linux/lib/ts_bm.c
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   1// SPDX-License-Identifier: GPL-2.0-or-later
   2/*
   3 * lib/ts_bm.c          Boyer-Moore text search implementation
   4 *
   5 * Authors:     Pablo Neira Ayuso <pablo@eurodev.net>
   6 *
   7 * ==========================================================================
   8 * 
   9 *   Implements Boyer-Moore string matching algorithm:
  10 *
  11 *   [1] A Fast String Searching Algorithm, R.S. Boyer and Moore.
  12 *       Communications of the Association for Computing Machinery, 
  13 *       20(10), 1977, pp. 762-772.
  14 *       http://www.cs.utexas.edu/users/moore/publications/fstrpos.pdf
  15 *
  16 *   [2] Handbook of Exact String Matching Algorithms, Thierry Lecroq, 2004
  17 *       http://www-igm.univ-mlv.fr/~lecroq/string/string.pdf
  18 *
  19 *   Note: Since Boyer-Moore (BM) performs searches for matchings from right 
  20 *   to left, it's still possible that a matching could be spread over 
  21 *   multiple blocks, in that case this algorithm won't find any coincidence.
  22 *   
  23 *   If you're willing to ensure that such thing won't ever happen, use the
  24 *   Knuth-Pratt-Morris (KMP) implementation instead. In conclusion, choose 
  25 *   the proper string search algorithm depending on your setting. 
  26 *
  27 *   Say you're using the textsearch infrastructure for filtering, NIDS or 
  28 *   any similar security focused purpose, then go KMP. Otherwise, if you 
  29 *   really care about performance, say you're classifying packets to apply
  30 *   Quality of Service (QoS) policies, and you don't mind about possible
  31 *   matchings spread over multiple fragments, then go BM.
  32 */
  33
  34#include <linux/kernel.h>
  35#include <linux/module.h>
  36#include <linux/types.h>
  37#include <linux/string.h>
  38#include <linux/ctype.h>
  39#include <linux/textsearch.h>
  40
  41/* Alphabet size, use ASCII */
  42#define ASIZE 256
  43
  44#if 0
  45#define DEBUGP printk
  46#else
  47#define DEBUGP(args, format...)
  48#endif
  49
  50struct ts_bm
  51{
  52        u8 *            pattern;
  53        unsigned int    patlen;
  54        unsigned int    bad_shift[ASIZE];
  55        unsigned int    good_shift[0];
  56};
  57
  58static unsigned int bm_find(struct ts_config *conf, struct ts_state *state)
  59{
  60        struct ts_bm *bm = ts_config_priv(conf);
  61        unsigned int i, text_len, consumed = state->offset;
  62        const u8 *text;
  63        int shift = bm->patlen - 1, bs;
  64        const u8 icase = conf->flags & TS_IGNORECASE;
  65
  66        for (;;) {
  67                text_len = conf->get_next_block(consumed, &text, conf, state);
  68
  69                if (unlikely(text_len == 0))
  70                        break;
  71
  72                while (shift < text_len) {
  73                        DEBUGP("Searching in position %d (%c)\n", 
  74                                shift, text[shift]);
  75                        for (i = 0; i < bm->patlen; i++) 
  76                                if ((icase ? toupper(text[shift-i])
  77                                    : text[shift-i])
  78                                        != 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, int flags)
 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                if (flags & TS_IGNORECASE)
 123                        bm->bad_shift[tolower(bm->pattern[i])]
 124                            = bm->patlen - 1 - i;
 125        }
 126
 127        /* Compute the good shift array, used to match reocurrences 
 128         * of a subpattern */
 129        bm->good_shift[0] = 1;
 130        for (i = 1; i < bm->patlen; i++)
 131                bm->good_shift[i] = bm->patlen;
 132        for (i = bm->patlen-1, g = 1; i > 0; g++, i--) {
 133                for (j = i-1; j >= 1-g ; j--)
 134                        if (subpattern(bm->pattern, i, j, g)) {
 135                                bm->good_shift[g] = bm->patlen-j-g;
 136                                break;
 137                        }
 138        }
 139}
 140
 141static struct ts_config *bm_init(const void *pattern, unsigned int len,
 142                                 gfp_t gfp_mask, int flags)
 143{
 144        struct ts_config *conf;
 145        struct ts_bm *bm;
 146        int i;
 147        unsigned int prefix_tbl_len = len * sizeof(unsigned int);
 148        size_t priv_size = sizeof(*bm) + len + prefix_tbl_len;
 149
 150        conf = alloc_ts_config(priv_size, gfp_mask);
 151        if (IS_ERR(conf))
 152                return conf;
 153
 154        conf->flags = flags;
 155        bm = ts_config_priv(conf);
 156        bm->patlen = len;
 157        bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len;
 158        if (flags & TS_IGNORECASE)
 159                for (i = 0; i < len; i++)
 160                        bm->pattern[i] = toupper(((u8 *)pattern)[i]);
 161        else
 162                memcpy(bm->pattern, pattern, len);
 163        compute_prefix_tbl(bm, flags);
 164
 165        return conf;
 166}
 167
 168static void *bm_get_pattern(struct ts_config *conf)
 169{
 170        struct ts_bm *bm = ts_config_priv(conf);
 171        return bm->pattern;
 172}
 173
 174static unsigned int bm_get_pattern_len(struct ts_config *conf)
 175{
 176        struct ts_bm *bm = ts_config_priv(conf);
 177        return bm->patlen;
 178}
 179
 180static struct ts_ops bm_ops = {
 181        .name             = "bm",
 182        .find             = bm_find,
 183        .init             = bm_init,
 184        .get_pattern      = bm_get_pattern,
 185        .get_pattern_len  = bm_get_pattern_len,
 186        .owner            = THIS_MODULE,
 187        .list             = LIST_HEAD_INIT(bm_ops.list)
 188};
 189
 190static int __init init_bm(void)
 191{
 192        return textsearch_register(&bm_ops);
 193}
 194
 195static void __exit exit_bm(void)
 196{
 197        textsearch_unregister(&bm_ops);
 198}
 199
 200MODULE_LICENSE("GPL");
 201
 202module_init(init_bm);
 203module_exit(exit_bm);
 204