bird/filter/trie.c
2009-08-27 19:01:04 +02:00

320 lines
8.8 KiB
C

/*
* Filters: Trie for prefix sets
*
* Copyright 2009 Ondrej Zajicek <santiago@crfreenet.org>
*
* Can be freely distributed and used under the terms of the GNU GPL.
*/
/**
* DOC: Trie for prefix sets
*
* We use a (compressed) trie to represent prefix sets. Every node
* in the trie represents one prefix (&addr/&plen) and &plen also
* indicates the index of the bit in the address that is used to
* branch at the node. If we need to represent just a set of
* prefixes, it would be simple, but we have to represent a
* set of prefix pattern. Each prefix pattern consists of
* &ppaddr/&pplen and two integers: &low and &high, and a prefix
* &paddr/&plen matches that pattern if the first MIN(&plen, &pplen)
* bits of &paddr and &ppaddr are the same and &low <= &plen <= &high.
*
* We use a bitmask (&accept) to represent accepted prefix lengths
* at a node. As there are 33 prefix lengths (0..32 for IPv4), but
* there is just one prefix of zero length in the whole trie so we
* have &zero flag in &f_trie (indicating whether the trie accepts
* prefix 0.0.0.0/0) as a special case, and &accept bitmask
* represents accepted prefix lengths from 1 to 32.
*
* There are two cases in prefix matching - a match when the length
* of the prefix is smaller that the length of the prefix pattern,
* (&plen < &pplen) and otherwise. The second case is simple - we
* just walk through the trie and look at every visited node
* whether that prefix accepts our prefix length (&plen). The
* first case is tricky - we don't want to examine every descendant
* of a final node, so (when we create the trie) we have to propagate
* that information from nodes to their ascendants.
*
* Suppose that we have two masks (M1 and M2) for a node. Mask M1
* represents accepted prefix lengths by just the node and mask M2
* represents accepted prefix lengths by the node or any of its
* descendants. Therefore M2 is a bitwise or of M1 and children's
* M2 and this is a maintained invariant during trie building.
* Basically, when we want to match a prefix, we walk through the trie,
* check mask M1 for our prefix length and when we came to
* final node, we check mask M2.
*
* There are two differences in the real implementation. First,
* we use a compressed trie so there is a case that we skip our
* final node (if it is not in the trie) and we came to node that
* is either extension of our prefix, or completely out of path
* In the first case, we also have to check M2.
*
* Second, we really need not to maintain two separate bitmasks.
* Checks for mask M1 are always larger than &applen and we need
* just the first &pplen bits of mask M2 (if trie compression
* hadn't been used it would suffice to know just $applen-th bit),
* so we have to store them together in &accept mask - the first
* &pplen bits of mask M2 and then mask M1.
*
* There are four cases when we walk through a trie:
*
* - we are in NULL
* - we are out of path (prefixes are inconsistent)
* - we are in the wanted (final) node (node length == &plen)
* - we are beyond the end of path (node length > &plen)
* - we are still on path and keep walking (node length < &plen)
*
* The walking code in add_node_to_trie() and trie_match_prefix()
* is structured according to these cases.
*/
#include "nest/bird.h"
#include "lib/string.h"
#include "conf/conf.h"
#include "filter/filter.h"
/**
* f_new_trie
*
* Allocates and returns a new empty trie.
*/
struct f_trie *
f_new_trie(void)
{
struct f_trie * ret;
ret = cfg_allocz(sizeof(struct f_trie));
return ret;
}
static inline struct f_trie_node *
new_node(int plen, ip_addr paddr, ip_addr pmask, ip_addr amask)
{
struct f_trie_node *n = cfg_allocz(sizeof(struct f_trie_node));
n->plen = plen;
n->addr = paddr;
n->mask = pmask;
n->accept = amask;
return n;
}
static inline void
attach_node(struct f_trie_node *parent, struct f_trie_node *child)
{
parent->c[ipa_getbit(child->addr, parent->plen) ? 1 : 0] = child;
}
static void
add_node_to_trie(struct f_trie *t, int plen, ip_addr ip, ip_addr amask)
{
ip_addr pmask = ipa_mkmask(plen);
ip_addr paddr = ipa_and(ip, pmask);
struct f_trie_node *o = NULL;
struct f_trie_node *n = &t->root;
while(n)
{
ip_addr cmask = ipa_and(n->mask, pmask);
if (ipa_compare(ipa_and(paddr, cmask), ipa_and(n->addr, cmask)))
{
/* We are out of path - we have to add branching node 'b'
between node 'o' and node 'n', and attach new node 'a'
as the other child of 'b'. */
int blen = ipa_pxlen(paddr, n->addr);
ip_addr bmask = ipa_mkmask(blen);
ip_addr baddr = ipa_and(ip, bmask);
/* Merge accept masks from children to get accept mask for node 'b' */
ip_addr baccm = ipa_and(ipa_or(amask, n->accept), bmask);
struct f_trie_node *a = new_node(plen, paddr, pmask, amask);
struct f_trie_node *b = new_node(blen, baddr, bmask, baccm);
attach_node(o, b);
attach_node(b, n);
attach_node(b, a);
return;
}
if (plen < n->plen)
{
/* We add new node 'a' between node 'o' and node 'n' */
amask = ipa_or(amask, ipa_and(n->accept, pmask));
struct f_trie_node *a = new_node(plen, paddr, pmask, amask);
attach_node(o, a);
attach_node(a, n);
return;
}
if (plen == n->plen)
{
/* We already found added node in trie. Just update accept mask */
n->accept = ipa_or(n->accept, amask);
return;
}
/* Update accept mask part M2 and go deeper */
n->accept = ipa_or(n->accept, ipa_and(amask, n->mask));
/* n->plen < plen and plen <= 32 */
o = n;
n = n->c[ipa_getbit(paddr, n->plen) ? 1 : 0];
}
/* We add new tail node 'a' after node 'o' */
struct f_trie_node *a = new_node(plen, paddr, pmask, amask);
attach_node(o, a);
}
/**
* trie_add_prefix
* @t: trie to add to
* @px: prefix to add
*
* Adds prefix (prefix pattern) @px to trie @t.
