ebt_limit.c

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/*
 *  ebt_limit
 *
 *  Authors:
 *  Tom Marshall <[email protected]>
 *
 *  Mostly copied from netfilter's ipt_limit.c, see that file for
 *  more explanation
 *
 *  September, 2003
 *
 */
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#include <linux/module.h>
#include <linux/netdevice.h>
#include <linux/spinlock.h>
#include <linux/netfilter/x_tables.h>
#include <linux/netfilter_bridge/ebtables.h>
#include <linux/netfilter_bridge/ebt_limit.h>

static DEFINE_SPINLOCK(limit_lock);

#define MAX_CPJ (0xFFFFFFFF / (HZ*60*60*24))

#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
ebt_limit_mt(const struct sk_buff *skb, struct xt_action_param *par)
{
    struct ebt_limit_info *info = (void *)par->matchinfo;
    unsigned long now = jiffies;

    spin_lock_bh(&limit_lock);
    info->credit += (now - xchg(&info->prev, now)) * CREDITS_PER_JIFFY;
    if (info->credit > info->credit_cap)
        info->credit = info->credit_cap;

    if (info->credit >= info->cost) {
        /* We're not limited. */
        info->credit -= info->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 / EBT_LIMIT_SCALE) * HZ * CREDITS_PER_JIFFY;

    return (user * HZ * CREDITS_PER_JIFFY) / EBT_LIMIT_SCALE;
}

static int ebt_limit_mt_check(const struct xt_mtchk_param *par)
{
    struct ebt_limit_info *info = par->matchinfo;

    /* Check for overflow. */
    if (info->burst == 0 ||
        user2credits(info->avg * info->burst) < user2credits(info->avg)) {
        pr_info("overflow, try lower: %u/%u\n",
            info->avg, info->burst);
        return -EINVAL;
    }

    /* User avg in seconds * EBT_LIMIT_SCALE: convert to jiffies * 128. */
    info->prev = jiffies;
    info->credit = user2credits(info->avg * info->burst);
    info->credit_cap = user2credits(info->avg * info->burst);
    info->cost = user2credits(info->avg);
    return 0;
}


#ifdef CONFIG_COMPAT
/*
 * no conversion function needed --
 * only avg/burst have meaningful values in userspace.
 */
struct ebt_compat_limit_info {
    compat_uint_t avg, burst;
    compat_ulong_t prev;
    compat_uint_t credit, credit_cap, cost;
};
#endif

static struct xt_match ebt_limit_mt_reg __read_mostly = {
    .name       = "limit",
    .revision   = 0,
    .family     = NFPROTO_BRIDGE,
    .match      = ebt_limit_mt,
    .checkentry = ebt_limit_mt_check,
    .matchsize  = sizeof(struct ebt_limit_info),
#ifdef CONFIG_COMPAT
    .compatsize = sizeof(struct ebt_compat_limit_info),
#endif
    .me     = THIS_MODULE,
};

static int __init ebt_limit_init(void)
{
    return xt_register_match(&ebt_limit_mt_reg);
}

static void __exit ebt_limit_fini(void)
{
    xt_unregister_match(&ebt_limit_mt_reg);
}

module_init(ebt_limit_init);
module_exit(ebt_limit_fini);
MODULE_DESCRIPTION("Ebtables: Rate-limit match");
MODULE_LICENSE("GPL");