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/config.h> 39 #include <linux/kernel.h> 40 #include <linux/module.h> 41 #include <linux/types.h> 42 #include <linux/string.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 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, bs; 68 69 for (;;) { 70 text_len = conf->get_next_block(consumed, &text, conf, state); 71 72 if (unlikely(text_len == 0)) 73 break; 74 75 while (shift < text_len) { 76 DEBUGP("Searching in position %d (%c)\n", 77 shift, text[shift]); 78 for (i = 0; i < bm->patlen; i++) 79 if (text[shift-i] != bm->pattern[bm->patlen-1-i]) 80 goto next; 81 82 /* London calling... */ 83 DEBUGP("found!\n"); 84 return consumed += (shift-(bm->patlen-1)); 85 86 next: bs = bm->bad_shift[text[shift-i]]; 87 88 /* Now jumping to... */ 89 shift = max_t(int, shift-i+bs, shift+bm->good_shift[i]); 90 } 91 consumed += text_len; 92 } 93 94 return UINT_MAX; 95 } 96 97 static int subpattern(u8 *pattern, int i, int j, int g) 98 { 99 int x = i+g-1, y = j+g-1, ret = 0; 100 101 while(pattern[x--] == pattern[y--]) { 102 if (y < 0) { 103 ret = 1; 104 break; 105 } 106 if (--g == 0) { 107 ret = pattern[i-1] != pattern[j-1]; 108 break; 109 } 110 } 111 112 return ret; 113 } 114 115 static void compute_prefix_tbl(struct ts_bm *bm, const u8 *pattern, 116 unsigned int len) 117 { 118 int i, j, g; 119 120 for (i = 0; i < ASIZE; i++) 121 bm->bad_shift[i] = len; 122 for (i = 0; i < len - 1; i++) 123 bm->bad_shift[pattern[i]] = len - 1 - i; 124 125 /* Compute the good shift array, used to match reocurrences 126 * of a subpattern */ 127 bm->good_shift[0] = 1; 128 for (i = 1; i < bm->patlen; i++) 129 bm->good_shift[i] = bm->patlen; 130 for (i = bm->patlen-1, g = 1; i > 0; g++, i--) { 131 for (j = i-1; j >= 1-g ; j--) 132 if (subpattern(bm->pattern, i, j, g)) { 133 bm->good_shift[g] = bm->patlen-j-g; 134 break; 135 } 136 } 137 } 138 139 static struct ts_config *bm_init(const void *pattern, unsigned int len, 140 gfp_t gfp_mask) 141 { 142 struct ts_config *conf; 143 struct ts_bm *bm; 144 unsigned int prefix_tbl_len = len * sizeof(unsigned int); 145 size_t priv_size = sizeof(*bm) + len + prefix_tbl_len; 146 147 conf = alloc_ts_config(priv_size, gfp_mask); 148 if (IS_ERR(conf)) 149 return conf; 150 151 bm = ts_config_priv(conf); 152 bm->patlen = len; 153 bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len; 154 compute_prefix_tbl(bm, pattern, len); 155 memcpy(bm->pattern, pattern, len); 156 157 return conf; 158 } 159 160 static void *bm_get_pattern(struct ts_config *conf) 161 { 162 struct ts_bm *bm = ts_config_priv(conf); 163 return bm->pattern; 164 } 165 166 static unsigned int bm_get_pattern_len(struct ts_config *conf) 167 { 168 struct ts_bm *bm = ts_config_priv(conf); 169 return bm->patlen; 170 } 171 172 static struct ts_ops bm_ops = { 173 .name = "bm", 174 .find = bm_find, 175 .init = bm_init, 176 .get_pattern = bm_get_pattern, 177 .get_pattern_len = bm_get_pattern_len, 178 .owner = THIS_MODULE, 179 .list = LIST_HEAD_INIT(bm_ops.list) 180 }; 181 182 static int __init init_bm(void) 183 { 184 return textsearch_register(&bm_ops); 185 } 186 187 static void __exit exit_bm(void) 188 { 189 textsearch_unregister(&bm_ops); 190 } 191 192 MODULE_LICENSE("GPL"); 193 194 module_init(init_bm); 195 module_exit(exit_bm); 196