*/
void
trie_add_prefix(struct f_trie *t, struct f_prefix *px)
{
int l, h;
int plen = px->len & LEN_MASK;
/* 'l' and 'h' are lower and upper bounds on accepted
prefix lengths, both inclusive. 0 <= l, h <= 32 */
f_prefix_get_bounds(px, &l, &h);
if (l == 0)
t->zero = 1;
else
l--;
ip_addr amask = ipa_xor(ipa_mkmask(l), ipa_mkmask(h));
/* MIN(plen, h) instead of just plen is a little trick. */
add_node_to_trie(t, MIN(plen, h), px->ip, amask);
}
/**
* trie_match_prefix
* @t: trie
* @px: prefix
*
* Tries to find a matching prefix pattern in the trie such that
* prefix @px matches that prefix pattern. Returns 1 if there
* is such prefix pattern in the trie.
*/
int
trie_match_prefix(struct f_trie *t, struct f_prefix *px)
{
int plen = px->len & LEN_MASK;
ip_addr pmask = ipa_mkmask(plen);
ip_addr paddr = ipa_and(px->ip, pmask);
if (plen == 0)
return t->zero;
int plentest = plen - 1;
struct f_trie_node *n = &t->root;
while(n)
{
ip_addr cmask = ipa_and(n->mask, pmask);
/* We are out of path */
if (ipa_compare(ipa_and(paddr, cmask), ipa_and(n->addr, cmask)))
return 0;
/* Check accept mask */
if (ipa_getbit(n->accept, plentest))
return 1;
/* We finished trie walk and still no match */
if (plen <= n->plen)
return 0;
/* Choose children */
n = n->c[(ipa_getbit(paddr, n->plen)) ? 1 : 0];
}
return 0;
}
static int
trie_node_same(struct f_trie_node *t1, struct f_trie_node *t2)
{
if ((t1 == NULL) && (t2 == NULL))
return 1;
if ((t1 == NULL) || (t2 == NULL))
return 0;
if ((t1->plen != t2->plen) ||
(! ipa_equal(t1->addr, t2->addr)) ||
(! ipa_equal(t1->accept, t2->accept)))
return 0;
return trie_node_same(t1->c[0], t2->c[0]) && trie_node_same(t1->c[1], t2->c[1]);
}
/**
* trie_same
* @t1: first trie to be compared
* @t2: second one
*
* Compares two tries and returns 1 if they are same
*/
int
trie_same(struct f_trie *t1, struct f_trie *t2)
{
return (t1->zero == t2->zero) && trie_node_same(&t1->root, &t2->root);
}
static int
trie_node_print(struct f_trie_node *t, char *buf, int blen)
{
if (t == NULL)
return 0;
int old_blen = blen;
int wb = 0; // bsnprintf(buf, blen, "%I/%d accept %I\n", t->addr, t->plen, t->accept);
debug("%I/%d accept %I\n", t->addr, t->plen, t->accept);
if ((wb < 0) || ((blen - wb) < 10))
{
bsnprintf(buf, blen, "...\n");
return -1;
}
buf += wb;
blen -= wb;
wb = trie_node_print(t->c[0], buf, blen);
if (wb < 0)
return -1;
buf += wb;
blen -= wb;
wb = trie_node_print(t->c[1], buf, blen);
if (wb < 0)
return -1;
blen -= wb;
return (old_blen - blen);
}
/**
* trie_print
* @t: trie to be printed
* @buf: buffer
* @blen: buffer length
*
* Prints the trie to the buffer, using at most blen bytes.
* Returns the number of used bytes, or -1 if there is not
* enough space in the buffer.
*/
int
trie_print(struct f_trie *t, char *buf, int blen)
{
return trie_node_print(&t->root, buf, blen);
}