1 // SPDX-License-Identifier: GPL-2.0-or-later 2 /* 3 * lib/ts_bm.c Boyer-Moore text search implementation 4 * 5 * Authors: Pablo Neira Ayuso <pablo@eurodev.net> 6 * 7 * ========================================================================== 8 * 9 * Implements Boyer-Moore string matching algorithm: 10 * 11 * [1] A Fast String Searching Algorithm, R.S. Boyer and Moore. 12 * Communications of the Association for Computing Machinery, 13 * 20(10), 1977, pp. 762-772. 14 * http://www.cs.utexas.edu/users/moore/publications/fstrpos.pdf 15 * 16 * [2] Handbook of Exact String Matching Algorithms, Thierry Lecroq, 2004 17 * http://www-igm.univ-mlv.fr/~lecroq/string/string.pdf 18 * 19 * Note: Since Boyer-Moore (BM) performs searches for matchings from right 20 * to left, it's still possible that a matching could be spread over 21 * multiple blocks, in that case this algorithm won't find any coincidence. 22 * 23 * If you're willing to ensure that such thing won't ever happen, use the 24 * Knuth-Pratt-Morris (KMP) implementation instead. In conclusion, choose 25 * the proper string search algorithm depending on your setting. 26 * 27 * Say you're using the textsearch infrastructure for filtering, NIDS or 28 * any similar security focused purpose, then go KMP. Otherwise, if you 29 * really care about performance, say you're classifying packets to apply 30 * Quality of Service (QoS) policies, and you don't mind about possible 31 * matchings spread over multiple fragments, then go BM. 32 */ 33 34 #include <linux/kernel.h> 35 #include <linux/module.h> 36 #include <linux/types.h> 37 #include <linux/string.h> 38 #include <linux/ctype.h> 39 #include <linux/textsearch.h> 40 41 /* Alphabet size, use ASCII */ 42 #define ASIZE 256 43 44 #if 0 45 #define DEBUGP printk 46 #else 47 #define DEBUGP(args, format...) 48 #endif 49 50 struct ts_bm 51 { 52 u8 * pattern; 53 unsigned int patlen; 54 unsigned int bad_shift[ASIZE]; 55 unsigned int good_shift[]; 56 }; 57 58 static unsigned int bm_find(struct ts_config *conf, struct ts_state *state) 59 { 60 struct ts_bm *bm = ts_config_priv(conf); 61 unsigned int i, text_len, consumed = state->offset; 62 const u8 *text; 63 int shift = bm->patlen - 1, bs; 64 const u8 icase = conf->flags & TS_IGNORECASE; 65 66 for (;;) { 67 text_len = conf->get_next_block(consumed, &text, conf, state); 68 69 if (unlikely(text_len == 0)) 70 break; 71 72 while (shift < text_len) { 73 DEBUGP("Searching in position %d (%c)\n", 74 shift, text[shift]); 75 for (i = 0; i < bm->patlen; i++) 76 if ((icase ? toupper(text[shift-i]) 77 : text[shift-i]) 78 != 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, int flags) 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 if (flags & TS_IGNORECASE) 123 bm->bad_shift[tolower(bm->pattern[i])] 124 = bm->patlen - 1 - i; 125 } 126 127 /* Compute the good shift array, used to match reocurrences 128 * of a subpattern */ 129 bm->good_shift[0] = 1; 130 for (i = 1; i < bm->patlen; i++) 131 bm->good_shift[i] = bm->patlen; 132 for (i = bm->patlen-1, g = 1; i > 0; g++, i--) { 133 for (j = i-1; j >= 1-g ; j--) 134 if (subpattern(bm->pattern, i, j, g)) { 135 bm->good_shift[g] = bm->patlen-j-g; 136 break; 137 } 138 } 139 } 140 141 static struct ts_config *bm_init(const void *pattern, unsigned int len, 142 gfp_t gfp_mask, int flags) 143 { 144 struct ts_config *conf; 145 struct ts_bm *bm; 146 int i; 147 unsigned int prefix_tbl_len = len * sizeof(unsigned int); 148 size_t priv_size = sizeof(*bm) + len + prefix_tbl_len; 149 150 conf = alloc_ts_config(priv_size, gfp_mask); 151 if (IS_ERR(conf)) 152 return conf; 153 154 conf->flags = flags; 155 bm = ts_config_priv(conf); 156 bm->patlen = len; 157 bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len; 158 if (flags & TS_IGNORECASE) 159 for (i = 0; i < len; i++) 160 bm->pattern[i] = toupper(((u8 *)pattern)[i]); 161 else 162 memcpy(bm->pattern, pattern, len); 163 compute_prefix_tbl(bm, flags); 164 165 return conf; 166 } 167 168 static void *bm_get_pattern(struct ts_config *conf) 169 { 170 struct ts_bm *bm = ts_config_priv(conf); 171 return bm->pattern; 172 } 173 174 static unsigned int bm_get_pattern_len(struct ts_config *conf) 175 { 176 struct ts_bm *bm = ts_config_priv(conf); 177 return bm->patlen; 178 } 179 180 static struct ts_ops bm_ops = { 181 .name = "bm", 182 .find = bm_find, 183 .init = bm_init, 184 .get_pattern = bm_get_pattern, 185 .get_pattern_len = bm_get_pattern_len, 186 .owner = THIS_MODULE, 187 .list = LIST_HEAD_INIT(bm_ops.list) 188 }; 189 190 static int __init init_bm(void) 191 { 192 return textsearch_register(&bm_ops); 193 } 194 195 static void __exit exit_bm(void) 196 { 197 textsearch_unregister(&bm_ops); 198 } 199 200 MODULE_LICENSE("GPL"); 201 202 module_init(init_bm); 203 module_exit(exit_bm); 204