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