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Diffstat (limited to 'lib/ts_bm.c')
| -rw-r--r-- | lib/ts_bm.c | 185 |
1 files changed, 185 insertions, 0 deletions
diff --git a/lib/ts_bm.c b/lib/ts_bm.c new file mode 100644 index 000000000000..2cc79112ecc3 --- /dev/null +++ b/lib/ts_bm.c @@ -0,0 +1,185 @@ +/* + * lib/ts_bm.c Boyer-Moore text search implementation + * + * This program is free software; you can redistribute it and/or + * modify it under the terms of the GNU General Public License + * as published by the Free Software Foundation; either version + * 2 of the License, or (at your option) any later version. + * + * Authors: Pablo Neira Ayuso <pablo@eurodev.net> + * + * ========================================================================== + * + * Implements Boyer-Moore string matching algorithm: + * + * [1] A Fast String Searching Algorithm, R.S. Boyer and Moore. + * Communications of the Association for Computing Machinery, + * 20(10), 1977, pp. 762-772. + * http://www.cs.utexas.edu/users/moore/publications/fstrpos.pdf + * + * [2] Handbook of Exact String Matching Algorithms, Thierry Lecroq, 2004 + * http://www-igm.univ-mlv.fr/~lecroq/string/string.pdf + * + * Note: Since Boyer-Moore (BM) performs searches for matchings from right + * to left, it's still possible that a matching could be spread over + * multiple blocks, in that case this algorithm won't find any coincidence. + * + * If you're willing to ensure that such thing won't ever happen, use the + * Knuth-Pratt-Morris (KMP) implementation instead. In conclusion, choose + * the proper string search algorithm depending on your setting. + * + * Say you're using the textsearch infrastructure for filtering, NIDS or + * any similar security focused purpose, then go KMP. Otherwise, if you + * really care about performance, say you're classifying packets to apply + * Quality of Service (QoS) policies, and you don't mind about possible + * matchings spread over multiple fragments, then go BM. + */ + +#include <linux/config.h> +#include <linux/kernel.h> +#include <linux/module.h> +#include <linux/types.h> +#include <linux/string.h> +#include <linux/textsearch.h> + +/* Alphabet size, use ASCII */ +#define ASIZE 256 + +#if 0 +#define DEBUGP printk +#else +#define DEBUGP(args, format...) +#endif + +struct ts_bm +{ + u8 * pattern; + unsigned int patlen; + unsigned int bad_shift[ASIZE]; + unsigned int good_shift[0]; +}; + +static unsigned int bm_find(struct ts_config *conf, struct ts_state *state) +{ + struct ts_bm *bm = ts_config_priv(conf); + unsigned int i, text_len, consumed = state->offset; + const u8 *text; + int shift = bm->patlen, bs; + + for (;;) { + text_len = conf->get_next_block(consumed, &text, conf, state); + + if (unlikely(text_len == 0)) + break; + + while (shift < text_len) { + DEBUGP("Searching in position %d (%c)\n", + shift, text[shift]); + for (i = 0; i < bm->patlen; i++) + if (text[shift-i] != bm->pattern[bm->patlen-1-i]) + goto next; + + /* London calling... */ + DEBUGP("found!\n"); + return consumed += (shift-(bm->patlen-1)); + +next: bs = bm->bad_shift[text[shift-i]]; + + /* Now jumping to... */ + shift = max_t(int, shift-i+bs, shift+bm->good_shift[i]); + } + consumed += text_len; + } + + return UINT_MAX; +} + +static void compute_prefix_tbl(struct ts_bm *bm, const u8 *pattern, + unsigned int len) +{ + int i, j, ended, l[ASIZE]; + + for (i = 0; i < ASIZE; i++) + bm->bad_shift[i] = len; + for (i = 0; i < len - 1; i++) + bm->bad_shift[pattern[i]] = len - 1 - i; + + /* Compute the good shift array, used to match reocurrences + * of a subpattern */ + for (i = 1; i < bm->patlen; i++) { + for (j = 0; j < bm->patlen && bm->pattern[bm->patlen - 1 - j] + == bm->pattern[bm->patlen - 1 - i - j]; j++); + l[i] = j; + } + + bm->good_shift[0] = 1; + for (i = 1; i < bm->patlen; i++) + bm->good_shift[i] = bm->patlen; + for (i = bm->patlen - 1; i > 0; i--) + bm->good_shift[l[i]] = i; + ended = 0; + for (i = 0; i < bm->patlen; i++) { + if (l[i] == bm->patlen - 1 - i) + ended = i; + if (ended) + bm->good_shift[i] = ended; + } +} + +static struct ts_config *bm_init(const void *pattern, unsigned int len, + int gfp_mask) +{ + struct ts_config *conf; + struct ts_bm *bm; + unsigned int prefix_tbl_len = len * sizeof(unsigned int); + size_t priv_size = sizeof(*bm) + len + prefix_tbl_len; + + conf = alloc_ts_config(priv_size, gfp_mask); + if (IS_ERR(conf)) + return conf; + + bm = ts_config_priv(conf); + bm->patlen = len; + bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len; + compute_prefix_tbl(bm, pattern, len); + memcpy(bm->pattern, pattern, len); + + return conf; +} + +static void *bm_get_pattern(struct ts_config *conf) +{ + struct ts_bm *bm = ts_config_priv(conf); + return bm->pattern; +} + +static unsigned int bm_get_pattern_len(struct ts_config *conf) +{ + struct ts_bm *bm = ts_config_priv(conf); + return bm->patlen; +} + +static struct ts_ops bm_ops = { + .name = "bm", + .find = bm_find, + .init = bm_init, + .get_pattern = bm_get_pattern, + .get_pattern_len = bm_get_pattern_len, + .owner = THIS_MODULE, + .list = LIST_HEAD_INIT(bm_ops.list) +}; + +static int __init init_bm(void) +{ + return textsearch_register(&bm_ops); +} + +static void __exit exit_bm(void) +{ + textsearch_unregister(&bm_ops); +} + +MODULE_LICENSE("GPL"); + +module_init(init_bm); +module_exit(exit_bm); |