net/netfilter/xt_limit.c

   1 /* Kernel module to control the rate
   2  *
   3  * 2 September 1999: Changed from the target RATE to the match
   4  *                   `limit', removed logging.  Did I mention that
   5  *                   Alexey is a fucking genius?
   6  *                   Rusty Russell (rusty@rustcorp.com.au).  */
   7
   8 /* (C) 1999 Jérôme de Vivie <devivie@info.enserb.u-bordeaux.fr>
   9  * (C) 1999 Hervé Eychenne <eychenne@info.enserb.u-bordeaux.fr>
  10  *
  11  * This program is free software; you can redistribute it and/or modify
  12  * it under the terms of the GNU General Public License version 2 as
  13  * published by the Free Software Foundation.
  14  */
  15
  16 #include <linux/module.h>
  17 #include <linux/skbuff.h>
  18 #include <linux/spinlock.h>
  19 #include <linux/interrupt.h>
  20
  21 #include <linux/netfilter/x_tables.h>
  22 #include <linux/netfilter/xt_limit.h>
  23
  24 MODULE_LICENSE("GPL");
  25 MODULE_AUTHOR("Herve Eychenne <rv@wallfire.org>");
  26 MODULE_DESCRIPTION("iptables rate limit match");
  27 MODULE_ALIAS("ipt_limit");
  28 MODULE_ALIAS("ip6t_limit");
  29
  30 /* The algorithm used is the Simple Token Bucket Filter (TBF)
  31  * see net/sched/sch_tbf.c in the linux source tree
  32  */
  33
  34 static DEFINE_SPINLOCK(limit_lock);
  35
  36 /* Rusty: This is my (non-mathematically-inclined) understanding of
  37    this algorithm.  The `average rate' in jiffies becomes your initial
  38    amount of credit `credit' and the most credit you can ever have
  39    `credit_cap'.  The `peak rate' becomes the cost of passing the
  40    test, `cost'.
  41
  42    `prev' tracks the last packet hit: you gain one credit per jiffy.
  43    If you get credit balance more than this, the extra credit is
  44    discarded.  Every time the match passes, you lose `cost' credits;
  45    if you don't have that many, the test fails.
  46
  47    See Alexey's formal explanation in net/sched/sch_tbf.c.
  48
  49    To get the maxmum range, we multiply by this factor (ie. you get N
  50    credits per jiffy).  We want to allow a rate as low as 1 per day
  51    (slowest userspace tool allows), which means
  52    CREDITS_PER_JIFFY*HZ*60*60*24 < 2^32. ie. */
  53 #define MAX_CPJ (0xFFFFFFFF / (HZ*60*60*24))
  54
  55 /* Repeated shift and or gives us all 1s, final shift and add 1 gives
  56  * us the power of 2 below the theoretical max, so GCC simply does a
  57  * shift. */
  58 #define _POW2_BELOW2(x) ((x)|((x)>>1))
  59 #define _POW2_BELOW4(x) (_POW2_BELOW2(x)|_POW2_BELOW2((x)>>2))
  60 #define _POW2_BELOW8(x) (_POW2_BELOW4(x)|_POW2_BELOW4((x)>>4))
  61 #define _POW2_BELOW16(x) (_POW2_BELOW8(x)|_POW2_BELOW8((x)>>8))
  62 #define _POW2_BELOW32(x) (_POW2_BELOW16(x)|_POW2_BELOW16((x)>>16))
  63 #define POW2_BELOW32(x) ((_POW2_BELOW32(x)>>1) + 1)
  64
  65 #define CREDITS_PER_JIFFY POW2_BELOW32(MAX_CPJ)
  66
  67 static int
  68 ipt_limit_match(const struct sk_buff *skb,
  69                 const struct net_device *in,
  70                 const struct net_device *out,
  71                 const void *matchinfo,
  72                 int offset,
  73                 unsigned int protoff,
  74                 int *hotdrop)
  75 {
  76         struct xt_rateinfo *r = ((struct xt_rateinfo *)matchinfo)->master;
  77         unsigned long now = jiffies;
  78
  79         spin_lock_bh(&limit_lock);
  80         r->credit += (now - xchg(&r->prev, now)) * CREDITS_PER_JIFFY;
  81         if (r->credit > r->credit_cap)
  82                 r->credit = r->credit_cap;
  83
  84         if (r->credit >= r->cost) {
  85                 /* We're not limited. */
  86                 r->credit -= r->cost;
  87                 spin_unlock_bh(&limit_lock);
  88                 return 1;
  89         }
  90
  91         spin_unlock_bh(&limit_lock);
  92         return 0;
  93 }
  94
  95 /* Precision saver. */
  96 static u_int32_t
  97 user2credits(u_int32_t user)
  98 {
  99         /* If multiplying would overflow... */
 100         if (user > 0xFFFFFFFF / (HZ*CREDITS_PER_JIFFY))
 101                 /* Divide first. */
 102                 return (user / XT_LIMIT_SCALE) * HZ * CREDITS_PER_JIFFY;
 103
 104         return (user * HZ * CREDITS_PER_JIFFY) / XT_LIMIT_SCALE;
 105 }
 106
 107 static int
 108 ipt_limit_checkentry(const char *tablename,
 109                      const void *inf,
 110                      void *matchinfo,
 111                      unsigned int matchsize,
 112                      unsigned int hook_mask)
 113 {
 114         struct xt_rateinfo *r = matchinfo;
 115
 116         if (matchsize != XT_ALIGN(sizeof(struct xt_rateinfo)))
 117                 return 0;
 118
 119         /* Check for overflow. */
 120         if (r->burst == 0
 121             || user2credits(r->avg * r->burst) < user2credits(r->avg)) {
 122                 printk("Overflow in xt_limit, try lower: %u/%u\n",
 123                        r->avg, r->burst);
 124                 return 0;
 125         }
 126
 127         /* User avg in seconds * XT_LIMIT_SCALE: convert to jiffies *
 128            128. */
 129         r->prev = jiffies;
 130         r->credit = user2credits(r->avg * r->burst);     /* Credits full. */
 131         r->credit_cap = user2credits(r->avg * r->burst); /* Credits full. */
 132         r->cost = user2credits(r->avg);
 133
 134         /* For SMP, we only want to use one set of counters. */
 135         r->master = r;
 136
 137         return 1;
 138 }
 139
 140 static struct xt_match ipt_limit_reg = {
 141         .name           = "limit",
 142         .match          = ipt_limit_match,
 143         .checkentry     = ipt_limit_checkentry,
 144         .me             = THIS_MODULE,
 145 };
 146 static struct xt_match limit6_reg = {
 147         .name           = "limit",
 148         .match          = ipt_limit_match,
 149         .checkentry     = ipt_limit_checkentry,
 150         .me             = THIS_MODULE,
 151 };
 152
 153 static int __init init(void)
 154 {
 155         int ret;
 156
 157         ret = xt_register_match(AF_INET, &ipt_limit_reg);
 158         if (ret)
 159                 return ret;
 160
 161         ret = xt_register_match(AF_INET6, &limit6_reg);
 162         if (ret)
 163                 xt_unregister_match(AF_INET, &ipt_limit_reg);
 164
 165         return ret;
 166 }
 167
 168 static void __exit fini(void)
 169 {
 170         xt_unregister_match(AF_INET, &ipt_limit_reg);
 171         xt_unregister_match(AF_INET6, &limit6_reg);
 172 }
 173
 174 module_init(init);
 175 module_exit(fini);