xt_limit.c | Explore in Territory

// SPDX-License-Identifier: GPL-2.0-only
/* (C) 1999 Jérôme de Vivie <[email protected]>
 * (C) 1999 Hervé Eychenne <[email protected]>
 * (C) 2006-2012 Patrick McHardy <[email protected]>
 */
#define pr_fmt(fmt) …

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

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

struct xt_limit_priv { … };

MODULE_LICENSE(…) …;
MODULE_AUTHOR(…) …;
MODULE_DESCRIPTION(…) …;
MODULE_ALIAS(…) …;
MODULE_ALIAS(…) …;

/* The algorithm used is the Simple Token Bucket Filter (TBF)
 * see net/sched/sch_tbf.c in the linux source tree
 */

/* 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 maximum 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 …

/* 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) …
#define _POW2_BELOW4(x) …
#define _POW2_BELOW8(x) …
#define _POW2_BELOW16(x) …
#define _POW2_BELOW32(x) …
#define POW2_BELOW32(x) …

#define CREDITS_PER_JIFFY …

static bool
limit_mt(const struct sk_buff *skb, struct xt_action_param *par)
{ … }

/* Precision saver. */
static u32 user2credits(u32 user)
{ … }

static int limit_mt_check(const struct xt_mtchk_param *par)
{ … }

static void limit_mt_destroy(const struct xt_mtdtor_param *par)
{ … }

#ifdef CONFIG_NETFILTER_XTABLES_COMPAT
struct compat_xt_rateinfo { … };

/* 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)
{ … }

static int limit_mt_compat_to_user(void __user *dst, const void *src)
{ … }
#endif /* CONFIG_NETFILTER_XTABLES_COMPAT */

static struct xt_match limit_mt_reg __read_mostly = …;

static int __init limit_mt_init(void)
{ … }

static void __exit limit_mt_exit(void)
{ … }

module_init(…) …;
module_exit(limit_mt_exit);
linux/net/netfilter/xt_limit.c
