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/textsearch.h> 43 44 /* Alphabet size, use ASCII */ 45 #define ASIZE 256 46 47 #if 0 48 #define DEBUGP printk 49 #else 50 #define DEBUGP(args, format...) 51 #endif 52 53 struct ts_bm 54 { 55 u8 * pattern; 56 unsigned int patlen; 57 unsigned int bad_shift[ASIZE]; 58 unsigned int good_shift[0]; 59 }; 60 61 static unsigned int bm_find(struct ts_config *conf, struct ts_state *state) 62 { 63 struct ts_bm *bm = ts_config_priv(conf); 64 unsigned int i, text_len, consumed = state->offset; 65 const u8 *text; 66 int shift = bm->patlen, bs; 67 68 for (;;) { 69 text_len = conf->get_next_block(consumed, &text, conf, state); 70 71 if (unlikely(text_len == 0)) 72 break; 73 74 while (shift < text_len) { 75 DEBUGP("Searching in position %d (%c)\n", 76 shift, text[shift]); 77 for (i = 0; i < bm->patlen; i++) 78 if (text[shift-i] != bm->pattern[bm->patlen-1-i]) 79 goto next; 80 81 /* London calling... */ 82 DEBUGP("found!\n"); 83 return consumed += (shift-(bm->patlen-1)); 84 85 next: bs = bm->bad_shift[text[shift-i]]; 86 87 /* Now jumping to... */ 88 shift = max_t(int, shift-i+bs, shift+bm->good_shift[i]); 89 } 90 consumed += text_len; 91 } 92 93 return UINT_MAX; 94 } 95 96 static int subpattern(u8 *pattern, int i, int j, int g) 97 { 98 int x = i+g-1, y = j+g-1, ret = 0; 99 100 while(pattern[x--] == pattern[y--]) { 101 if (y < 0) { 102 ret = 1; 103 break; 104 } 105 if (--g == 0) { 106 ret = pattern[i-1] != pattern[j-1]; 107 break; 108 } 109 } 110 111 return ret; 112 } 113 114 static void compute_prefix_tbl(struct ts_bm *bm) 115 { 116 int i, j, g; 117 118 for (i = 0; i < ASIZE; i++) 119 bm->bad_shift[i] = bm->patlen; 120 for (i = 0; i < bm->patlen - 1; i++) 121 bm->bad_shift[bm->pattern[i]] = bm->patlen - 1 - i; 122 123 /* Compute the good shift array, used to match reocurrences 124 * of a subpattern */ 125 bm->good_shift[0] = 1; 126 for (i = 1; i < bm->patlen; i++) 127 bm->good_shift[i] = bm->patlen; 128 for (i = bm->patlen-1, g = 1; i > 0; g++, i--) { 129 for (j = i-1; j >= 1-g ; j--) 130 if (subpattern(bm->pattern, i, j, g)) { 131 bm->good_shift[g] = bm->patlen-j-g; 132 break; 133 } 134 } 135 } 136 137 static struct ts_config *bm_init(const void *pattern, unsigned int len, 138 gfp_t gfp_mask) 139 { 140 struct ts_config *conf; 141 struct ts_bm *bm; 142 unsigned int prefix_tbl_len = len * sizeof(unsigned int); 143 size_t priv_size = sizeof(*bm) + len + prefix_tbl_len; 144 145 conf = alloc_ts_config(priv_size, gfp_mask); 146 if (IS_ERR(conf)) 147 return conf; 148 149 bm = ts_config_priv(conf); 150 bm->patlen = len; 151 bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len; 152 memcpy(bm->pattern, pattern, len); 153 compute_prefix_tbl(bm); 154 155 return conf; 156 } 157 158 static void *bm_get_pattern(struct ts_config *conf) 159 { 160 struct ts_bm *bm = ts_config_priv(conf); 161 return bm->pattern; 162 } 163 164 static unsigned int bm_get_pattern_len(struct ts_config *conf) 165 { 166 struct ts_bm *bm = ts_config_priv(conf); 167 return bm->patlen; 168 } 169 170 static struct ts_ops bm_ops = { 171 .name = "bm", 172 .find = bm_find, 173 .init = bm_init, 174 .get_pattern = bm_get_pattern, 175 .get_pattern_len = bm_get_pattern_len, 176 .owner = THIS_MODULE, 177 .list = LIST_HEAD_INIT(bm_ops.list) 178 }; 179 180 static int __init init_bm(void) 181 { 182 return textsearch_register(&bm_ops); 183 } 184 185 static void __exit exit_bm(void) 186 { 187 textsearch_unregister(&bm_ops); 188 } 189 190 MODULE_LICENSE("GPL"); 191 192 module_init(init_bm); 193 module_exit(exit_bm); 194