mtdtor_param *par)
  {
  	const struct xt_quota_info *q = par->matchinfo;
  
  	kfree(q->master);
  }
  
  static struct xt_match quota_mt_reg __read_mostly = {
  	.name       = "quota",
  	.revision   = 0,
  	.family     = NFPROTO_UNSPEC,
  	.match      = quota_mt,
  	.checkentry = quota_mt_check,
  	.destroy    = quota_mt_destroy,
  	.matchsize  = sizeof(struct xt_quota_info),
  	.me         = THIS_MODULE,
  };
  
  static int __init quota_mt_init(void)
  {
  	return xt_register_match(&quota_mt_reg);
  }
  
  static void __exit quota_mt_exit(void)
  {
  	xt_unregister_match(&quota_mt_reg);
  }
  
  module_init(quota_mt_init);
  module_exit(quota_mt_exit);
  