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 matchpat(const u8 *pattern, unsigned int patlen, 59 const u8 *text, bool icase) 60 { 61 unsigned int i; 62 63 for (i = 0; i < patlen; i++) { 64 u8 t = *(text-i); 65 66 if (icase) 67 t = toupper(t); 68 69 if (t != *(pattern-i)) 70 break; 71 } 72 73 return i; 74 } 75 76 static unsigned int bm_find(struct ts_config *conf, struct ts_state *state) 77 { 78 struct ts_bm *bm = ts_config_priv(conf); 79 unsigned int i, text_len, consumed = state->offset; 80 const u8 *text; 81 int bs; 82 const u8 icase = conf->flags & TS_IGNORECASE; 83 84 for (;;) { 85 int shift = bm->patlen - 1; 86 87 text_len = conf->get_next_block(consumed, &text, conf, state); 88 89 if (unlikely(text_len == 0)) 90 break; 91 92 while (shift < text_len) { 93 DEBUGP("Searching in position %d (%c)\n", 94 shift, text[shift]); 95 96 i = matchpat(&bm->pattern[bm->patlen-1], bm->patlen, 97 &text[shift], icase); 98 if (i == bm->patlen) { 99 /* London calling... */ 100 DEBUGP("found!\n"); 101 return consumed + (shift-(bm->patlen-1)); 102 } 103 104 bs = bm->bad_shift[text[shift-i]]; 105 106 /* Now jumping to... */ 107 shift = max_t(int, shift-i+bs, shift+bm->good_shift[i]); 108 } 109 consumed += text_len; 110 } 111 112 return UINT_MAX; 113 } 114 115 static int subpattern(u8 *pattern, int i, int j, int g) 116 { 117 int x = i+g-1, y = j+g-1, ret = 0; 118 119 while(pattern[x--] == pattern[y--]) { 120 if (y < 0) { 121 ret = 1; 122 break; 123 } 124 if (--g == 0) { 125 ret = pattern[i-1] != pattern[j-1]; 126 break; 127 } 128 } 129 130 return ret; 131 } 132 133 static void compute_prefix_tbl(struct ts_bm *bm, int flags) 134 { 135 int i, j, g; 136 137 for (i = 0; i < ASIZE; i++) 138 bm->bad_shift[i] = bm->patlen; 139 for (i = 0; i < bm->patlen - 1; i++) { 140 bm->bad_shift[bm->pattern[i]] = bm->patlen - 1 - i; 141 if (flags & TS_IGNORECASE) 142 bm->bad_shift[tolower(bm->pattern[i])] 143 = bm->patlen - 1 - i; 144 } 145 146 /* Compute the good shift array, used to match reocurrences 147 * of a subpattern */ 148 bm->good_shift[0] = 1; 149 for (i = 1; i < bm->patlen; i++) 150 bm->good_shift[i] = bm->patlen; 151 for (i = bm->patlen-1, g = 1; i > 0; g++, i--) { 152 for (j = i-1; j >= 1-g ; j--) 153 if (subpattern(bm->pattern, i, j, g)) { 154 bm->good_shift[g] = bm->patlen-j-g; 155 break; 156 } 157 } 158 } 159 160 static struct ts_config *bm_init(const void *pattern, unsigned int len, 161 gfp_t gfp_mask, int flags) 162 { 163 struct ts_config *conf; 164 struct ts_bm *bm; 165 int i; 166 unsigned int prefix_tbl_len; 167 size_t priv_size; 168 169 /* Zero-length patterns would underflow bm_find()'s initial shift. */ 170 if (unlikely(!len)) 171 return ERR_PTR(-EINVAL); 172 173 /* 174 * bm->pattern is stored immediately after the good_shift[] table. 175 * Reject lengths that would wrap while sizing either region. 176 */ 177 if (unlikely(check_mul_overflow(len, sizeof(*bm->good_shift), 178 &prefix_tbl_len) || 179 check_add_overflow(sizeof(*bm), (size_t)len, &priv_size) || 180 check_add_overflow(priv_size, prefix_tbl_len, &priv_size))) 181 return ERR_PTR(-EINVAL); 182 183 conf = alloc_ts_config(priv_size, gfp_mask); 184 if (IS_ERR(conf)) 185 return conf; 186 187 conf->flags = flags; 188 bm = ts_config_priv(conf); 189 bm->patlen = len; 190 bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len; 191 if (flags & TS_IGNORECASE) 192 for (i = 0; i < len; i++) 193 bm->pattern[i] = toupper(((u8 *)pattern)[i]); 194 else 195 memcpy(bm->pattern, pattern, len); 196 compute_prefix_tbl(bm, flags); 197 198 return conf; 199 } 200 201 static void *bm_get_pattern(struct ts_config *conf) 202 { 203 struct ts_bm *bm = ts_config_priv(conf); 204 return bm->pattern; 205 } 206 207 static unsigned int bm_get_pattern_len(struct ts_config *conf) 208 { 209 struct ts_bm *bm = ts_config_priv(conf); 210 return bm->patlen; 211 } 212 213 static struct ts_ops bm_ops = { 214 .name = "bm", 215 .find = bm_find, 216 .init = bm_init, 217 .get_pattern = bm_get_pattern, 218 .get_pattern_len = bm_get_pattern_len, 219 .owner = THIS_MODULE, 220 .list = LIST_HEAD_INIT(bm_ops.list) 221 }; 222 223 static int __init init_bm(void) 224 { 225 return textsearch_register(&bm_ops); 226 } 227 228 static void __exit exit_bm(void) 229 { 230 textsearch_unregister(&bm_ops); 231 } 232 233 MODULE_DESCRIPTION("Boyer-Moore text search implementation"); 234 MODULE_LICENSE("GPL"); 235 236 module_init(init_bm); 237 module_exit(exit_bm); 238