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/config.h>
  39 #include <linux/kernel.h>
  40 #include <linux/module.h>
  41 #include <linux/types.h>
  42 #include <linux/string.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, bs;
  68
  69         for (;;) {
  70                 text_len = conf->get_next_block(consumed, &text, conf, state);
  71
  72                 if (unlikely(text_len == 0))
  73                         break;
  74
  75                 while (shift < text_len) {
  76                         DEBUGP("Searching in position %d (%c)\n",
  77                                 shift, text[shift]);
  78                         for (i = 0; i < bm->patlen; i++)
  79                              if (text[shift-i] != bm->pattern[bm->patlen-1-i])
  80                                      goto next;
  81
  82                         /* London calling... */
  83                         DEBUGP("found!\n");
  84                         return consumed += (shift-(bm->patlen-1));
  85
  86 next:                   bs = bm->bad_shift[text[shift-i]];
  87
  88                         /* Now jumping to... */
  89                         shift = max_t(int, shift-i+bs, shift+bm->good_shift[i]);
  90                 }
  91                 consumed += text_len;
  92         }
  93
  94         return UINT_MAX;
  95 }
  96
  97 static void compute_prefix_tbl(struct ts_bm *bm, const u8 *pattern,
  98                                unsigned int len)
  99 {
 100         int i, j, ended, l[ASIZE];
 101
 102         for (i = 0; i < ASIZE; i++)
 103                 bm->bad_shift[i] = len;
 104         for (i = 0; i < len - 1; i++)
 105                 bm->bad_shift[pattern[i]] = len - 1 - i;
 106
 107         /* Compute the good shift array, used to match reocurrences
 108          * of a subpattern */
 109         for (i = 1; i < bm->patlen; i++) {
 110                 for (j = 0; j < bm->patlen && bm->pattern[bm->patlen - 1 - j]
 111                                 == bm->pattern[bm->patlen - 1 - i - j]; j++);
 112                 l[i] = j;
 113         }
 114
 115         bm->good_shift[0] = 1;
 116         for (i = 1; i < bm->patlen; i++)
 117                 bm->good_shift[i] = bm->patlen;
 118         for (i = bm->patlen - 1; i > 0; i--)
 119                 bm->good_shift[l[i]] = i;
 120         ended = 0;
 121         for (i = 0; i < bm->patlen; i++) {
 122                 if (l[i] == bm->patlen - 1 - i)
 123                         ended = i;
 124                 if (ended)
 125                         bm->good_shift[i] = ended;
 126         }
 127 }
 128
 129 static struct ts_config *bm_init(const void *pattern, unsigned int len,
 130                                  gfp_t gfp_mask)
 131 {
 132         struct ts_config *conf;
 133         struct ts_bm *bm;
 134         unsigned int prefix_tbl_len = len * sizeof(unsigned int);
 135         size_t priv_size = sizeof(*bm) + len + prefix_tbl_len;
 136
 137         conf = alloc_ts_config(priv_size, gfp_mask);
 138         if (IS_ERR(conf))
 139                 return conf;
 140
 141         bm = ts_config_priv(conf);
 142         bm->patlen = len;
 143         bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len;
 144         compute_prefix_tbl(bm, pattern, len);
 145         memcpy(bm->pattern, pattern, len);
 146
 147         return conf;
 148 }
 149
 150 static void *bm_get_pattern(struct ts_config *conf)
 151 {
 152         struct ts_bm *bm = ts_config_priv(conf);
 153         return bm->pattern;
 154 }
 155
 156 static unsigned int bm_get_pattern_len(struct ts_config *conf)
 157 {
 158         struct ts_bm *bm = ts_config_priv(conf);
 159         return bm->patlen;
 160 }
 161
 162 static struct ts_ops bm_ops = {
 163         .name             = "bm",
 164         .find             = bm_find,
 165         .init             = bm_init,
 166         .get_pattern      = bm_get_pattern,
 167         .get_pattern_len  = bm_get_pattern_len,
 168         .owner            = THIS_MODULE,
 169         .list             = LIST_HEAD_INIT(bm_ops.list)
 170 };
 171
 172 static int __init init_bm(void)
 173 {
 174         return textsearch_register(&bm_ops);
 175 }
 176
 177 static void __exit exit_bm(void)
 178 {
 179         textsearch_unregister(&bm_ops);
 180 }
 181
 182 MODULE_LICENSE("GPL");
 183
 184 module_init(init_bm);
 185 module_exit(exit_bm);