Annotation of embedaddon/bird/filter/trie.c, revision 1.1.1.1
1.1 misho 1: /*
2: * Filters: Trie for prefix sets
3: *
4: * Copyright 2009 Ondrej Zajicek <santiago@crfreenet.org>
5: *
6: * Can be freely distributed and used under the terms of the GNU GPL.
7: */
8:
9: /**
10: * DOC: Trie for prefix sets
11: *
12: * We use a (compressed) trie to represent prefix sets. Every node
13: * in the trie represents one prefix (&addr/&plen) and &plen also
14: * indicates the index of the bit in the address that is used to
15: * branch at the node. If we need to represent just a set of
16: * prefixes, it would be simple, but we have to represent a
17: * set of prefix patterns. Each prefix pattern consists of
18: * &ppaddr/&pplen and two integers: &low and &high, and a prefix
19: * &paddr/&plen matches that pattern if the first MIN(&plen, &pplen)
20: * bits of &paddr and &ppaddr are the same and &low <= &plen <= &high.
21: *
22: * We use a bitmask (&accept) to represent accepted prefix lengths
23: * at a node. As there are 33 prefix lengths (0..32 for IPv4), but
24: * there is just one prefix of zero length in the whole trie so we
25: * have &zero flag in &f_trie (indicating whether the trie accepts
26: * prefix 0.0.0.0/0) as a special case, and &accept bitmask
27: * represents accepted prefix lengths from 1 to 32.
28: *
29: * There are two cases in prefix matching - a match when the length
30: * of the prefix is smaller that the length of the prefix pattern,
31: * (&plen < &pplen) and otherwise. The second case is simple - we
32: * just walk through the trie and look at every visited node
33: * whether that prefix accepts our prefix length (&plen). The
34: * first case is tricky - we don't want to examine every descendant
35: * of a final node, so (when we create the trie) we have to propagate
36: * that information from nodes to their ascendants.
37: *
38: * Suppose that we have two masks (M1 and M2) for a node. Mask M1
39: * represents accepted prefix lengths by just the node and mask M2
40: * represents accepted prefix lengths by the node or any of its
41: * descendants. Therefore M2 is a bitwise or of M1 and children's
42: * M2 and this is a maintained invariant during trie building.
43: * Basically, when we want to match a prefix, we walk through the trie,
44: * check mask M1 for our prefix length and when we came to
45: * final node, we check mask M2.
46: *
47: * There are two differences in the real implementation. First,
48: * we use a compressed trie so there is a case that we skip our
49: * final node (if it is not in the trie) and we came to node that
50: * is either extension of our prefix, or completely out of path
51: * In the first case, we also have to check M2.
52: *
53: * Second, we really need not to maintain two separate bitmasks.
54: * Checks for mask M1 are always larger than &applen and we need
55: * just the first &pplen bits of mask M2 (if trie compression
56: * hadn't been used it would suffice to know just $applen-th bit),
57: * so we have to store them together in &accept mask - the first
58: * &pplen bits of mask M2 and then mask M1.
59: *
60: * There are four cases when we walk through a trie:
61: *
62: * - we are in NULL
63: * - we are out of path (prefixes are inconsistent)
64: * - we are in the wanted (final) node (node length == &plen)
65: * - we are beyond the end of path (node length > &plen)
66: * - we are still on path and keep walking (node length < &plen)
67: *
68: * The walking code in trie_match_prefix() is structured according to
69: * these cases.
70: */
71:
72: #include "nest/bird.h"
73: #include "lib/string.h"
74: #include "conf/conf.h"
75: #include "filter/filter.h"
76:
77: /**
78: * f_new_trie - allocates and returns a new empty trie
79: * @lp: linear pool to allocate items from
80: * @node_size: node size to be used (&f_trie_node and user data)
81: */
82: struct f_trie *
83: f_new_trie(linpool *lp, uint node_size)
84: {
85: struct f_trie * ret;
86: ret = lp_allocz(lp, sizeof(struct f_trie) + node_size);
87: ret->lp = lp;
88: ret->node_size = node_size;
89: return ret;
90: }
91:
92: static inline struct f_trie_node *
93: new_node(struct f_trie *t, int plen, ip_addr paddr, ip_addr pmask, ip_addr amask)
94: {
95: struct f_trie_node *n = lp_allocz(t->lp, t->node_size);
96: n->plen = plen;
97: n->addr = paddr;
98: n->mask = pmask;
99: n->accept = amask;
100: return n;
101: }
102:
103: static inline void
104: attach_node(struct f_trie_node *parent, struct f_trie_node *child)
105: {
106: parent->c[ipa_getbit(child->addr, parent->plen) ? 1 : 0] = child;
107: }
108:
109: /**
110: * trie_add_prefix
111: * @t: trie to add to
112: * @px: prefix address
113: * @plen: prefix length
114: * @l: prefix lower bound
115: * @h: prefix upper bound
116: *
117: * Adds prefix (prefix pattern) @px/@plen to trie @t. @l and @h are lower
118: * and upper bounds on accepted prefix lengths, both inclusive.
119: * 0 <= l, h <= 32 (128 for IPv6).
120: *
121: * Returns a pointer to the allocated node. The function can return a pointer to
122: * an existing node if @px and @plen are the same. If px/plen == 0/0 (or ::/0),
123: * a pointer to the root node is returned.
