1 /*
2 * lib/ts_bm.c Boyer-Moore text search implementation
3 *
4 * This program is free software; you can redistribute it and/or
5 * modify it under the terms of the GNU General Public License
6 * as published by the Free Software Foundation; either version
7 * 2 of the License, or (at your option) any later version.
8 *
9 * Authors: Pablo Neira Ayuso <pablo@eurodev.net>
10 *
11 * ==========================================================================
12 *
13 * Implements Boyer-Moore string matching algorithm:
14 *
15 * [1] A Fast String Searching Algorithm, R.S. Boyer and Moore.
16 * Communications of the Association for Computing Machinery,
17 * 20(10), 1977, pp. 762-772.
18 * http://www.cs.utexas.edu/users/moore/publications/fstrpos.pdf
19 *
20 * [2] Handbook of Exact String Matching Algorithms, Thierry Lecroq, 2004
21 * http://www-igm.univ-mlv.fr/~lecroq/string/string.pdf
22 *
23 * Note: Since Boyer-Moore (BM) performs searches for matchings from right
24 * to left, it's still possible that a matching could be spread over
25 * multiple blocks, in that case this algorithm won't find any coincidence.
26 *
27 * If you're willing to ensure that such thing won't ever happen, use the
28 * Knuth-Pratt-Morris (KMP) implementation instead. In conclusion, choose
29 * the proper string search algorithm depending on your setting.
30 *
31 * Say you're using the textsearch infrastructure for filtering, NIDS or
32 * any similar security focused purpose, then go KMP. Otherwise, if you
33 * really care about performance, say you're classifying packets to apply
34 * Quality of Service (QoS) policies, and you don't mind about possible
35 * matchings spread over multiple fragments, then go BM.
36 */
37
38 #include <linux/kernel.h>
39 #include <linux/module.h>
40 #include <linux/types.h>
41 #include <linux/string.h>
42 #include <linux/ctype.h>
43 #include <linux/textsearch.h>
44
45 /* Alphabet size, use ASCII */
46 #define ASIZE 256
47
48 #if 0
49 #define DEBUGP printk
50 #else
51 #define DEBUGP(args, format...)
52 #endif
53
54 struct ts_bm
55 {
56 u8 * pattern;
57 unsigned int patlen;
58 unsigned int bad_shift[ASIZE];
59 unsigned int good_shift[0];
60 };
61
bm_find(struct ts_config * conf,struct ts_state * state)62 static unsigned int bm_find(struct ts_config *conf, struct ts_state *state)
63 {
64 struct ts_bm *bm = ts_config_priv(conf);
65 unsigned int i, text_len, consumed = state->offset;
66 const u8 *text;
67 int shift = bm->patlen - 1, bs;
68 const u8 icase = conf->flags & TS_IGNORECASE;
69
70 for (;;) {
71 text_len = conf->get_next_block(consumed, &text, conf, state);
72
73 if (unlikely(text_len == 0))
74 break;
75
76 while (shift < text_len) {
77 DEBUGP("Searching in position %d (%c)\n",
78 shift, text[shift]);
79 for (i = 0; i < bm->patlen; i++)
80 if ((icase ? toupper(text[shift-i])
81 : text[shift-i])
82 != bm->pattern[bm->patlen-1-i])
83 goto next;
84
85 /* London calling... */
86 DEBUGP("found!\n");
87 return consumed += (shift-(bm->patlen-1));
88
89 next: bs = bm->bad_shift[text[shift-i]];
90
91 /* Now jumping to... */
92 shift = max_t(int, shift-i+bs, shift+bm->good_shift[i]);
93 }
94 consumed += text_len;
95 }
96
97 return UINT_MAX;
98 }
99
subpattern(u8 * pattern,int i,int j,int g)100 static int subpattern(u8 *pattern, int i, int j, int g)
101 {
102 int x = i+g-1, y = j+g-1, ret = 0;
103
104 while(pattern[x--] == pattern[y--]) {
105 if (y < 0) {
106 ret = 1;
107 break;
108 }
109 if (--g == 0) {
110 ret = pattern[i-1] != pattern[j-1];
111 break;
112 }
113 }
114
115 return ret;
116 }
117
compute_prefix_tbl(struct ts_bm * bm,int flags)118 static void compute_prefix_tbl(struct ts_bm *bm, int flags)
119 {
120 int i, j, g;
121
122 for (i = 0; i < ASIZE; i++)
123 bm->bad_shift[i] = bm->patlen;
124 for (i = 0; i < bm->patlen - 1; i++) {
125 bm->bad_shift[bm->pattern[i]] = bm->patlen - 1 - i;
126 if (flags & TS_IGNORECASE)
127 bm->bad_shift[tolower(bm->pattern[i])]
128 = bm->patlen - 1 - i;
129 }
130
131 /* Compute the good shift array, used to match reocurrences
132 * of a subpattern */
133 bm->good_shift[0] = 1;
134 for (i = 1; i < bm->patlen; i++)
135 bm->good_shift[i] = bm->patlen;
136 for (i = bm->patlen-1, g = 1; i > 0; g++, i--) {
137 for (j = i-1; j >= 1-g ; j--)
138 if (subpattern(bm->pattern, i, j, g)) {
139 bm->good_shift[g] = bm->patlen-j-g;
140 break;
141 }
142 }
143 }
144
bm_init(const void * pattern,unsigned int len,gfp_t gfp_mask,int flags)145 static struct ts_config *bm_init(const void *pattern, unsigned int len,
146 gfp_t gfp_mask, int flags)
147 {
148 struct ts_config *conf;
149 struct ts_bm *bm;
150 int i;
151 unsigned int prefix_tbl_len = len * sizeof(unsigned int);
152 size_t priv_size = sizeof(*bm) + len + prefix_tbl_len;
153
154 conf = alloc_ts_config(priv_size, gfp_mask);
155 if (IS_ERR(conf))
156 return conf;
157
158 conf->flags = flags;
159 bm = ts_config_priv(conf);
160 bm->patlen = len;
161 bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len;
162 if (flags & TS_IGNORECASE)
163 for (i = 0; i < len; i++)
164 bm->pattern[i] = toupper(((u8 *)pattern)[i]);
165 else
166 memcpy(bm->pattern, pattern, len);
167 compute_prefix_tbl(bm, flags);
168
169 return conf;
170 }
171
bm_get_pattern(struct ts_config * conf)172 static void *bm_get_pattern(struct ts_config *conf)
173 {
174 struct ts_bm *bm = ts_config_priv(conf);
175 return bm->pattern;
176 }
177
bm_get_pattern_len(struct ts_config * conf)178 static unsigned int bm_get_pattern_len(struct ts_config *conf)
179 {
180 struct ts_bm *bm = ts_config_priv(conf);
181 return bm->patlen;
182 }
183
184 static struct ts_ops bm_ops = {
185 .name = "bm",
186 .find = bm_find,
187 .init = bm_init,
188 .get_pattern = bm_get_pattern,
189 .get_pattern_len = bm_get_pattern_len,
190 .owner = THIS_MODULE,
191 .list = LIST_HEAD_INIT(bm_ops.list)
192 };
193
init_bm(void)194 static int __init init_bm(void)
195 {
196 return textsearch_register(&bm_ops);
197 }
198
exit_bm(void)199 static void __exit exit_bm(void)
200 {
201 textsearch_unregister(&bm_ops);
202 }
203
204 MODULE_LICENSE("GPL");
205
206 module_init(init_bm);
207 module_exit(exit_bm);
208