/* (C) 1999 Jérôme de Vivie <devivie@info.enserb.u-bordeaux.fr>
   * (C) 1999 Hervé Eychenne <eychenne@info.enserb.u-bordeaux.fr>

   *
   * This program is free software; you can redistribute it and/or modify
   * it under the terms of the GNU General Public License version 2 as
   * published by the Free Software Foundation.
   */

  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

  #include <linux/slab.h>

  #include <linux/module.h>
  #include <linux/skbuff.h>
  #include <linux/spinlock.h>
  #include <linux/interrupt.h>

  #include <linux/netfilter/x_tables.h>
  #include <linux/netfilter/xt_limit.h>

  struct xt_limit_priv {
  	unsigned long prev;
  	uint32_t credit;
  };

  MODULE_LICENSE("GPL");
  MODULE_AUTHOR("Herve Eychenne <rv@wallfire.org>");

  MODULE_DESCRIPTION("Xtables: rate-limit match");

  MODULE_ALIAS("ipt_limit");
  MODULE_ALIAS("ip6t_limit");

  
  /* The algorithm used is the Simple Token Bucket Filter (TBF)
   * see net/sched/sch_tbf.c in the linux source tree
   */
  
  static DEFINE_SPINLOCK(limit_lock);
  
  /* Rusty: This is my (non-mathematically-inclined) understanding of
     this algorithm.  The `average rate' in jiffies becomes your initial
     amount of credit `credit' and the most credit you can ever have
     `credit_cap'.  The `peak rate' becomes the cost of passing the
     test, `cost'.
  
     `prev' tracks the last packet hit: you gain one credit per jiffy.
     If you get credit balance more than this, the extra credit is
     discarded.  Every time the match passes, you lose `cost' credits;
     if you don't have that many, the test fails.
  
     See Alexey's formal explanation in net/sched/sch_tbf.c.
  
     To get the maxmum range, we multiply by this factor (ie. you get N
     credits per jiffy).  We want to allow a rate as low as 1 per day
     (slowest userspace tool allows), which means
     CREDITS_PER_JIFFY*HZ*60*60*24 < 2^32. ie. */
  #define MAX_CPJ (0xFFFFFFFF / (HZ*60*60*24))
  
  /* Repeated shift and or gives us all 1s, final shift and add 1 gives
   * us the power of 2 below the theoretical max, so GCC simply does a
   * shift. */
  #define _POW2_BELOW2(x) ((x)|((x)>>1))
  #define _POW2_BELOW4(x) (_POW2_BELOW2(x)|_POW2_BELOW2((x)>>2))
  #define _POW2_BELOW8(x) (_POW2_BELOW4(x)|_POW2_BELOW4((x)>>4))
  #define _POW2_BELOW16(x) (_POW2_BELOW8(x)|_POW2_BELOW8((x)>>8))
  #define _POW2_BELOW32(x) (_POW2_BELOW16(x)|_POW2_BELOW16((x)>>16))
  #define POW2_BELOW32(x) ((_POW2_BELOW32(x)>>1) + 1)
  
  #define CREDITS_PER_JIFFY POW2_BELOW32(MAX_CPJ)

  static bool

  limit_mt(const struct sk_buff *skb, struct xt_action_param *par)

  {

  	const struct xt_rateinfo *r = par->matchinfo;
  	struct xt_limit_priv *priv = r->master;

  	unsigned long now = jiffies;
  
  	spin_lock_bh(&limit_lock);

  	priv->credit += (now - xchg(&priv->prev, now)) * CREDITS_PER_JIFFY;
  	if (priv->credit > r->credit_cap)
  		priv->credit = r->credit_cap;

  	if (priv->credit >= r->cost) {

  		/* We're not limited. */

  		priv->credit -= r->cost;

  		spin_unlock_bh(&limit_lock);

  		return true;

  	}

  	spin_unlock_bh(&limit_lock);

  	return false;

