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