124: */
125:
126: void *
127: trie_add_prefix(struct f_trie *t, ip_addr px, int plen, int l, int h)
128: {
129: if (l == 0)
130: t->zero = 1;
131: else
132: l--;
133:
134: if (h < plen)
135: plen = h;
136:
137: ip_addr amask = ipa_xor(ipa_mkmask(l), ipa_mkmask(h));
138: ip_addr pmask = ipa_mkmask(plen);
139: ip_addr paddr = ipa_and(px, pmask);
140: struct f_trie_node *o = NULL;
141: struct f_trie_node *n = t->root;
142:
143: while(n)
144: {
145: ip_addr cmask = ipa_and(n->mask, pmask);
146:
147: if (ipa_compare(ipa_and(paddr, cmask), ipa_and(n->addr, cmask)))
148: {
149: /* We are out of path - we have to add branching node 'b'
150: between node 'o' and node 'n', and attach new node 'a'
151: as the other child of 'b'. */
152: int blen = ipa_pxlen(paddr, n->addr);
153: ip_addr bmask = ipa_mkmask(blen);
154: ip_addr baddr = ipa_and(px, bmask);
155:
156: /* Merge accept masks from children to get accept mask for node 'b' */
157: ip_addr baccm = ipa_and(ipa_or(amask, n->accept), bmask);
158:
159: struct f_trie_node *a = new_node(t, plen, paddr, pmask, amask);
160: struct f_trie_node *b = new_node(t, blen, baddr, bmask, baccm);
161: attach_node(o, b);
162: attach_node(b, n);
163: attach_node(b, a);
164: return a;
165: }
166:
167: if (plen < n->plen)
168: {
169: /* We add new node 'a' between node 'o' and node 'n' */
170: amask = ipa_or(amask, ipa_and(n->accept, pmask));
171: struct f_trie_node *a = new_node(t, plen, paddr, pmask, amask);
172: attach_node(o, a);
173: attach_node(a, n);
174: return a;
175: }
176:
177: if (plen == n->plen)
178: {
179: /* We already found added node in trie. Just update accept mask */
180: n->accept = ipa_or(n->accept, amask);
181: return n;
182: }
183:
184: /* Update accept mask part M2 and go deeper */
185: n->accept = ipa_or(n->accept, ipa_and(amask, n->mask));
186:
187: /* n->plen < plen and plen <= 32 (128) */
188: o = n;
189: n = n->c[ipa_getbit(paddr, n->plen) ? 1 : 0];
190: }
191:
192: /* We add new tail node 'a' after node 'o' */
193: struct f_trie_node *a = new_node(t, plen, paddr, pmask, amask);
194: attach_node(o, a);
195:
196: return a;
197: }
198:
199: /**
200: * trie_match_prefix
201: * @t: trie
202: * @px: prefix address
203: * @plen: prefix length
204: *
205: * Tries to find a matching prefix pattern in the trie such that
206: * prefix @px/@plen matches that prefix pattern. Returns 1 if there
207: * is such prefix pattern in the trie.
208: */
209: int
210: trie_match_prefix(struct f_trie *t, ip_addr px, int plen)
211: {
212: ip_addr pmask = ipa_mkmask(plen);
213: ip_addr paddr = ipa_and(px, pmask);
214:
215: if (plen == 0)
216: return t->zero;
217:
218: int plentest = plen - 1;
219: struct f_trie_node *n = t->root;
220:
221: while(n)
222: {
223: ip_addr cmask = ipa_and(n->mask, pmask);
224:
225: /* We are out of path */
226: if (ipa_compare(ipa_and(paddr, cmask), ipa_and(n->addr, cmask)))
227: return 0;
228:
229: /* Check accept mask */
230: if (ipa_getbit(n->accept, plentest))
231: return 1;
232:
233: /* We finished trie walk and still no match */
234: if (plen <= n->plen)
235: return 0;
236:
237: /* Choose children */
238: n = n->c[(ipa_getbit(paddr, n->plen)) ? 1 : 0];
239: }
240:
241: return 0;
242: }
243:
244: static int
245: trie_node_same(struct f_trie_node *t1, struct f_trie_node *t2)
246: {
247: if ((t1 == NULL) && (t2 == NULL))
248: return 1;
249:
250: if ((t1 == NULL) || (t2 == NULL))
251: return 0;
252:
253: if ((t1->plen != t2->plen) ||
254: (! ipa_equal(t1->addr, t2->addr)) ||
255: (! ipa_equal(t1->accept, t2->accept)))
256: return 0;
257:
258: return trie_node_same(t1->c[0], t2->c[0]) && trie_node_same(t1->c[1], t2->c[1]);
259: }
260:
261: /**
262: * trie_same
263: * @t1: first trie to be compared
264: * @t2: second one
265: *
266: * Compares two tries and returns 1 if they are same
267: */
268: int
269: trie_same(struct f_trie *t1, struct f_trie *t2)
270: {
271: return (t1->zero == t2->zero) && trie_node_same(t1->root, t2->root);
272: }
273:
274: static void
275: trie_node_format(struct f_trie_node *t, buffer *buf)
276: {
277: if (t == NULL)
278: return;
279:
280: if (ipa_nonzero(t->accept))
281: buffer_print(buf, "%I/%d{%I}, ", t->addr, t->plen, t->accept);
282:
283: trie_node_format(t->c[0], buf);
284: trie_node_format(t->c[1], buf);
285: }
286:
287: /**
288: * trie_format
289: * @t: trie to be formatted
290: * @buf: destination buffer
291: *
292: * Prints the trie to the supplied buffer.
293: */
294: void
295: trie_format(struct f_trie *t, buffer *buf)
296: {
297: buffer_puts(buf, "[");
298:
299: if (t->zero)
300: buffer_print(buf, "%I/%d, ", IPA_NONE, 0);
301: trie_node_format(t->root, buf);
302:
303: if (buf->pos == buf->end)
304: return;
305:
306: /* Undo last separator */
307: if (buf->pos[-1] != '[')
308: buf->pos -= 2;
309:
310: buffer_puts(buf, "]");
311: }
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