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Linux/lib/ts_bm.c

  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);
208 

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