The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/lib/ts_bm.c

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    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/ctype.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 - 1, bs;
   68         const u8 icase = conf->flags & TS_IGNORECASE;
   69 
   70         for (;;) {
   71                 text_len = conf->get_next_block(consumed, &text, conf, state);
   72 
   73                 if (unlikely(text_len == 0))
   74                         break;
   75 
   76                 while (shift < text_len) {
   77                         DEBUGP("Searching in position %d (%c)\n", 
   78                                 shift, text[shift]);
   79                         for (i = 0; i < bm->patlen; i++) 
   80                                 if ((icase ? toupper(text[shift-i])
   81                                     : text[shift-i])
   82                                         != bm->pattern[bm->patlen-1-i])
   83                                      goto next;
   84 
   85                         /* London calling... */
   86                         DEBUGP("found!\n");
   87                         return consumed += (shift-(bm->patlen-1));
   88 
   89 next:                   bs = bm->bad_shift[text[shift-i]];
   90 
   91                         /* Now jumping to... */
   92                         shift = max_t(int, shift-i+bs, shift+bm->good_shift[i]);
   93                 }
   94                 consumed += text_len;
   95         }
   96 
   97         return UINT_MAX;
   98 }
   99 
  100 static int subpattern(u8 *pattern, int i, int j, int g)
  101 {
  102         int x = i+g-1, y = j+g-1, ret = 0;
  103 
  104         while(pattern[x--] == pattern[y--]) {
  105                 if (y < 0) {
  106                         ret = 1;
  107                         break;
  108                 }
  109                 if (--g == 0) {
  110                         ret = pattern[i-1] != pattern[j-1];
  111                         break;
  112                 }
  113         }
  114 
  115         return ret;
  116 }
  117 
  118 static void compute_prefix_tbl(struct ts_bm *bm, int flags)
  119 {
  120         int i, j, g;
  121 
  122         for (i = 0; i < ASIZE; i++)
  123                 bm->bad_shift[i] = bm->patlen;
  124         for (i = 0; i < bm->patlen - 1; i++) {
  125                 bm->bad_shift[bm->pattern[i]] = bm->patlen - 1 - i;
  126                 if (flags & TS_IGNORECASE)
  127                         bm->bad_shift[tolower(bm->pattern[i])]
  128                             = bm->patlen - 1 - i;
  129         }
  130 
  131         /* Compute the good shift array, used to match reocurrences 
  132          * of a subpattern */
  133         bm->good_shift[0] = 1;
  134         for (i = 1; i < bm->patlen; i++)
  135                 bm->good_shift[i] = bm->patlen;
  136         for (i = bm->patlen-1, g = 1; i > 0; g++, i--) {
  137                 for (j = i-1; j >= 1-g ; j--)
  138                         if (subpattern(bm->pattern, i, j, g)) {
  139                                 bm->good_shift[g] = bm->patlen-j-g;
  140                                 break;
  141                         }
  142         }
  143 }
  144 
  145 static struct ts_config *bm_init(const void *pattern, unsigned int len,
  146                                  gfp_t gfp_mask, int flags)
  147 {
  148         struct ts_config *conf;
  149         struct ts_bm *bm;
  150         int i;
  151         unsigned int prefix_tbl_len = len * sizeof(unsigned int);
  152         size_t priv_size = sizeof(*bm) + len + prefix_tbl_len;
  153 
  154         conf = alloc_ts_config(priv_size, gfp_mask);
  155         if (IS_ERR(conf))
  156                 return conf;
  157 
  158         conf->flags = flags;
  159         bm = ts_config_priv(conf);
  160         bm->patlen = len;
  161         bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len;
  162         if (flags & TS_IGNORECASE)
  163                 for (i = 0; i < len; i++)
  164                         bm->pattern[i] = toupper(((u8 *)pattern)[i]);
  165         else
  166                 memcpy(bm->pattern, pattern, len);
  167         compute_prefix_tbl(bm, flags);
  168 
  169         return conf;
  170 }
  171 
  172 static void *bm_get_pattern(struct ts_config *conf)
  173 {
  174         struct ts_bm *bm = ts_config_priv(conf);
  175         return bm->pattern;
  176 }
  177 
  178 static unsigned int bm_get_pattern_len(struct ts_config *conf)
  179 {
  180         struct ts_bm *bm = ts_config_priv(conf);
  181         return bm->patlen;
  182 }
  183 
  184 static struct ts_ops bm_ops = {
  185         .name             = "bm",
  186         .find             = bm_find,
  187         .init             = bm_init,
  188         .get_pattern      = bm_get_pattern,
  189         .get_pattern_len  = bm_get_pattern_len,
  190         .owner            = THIS_MODULE,
  191         .list             = LIST_HEAD_INIT(bm_ops.list)
  192 };
  193 
  194 static int __init init_bm(void)
  195 {
  196         return textsearch_register(&bm_ops);
  197 }
  198 
  199 static void __exit exit_bm(void)
  200 {
  201         textsearch_unregister(&bm_ops);
  202 }
  203 
  204 MODULE_LICENSE("GPL");
  205 
  206 module_init(init_bm);
  207 module_exit(exit_bm);

Cache object: d92853b11cccb86d8a81a5e7077c5543


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