lib/ts_bm.c

   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/textsearch.h>
  43
  44 /* Alphabet size, use ASCII */
  45 #define ASIZE 256
  46
  47 #if 0
  48 #define DEBUGP printk
  49 #else
  50 #define DEBUGP(args, format...)
  51 #endif
  52
  53 struct ts_bm
  54 {
  55         u8 *            pattern;
  56         unsigned int    patlen;
  57         unsigned int    bad_shift[ASIZE];
  58         unsigned int    good_shift[0];
  59 };
  60
  61 static unsigned int bm_find(struct ts_config *conf, struct ts_state *state)
  62 {
  63         struct ts_bm *bm = ts_config_priv(conf);
  64         unsigned int i, text_len, consumed = state->offset;
  65         const u8 *text;
  66         int shift = bm->patlen, bs;
  67
  68         for (;;) {
  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 (text[shift-i] != 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)
 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
 123         /* Compute the good shift array, used to match reocurrences
 124          * of a subpattern */
 125         bm->good_shift[0] = 1;
 126         for (i = 1; i < bm->patlen; i++)
 127                 bm->good_shift[i] = bm->patlen;
 128         for (i = bm->patlen-1, g = 1; i > 0; g++, i--) {
 129                 for (j = i-1; j >= 1-g ; j--)
 130                         if (subpattern(bm->pattern, i, j, g)) {
 131                                 bm->good_shift[g] = bm->patlen-j-g;
 132                                 break;
 133                         }
 134         }
 135 }
 136
 137 static struct ts_config *bm_init(const void *pattern, unsigned int len,
 138                                  gfp_t gfp_mask)
 139 {
 140         struct ts_config *conf;
 141         struct ts_bm *bm;
 142         unsigned int prefix_tbl_len = len * sizeof(unsigned int);
 143         size_t priv_size = sizeof(*bm) + len + prefix_tbl_len;
 144
 145         conf = alloc_ts_config(priv_size, gfp_mask);
 146         if (IS_ERR(conf))
 147                 return conf;
 148
 149         bm = ts_config_priv(conf);
 150         bm->patlen = len;
 151         bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len;
 152         memcpy(bm->pattern, pattern, len);
 153         compute_prefix_tbl(bm);
 154
 155         return conf;
 156 }
 157
 158 static void *bm_get_pattern(struct ts_config *conf)
 159 {
 160         struct ts_bm *bm = ts_config_priv(conf);
 161         return bm->pattern;
 162 }
 163
 164 static unsigned int bm_get_pattern_len(struct ts_config *conf)
 165 {
 166         struct ts_bm *bm = ts_config_priv(conf);
 167         return bm->patlen;
 168 }
 169
 170 static struct ts_ops bm_ops = {
 171         .name             = "bm",
 172         .find             = bm_find,
 173         .init             = bm_init,
 174         .get_pattern      = bm_get_pattern,
 175         .get_pattern_len  = bm_get_pattern_len,
 176         .owner            = THIS_MODULE,
 177         .list             = LIST_HEAD_INIT(bm_ops.list)
 178 };
 179
 180 static int __init init_bm(void)
 181 {
 182         return textsearch_register(&bm_ops);
 183 }
 184
 185 static void __exit exit_bm(void)
 186 {
 187         textsearch_unregister(&bm_ops);
 188 }
 189
 190 MODULE_LICENSE("GPL");
 191
 192 module_init(init_bm);
 193 module_exit(exit_bm);