root/lib/ts_bm.c

DEFINITIONS

1. bm_find
2. subpattern
3. compute_prefix_tbl
4. bm_init
5. bm_get_pattern
6. bm_get_pattern_len
7. init_bm
8. exit_bm

   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 
  50 struct ts_bm
  51 {
  52         u8 *            pattern;
  53         unsigned int    patlen;
  54         unsigned int    bad_shift[ASIZE];
  55         unsigned int    good_shift[0];
  56 };
  57 
  58 static 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 
  85 next:                   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 
  96 static 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 
 114 static 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 
 141 static 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 
 168 static void *bm_get_pattern(struct ts_config *conf)
 169 {
 170         struct ts_bm *bm = ts_config_priv(conf);
 171         return bm->pattern;
 172 }
 173 
 174 static 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 
 180 static 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 
 190 static int __init init_bm(void)
 191 {
 192         return textsearch_register(&bm_ops);
 193 }
 194 
 195 static void __exit exit_bm(void)
 196 {
 197         textsearch_unregister(&bm_ops);
 198 }
 199 
 200 MODULE_LICENSE("GPL");
 201 
 202 module_init(init_bm);
 203 module_exit(exit_bm);