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 void compute_prefix_tbl(struct ts_bm *bm, const u8 *pattern, 98 unsigned int len) 99 { 100 int i, j, ended, l[ASIZE]; 101 102 for (i = 0; i < ASIZE; i++) 103 bm->bad_shift[i] = len; 104 for (i = 0; i < len - 1; i++) 105 bm->bad_shift[pattern[i]] = len - 1 - i; 106 107 /* Compute the good shift array, used to match reocurrences 108 * of a subpattern */ 109 for (i = 1; i < bm->patlen; i++) { 110 for (j = 0; j < bm->patlen && bm->pattern[bm->patlen - 1 - j] 111 == bm->pattern[bm->patlen - 1 - i - j]; j++); 112 l[i] = j; 113 } 114 115 bm->good_shift[0] = 1; 116 for (i = 1; i < bm->patlen; i++) 117 bm->good_shift[i] = bm->patlen; 118 for (i = bm->patlen - 1; i > 0; i--) 119 bm->good_shift[l[i]] = i; 120 ended = 0; 121 for (i = 0; i < bm->patlen; i++) { 122 if (l[i] == bm->patlen - 1 - i) 123 ended = i; 124 if (ended) 125 bm->good_shift[i] = ended; 126 } 127 } 128 129 static struct ts_config *bm_init(const void *pattern, unsigned int len, 130 gfp_t gfp_mask) 131 { 132 struct ts_config *conf; 133 struct ts_bm *bm; 134 unsigned int prefix_tbl_len = len * sizeof(unsigned int); 135 size_t priv_size = sizeof(*bm) + len + prefix_tbl_len; 136 137 conf = alloc_ts_config(priv_size, gfp_mask); 138 if (IS_ERR(conf)) 139 return conf; 140 141 bm = ts_config_priv(conf); 142 bm->patlen = len; 143 bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len; 144 compute_prefix_tbl(bm, pattern, len); 145 memcpy(bm->pattern, pattern, len); 146 147 return conf; 148 } 149 150 static void *bm_get_pattern(struct ts_config *conf) 151 { 152 struct ts_bm *bm = ts_config_priv(conf); 153 return bm->pattern; 154 } 155 156 static unsigned int bm_get_pattern_len(struct ts_config *conf) 157 { 158 struct ts_bm *bm = ts_config_priv(conf); 159 return bm->patlen; 160 } 161 162 static struct ts_ops bm_ops = { 163 .name = "bm", 164 .find = bm_find, 165 .init = bm_init, 166 .get_pattern = bm_get_pattern, 167 .get_pattern_len = bm_get_pattern_len, 168 .owner = THIS_MODULE, 169 .list = LIST_HEAD_INIT(bm_ops.list) 170 }; 171 172 static int __init init_bm(void) 173 { 174 return textsearch_register(&bm_ops); 175 } 176 177 static void __exit exit_bm(void) 178 { 179 textsearch_unregister(&bm_ops); 180 } 181 182 MODULE_LICENSE("GPL"); 183 184 module_init(init_bm); 185 module_exit(exit_bm); 186