linux/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
  54struct ts_bm
  55{
  56        u8 *            pattern;
  57        unsigned int    patlen;
  58        unsigned int    bad_shift[ASIZE];
  59        unsigned int    good_shift[0];
  60};
  61
  62static 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
  89next:                   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
 100static 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
 118static 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
 145static 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
 172static void *bm_get_pattern(struct ts_config *conf)
 173{
 174        struct ts_bm *bm = ts_config_priv(conf);
 175        return bm->pattern;
 176}
 177
 178static 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
 184static 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
 194static int __init init_bm(void)
 195{
 196        return textsearch_register(&bm_ops);
 197}
 198
 199static void __exit exit_bm(void)
 200{
 201        textsearch_unregister(&bm_ops);
 202}
 203
 204MODULE_LICENSE("GPL");
 205
 206module_init(init_bm);
 207module_exit(exit_bm);
 208
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