  }
  
  /* Precision saver. */
  static u_int32_t
  user2credits(u_int32_t user)
  {
  	/* If multiplying would overflow... */
  	if (user > 0xFFFFFFFF / (HZ*CREDITS_PER_JIFFY))
  		/* Divide first. */

  		return (user / XT_LIMIT_SCALE) * HZ * CREDITS_PER_JIFFY;

  	return (user * HZ * CREDITS_PER_JIFFY) / XT_LIMIT_SCALE;

  }

  static int limit_mt_check(const struct xt_mtchk_param *par)

  {

  	struct xt_rateinfo *r = par->matchinfo;

  	struct xt_limit_priv *priv;

  	/* Check for overflow. */
  	if (r->burst == 0
  	    || user2credits(r->avg * r->burst) < user2credits(r->avg)) {

  		pr_info("Overflow, try lower: %u/%u
  ",
  			r->avg, r->burst);

  		return -ERANGE;

  	}

  	priv = kmalloc(sizeof(*priv), GFP_KERNEL);
  	if (priv == NULL)

  		return -ENOMEM;

  
  	/* For SMP, we only want to use one set of state. */
  	r->master = priv;

  	if (r->cost == 0) {
  		/* User avg in seconds * XT_LIMIT_SCALE: convert to jiffies *
  		   128. */

  		priv->prev = jiffies;
  		priv->credit = user2credits(r->avg * r->burst); /* Credits full. */

  		r->credit_cap = user2credits(r->avg * r->burst); /* Credits full. */
  		r->cost = user2credits(r->avg);
  	}

  	return 0;

  }

  static void limit_mt_destroy(const struct xt_mtdtor_param *par)
  {
  	const struct xt_rateinfo *info = par->matchinfo;
  
  	kfree(info->master);
  }

  #ifdef CONFIG_COMPAT
  struct compat_xt_rateinfo {
  	u_int32_t avg;
  	u_int32_t burst;
  
  	compat_ulong_t prev;
  	u_int32_t credit;
  	u_int32_t credit_cap, cost;
  
  	u_int32_t master;
  };
  
  /* To keep the full "prev" timestamp, the upper 32 bits are stored in the
   * master pointer, which does not need to be preserved. */

  static void limit_mt_compat_from_user(void *dst, const void *src)

  {

  	const struct compat_xt_rateinfo *cm = src;

  	struct xt_rateinfo m = {
  		.avg		= cm->avg,
  		.burst		= cm->burst,
  		.prev		= cm->prev | (unsigned long)cm->master << 32,
  		.credit		= cm->credit,
  		.credit_cap	= cm->credit_cap,
  		.cost		= cm->cost,
  	};
  	memcpy(dst, &m, sizeof(m));
  }

  static int limit_mt_compat_to_user(void __user *dst, const void *src)

  {

  	const struct xt_rateinfo *m = src;

  	struct compat_xt_rateinfo cm = {
  		.avg		= m->avg,
  		.burst		= m->burst,
  		.prev		= m->prev,
  		.credit		= m->credit,
  		.credit_cap	= m->credit_cap,
  		.cost		= m->cost,
  		.master		= m->prev >> 32,
  	};
  	return copy_to_user(dst, &cm, sizeof(cm)) ? -EFAULT : 0;
  }
  #endif /* CONFIG_COMPAT */

  static struct xt_match limit_mt_reg __read_mostly = {
  	.name             = "limit",
  	.revision         = 0,
  	.family           = NFPROTO_UNSPEC,
  	.match            = limit_mt,
  	.checkentry       = limit_mt_check,

  	.destroy          = limit_mt_destroy,

  	.matchsize        = sizeof(struct xt_rateinfo),

  #ifdef CONFIG_COMPAT

  	.compatsize       = sizeof(struct compat_xt_rateinfo),
  	.compat_from_user = limit_mt_compat_from_user,
  	.compat_to_user   = limit_mt_compat_to_user,

  #endif

  	.me               = THIS_MODULE,

  };

  static int __init limit_mt_init(void)

  {

  	return xt_register_match(&limit_mt_reg);

  }

  static void __exit limit_mt_exit(void)

  {

  	xt_unregister_match(&limit_mt_reg);

  }

  module_init(limit_mt_init);
  module_exit(limit_mt_exit);