net/ipv4/netfilter/ipt_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_ipv4/ip_tables.h>
  22 #include <linux/netfilter_ipv4/ipt_limit.h>
  23
  24 MODULE_LICENSE("GPL");
  25 MODULE_AUTHOR("Herve Eychenne <rv@wallfire.org>");
  26 MODULE_DESCRIPTION("iptables rate limit match");
  27
  28 /* The algorithm used is the Simple Token Bucket Filter (TBF)
  29  * see net/sched/sch_tbf.c in the linux source tree
  30  */
  31
  32 static DEFINE_SPINLOCK(limit_lock);
  33
  34 /* Rusty: This is my (non-mathematically-inclined) understanding of
  35    this algorithm.  The `average rate' in jiffies becomes your initial
  36    amount of credit `credit' and the most credit you can ever have
  37    `credit_cap'.  The `peak rate' becomes the cost of passing the
  38    test, `cost'.
  39
  40    `prev' tracks the last packet hit: you gain one credit per jiffy.
  41    If you get credit balance more than this, the extra credit is
  42    discarded.  Every time the match passes, you lose `cost' credits;
  43    if you don't have that many, the test fails.
  44
  45    See Alexey's formal explanation in net/sched/sch_tbf.c.
  46
  47    To get the maxmum range, we multiply by this factor (ie. you get N
  48    credits per jiffy).  We want to allow a rate as low as 1 per day
  49    (slowest userspace tool allows), which means
  50    CREDITS_PER_JIFFY*HZ*60*60*24 < 2^32. ie. */
  51 #define MAX_CPJ (0xFFFFFFFF / (HZ*60*60*24))
  52
  53 /* Repeated shift and or gives us all 1s, final shift and add 1 gives
  54  * us the power of 2 below the theoretical max, so GCC simply does a
  55  * shift. */
  56 #define _POW2_BELOW2(x) ((x)|((x)>>1))
  57 #define _POW2_BELOW4(x) (_POW2_BELOW2(x)|_POW2_BELOW2((x)>>2))
  58 #define _POW2_BELOW8(x) (_POW2_BELOW4(x)|_POW2_BELOW4((x)>>4))
  59 #define _POW2_BELOW16(x) (_POW2_BELOW8(x)|_POW2_BELOW8((x)>>8))
  60 #define _POW2_BELOW32(x) (_POW2_BELOW16(x)|_POW2_BELOW16((x)>>16))
  61 #define POW2_BELOW32(x) ((_POW2_BELOW32(x)>>1) + 1)
  62
  63 #define CREDITS_PER_JIFFY POW2_BELOW32(MAX_CPJ)
  64
  65 static int
  66 ipt_limit_match(const struct sk_buff *skb,
  67                 const struct net_device *in,
  68                 const struct net_device *out,
  69                 const void *matchinfo,
  70                 int offset,
  71                 int *hotdrop)
  72 {
  73         struct ipt_rateinfo *r = ((struct ipt_rateinfo *)matchinfo)->master;
  74         unsigned long now = jiffies;
  75
  76         spin_lock_bh(&limit_lock);
  77         r->credit += (now - xchg(&r->prev, now)) * CREDITS_PER_JIFFY;
  78         if (r->credit > r->credit_cap)
  79                 r->credit = r->credit_cap;
  80
  81         if (r->credit >= r->cost) {
  82                 /* We're not limited. */
  83                 r->credit -= r->cost;
  84                 spin_unlock_bh(&limit_lock);
  85                 return 1;
  86         }
  87
  88         spin_unlock_bh(&limit_lock);
  89         return 0;
  90 }
  91
  92 /* Precision saver. */
  93 static u_int32_t
  94 user2credits(u_int32_t user)
  95 {
  96         /* If multiplying would overflow... */
  97         if (user > 0xFFFFFFFF / (HZ*CREDITS_PER_JIFFY))
  98                 /* Divide first. */
  99                 return (user / IPT_LIMIT_SCALE) * HZ * CREDITS_PER_JIFFY;
 100
 101         return (user * HZ * CREDITS_PER_JIFFY) / IPT_LIMIT_SCALE;
 102 }
 103
 104 static int
 105 ipt_limit_checkentry(const char *tablename,
 106                      const struct ipt_ip *ip,
 107                      void *matchinfo,
 108                      unsigned int matchsize,
 109                      unsigned int hook_mask)
 110 {
 111         struct ipt_rateinfo *r = matchinfo;
 112
 113         if (matchsize != IPT_ALIGN(sizeof(struct ipt_rateinfo)))
 114                 return 0;
 115
 116         /* Check for overflow. */
 117         if (r->burst == 0
 118             || user2credits(r->avg * r->burst) < user2credits(r->avg)) {
 119                 printk("Overflow in ipt_limit, try lower: %u/%u\n",
 120                        r->avg, r->burst);
 121                 return 0;
 122         }
 123
 124         /* User avg in seconds * IPT_LIMIT_SCALE: convert to jiffies *
 125            128. */
 126         r->prev = jiffies;
 127         r->credit = user2credits(r->avg * r->burst);     /* Credits full. */
 128         r->credit_cap = user2credits(r->avg * r->burst); /* Credits full. */
 129         r->cost = user2credits(r->avg);
 130
 131         /* For SMP, we only want to use one set of counters. */
 132         r->master = r;
 133
 134         return 1;
 135 }
 136
 137 static struct ipt_match ipt_limit_reg = {
 138         .name           = "limit",
 139         .match          = ipt_limit_match,
 140         .checkentry     = ipt_limit_checkentry,
 141         .me             = THIS_MODULE,
 142 };
 143
 144 static int __init init(void)
 145 {
 146         if (ipt_register_match(&ipt_limit_reg))
 147                 return -EINVAL;
 148         return 0;
 149 }
 150
 151 static void __exit fini(void)
 152 {
 153         ipt_unregister_match(&ipt_limit_reg);
 154 }
 155
 156 module_init(init);
 157 module_exit(fini);