Root/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
54struct ts_bm
55{
56    u8 * pattern;
57    unsigned int patlen;
58    unsigned int bad_shift[ASIZE];
59    unsigned int good_shift[0];
60};
61
62static 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
89next: 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
100static 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
118static 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
145static 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
172static void *bm_get_pattern(struct ts_config *conf)
173{
174    struct ts_bm *bm = ts_config_priv(conf);
175    return bm->pattern;
176}
177
178static 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
184static 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
194static int __init init_bm(void)
195{
196    return textsearch_register(&bm_ops);
197}
198
199static void __exit exit_bm(void)
200{
201    textsearch_unregister(&bm_ops);
202}
203
204MODULE_LICENSE("GPL");
205
206module_init(init_bm);
207module_exit(exit_bm);
208

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