/* * Filters: Trie for prefix sets * * Copyright 2009 Ondrej Zajicek * * 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); }