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 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 } 99 100 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 } 117 118 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 } 144 145 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 } 171 172 static void *bm_get_pattern(struct ts_config *conf) 173 { 174 struct ts_bm *bm = ts_config_priv(conf); 175 return bm->pattern; 176 } 177 178 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 }; 193 194 static int __init init_bm(void) 195 { 196 return textsearch_register(&bm_ops); 197 } 198 199 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