LXR linux/lib/ts

   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
  53struct ts_bm
  54{
  55        u8 *            pattern;
  56        unsigned int    patlen;
  57        unsigned int    bad_shift[ASIZE];
  58        unsigned int    good_shift[0];
  59};
  60
  61static 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
  85next:                   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
  96static 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
 114static 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
 137static 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
 158static void *bm_get_pattern(struct ts_config *conf)
 159{
 160        struct ts_bm *bm = ts_config_priv(conf);
 161        return bm->pattern;
 162}
 163
 164static 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
 170static 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
 180static int __init init_bm(void)
 181{
 182        return textsearch_register(&bm_ops);
 183}
 184
 185static void __exit exit_bm(void)
 186{
 187        textsearch_unregister(&bm_ops);
 188}
 189
 190MODULE_LICENSE("GPL");
 191
 192module_init(init_bm);
 193module_exit(exit_bm);
 194