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 * https://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 bs; 64 const u8 icase = conf->flags & TS_IGNORECASE; 65 66 for (;;) { 67 int shift = bm->patlen - 1; 68 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 ((icase ? toupper(text[shift-i]) 79 : text[shift-i]) 80 != bm->pattern[bm->patlen-1-i]) 81 goto next; 82 83 /* London calling... */ 84 DEBUGP("found!\n"); 85 return consumed + (shift-(bm->patlen-1)); 86 87 next: bs = bm->bad_shift[text[shift-i]]; 88 89 /* Now jumping to... */ 90 shift = max_t(int, shift-i+bs, shift+bm->good_shift[i]); 91 } 92 consumed += text_len; 93 } 94 95 return UINT_MAX; 96 } 97 98 static int subpattern(u8 *pattern, int i, int j, int g) 99 { 100 int x = i+g-1, y = j+g-1, ret = 0; 101 102 while(pattern[x--] == pattern[y--]) { 103 if (y < 0) { 104 ret = 1; 105 break; 106 } 107 if (--g == 0) { 108 ret = pattern[i-1] != pattern[j-1]; 109 break; 110 } 111 } 112 113 return ret; 114 } 115 116 static void compute_prefix_tbl(struct ts_bm *bm, int flags) 117 { 118 int i, j, g; 119 120 for (i = 0; i < ASIZE; i++) 121 bm->bad_shift[i] = bm->patlen; 122 for (i = 0; i < bm->patlen - 1; i++) { 123 bm->bad_shift[bm->pattern[i]] = bm->patlen - 1 - i; 124 if (flags & TS_IGNORECASE) 125 bm->bad_shift[tolower(bm->pattern[i])] 126 = bm->patlen - 1 - i; 127 } 128 129 /* Compute the good shift array, used to match reocurrences 130 * of a subpattern */ 131 bm->good_shift[0] = 1; 132 for (i = 1; i < bm->patlen; i++) 133 bm->good_shift[i] = bm->patlen; 134 for (i = bm->patlen-1, g = 1; i > 0; g++, i--) { 135 for (j = i-1; j >= 1-g ; j--) 136 if (subpattern(bm->pattern, i, j, g)) { 137 bm->good_shift[g] = bm->patlen-j-g; 138 break; 139 } 140 } 141 } 142 143 static struct ts_config *bm_init(const void *pattern, unsigned int len, 144 gfp_t gfp_mask, int flags) 145 { 146 struct ts_config *conf; 147 struct ts_bm *bm; 148 int i; 149 unsigned int prefix_tbl_len = len * sizeof(unsigned int); 150 size_t priv_size = sizeof(*bm) + len + prefix_tbl_len; 151 152 conf = alloc_ts_config(priv_size, gfp_mask); 153 if (IS_ERR(conf)) 154 return conf; 155 156 conf->flags = flags; 157 bm = ts_config_priv(conf); 158 bm->patlen = len; 159 bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len; 160 if (flags & TS_IGNORECASE) 161 for (i = 0; i < len; i++) 162 bm->pattern[i] = toupper(((u8 *)pattern)[i]); 163 else 164 memcpy(bm->pattern, pattern, len); 165 compute_prefix_tbl(bm, flags); 166 167 return conf; 168 } 169 170 static void *bm_get_pattern(struct ts_config *conf) 171 { 172 struct ts_bm *bm = ts_config_priv(conf); 173 return bm->pattern; 174 } 175 176 static unsigned int bm_get_pattern_len(struct ts_config *conf) 177 { 178 struct ts_bm *bm = ts_config_priv(conf); 179 return bm->patlen; 180 } 181 182 static struct ts_ops bm_ops = { 183 .name = "bm", 184 .find = bm_find, 185 .init = bm_init, 186 .get_pattern = bm_get_pattern, 187 .get_pattern_len = bm_get_pattern_len, 188 .owner = THIS_MODULE, 189 .list = LIST_HEAD_INIT(bm_ops.list) 190 }; 191 192 static int __init init_bm(void) 193 { 194 return textsearch_register(&bm_ops); 195 } 196 197 static void __exit exit_bm(void) 198 { 199 textsearch_unregister(&bm_ops); 200 } 201 202 MODULE_LICENSE("GPL"); 203 204 module_init(init_bm); 205 module_exit(exit_bm); 206