ts_bm.c

Go to the documentation of this file.

/*
 * 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 <[email protected]>
 *
 * ==========================================================================
 * 
 *   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/kernel.h>
#include <linux/module.h>
#include <linux/types.h>
#include <linux/string.h>
#include <linux/ctype.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 - 1, bs;
    const u8 icase = conf->flags & TS_IGNORECASE;

    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 ((icase ? toupper(text[shift-i])
                    : 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 int subpattern(u8 *pattern, int i, int j, int g)
{
    int x = i+g-1, y = j+g-1, ret = 0;

    while(pattern[x--] == pattern[y--]) {
        if (y < 0) {
            ret = 1;
            break;
        }
        if (--g == 0) {
            ret = pattern[i-1] != pattern[j-1];
            break;
        }
    }

    return ret;
}

static void compute_prefix_tbl(struct ts_bm *bm, int flags)
{
    int i, j, g;

    for (i = 0; i < ASIZE; i++)
        bm->bad_shift[i] = bm->patlen;
    for (i = 0; i < bm->patlen - 1; i++) {
        bm->bad_shift[bm->pattern[i]] = bm->patlen - 1 - i;
        if (flags & TS_IGNORECASE)
            bm->bad_shift[tolower(bm->pattern[i])]
                = bm->patlen - 1 - i;
    }

    /* Compute the good shift array, used to match reocurrences 
     * of a subpattern */
    bm->good_shift[0] = 1;
    for (i = 1; i < bm->patlen; i++)
        bm->good_shift[i] = bm->patlen;
        for (i = bm->patlen-1, g = 1; i > 0; g++, i--) {
        for (j = i-1; j >= 1-g ; j--)
            if (subpattern(bm->pattern, i, j, g)) {
                bm->good_shift[g] = bm->patlen-j-g;
                break;
            }
    }
}

static struct ts_config *bm_init(const void *pattern, unsigned int len,
                 gfp_t gfp_mask, int flags)
{
    struct ts_config *conf;
    struct ts_bm *bm;
    int i;
    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;

    conf->flags = flags;
    bm = ts_config_priv(conf);
    bm->patlen = len;
    bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len;
    if (flags & TS_IGNORECASE)
        for (i = 0; i < len; i++)
            bm->pattern[i] = toupper(((u8 *)pattern)[i]);
    else
        memcpy(bm->pattern, pattern, len);
    compute_prefix_tbl(bm, flags);

    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);