1// SPDX-License-Identifier: GPL-2.0-or-later
2/*
3 * lib/ts_bm.c		Boyer-Moore text search implementation
4 *
5 * Authors:	Pablo Neira Ayuso <pablo@eurodev.net>
6 *
7 * ==========================================================================
8 *
9 *   Implements Boyer-Moore string matching algorithm:
10 *
11 *   [1] A Fast String Searching Algorithm, R.S. Boyer and Moore.
12 *       Communications of the Association for Computing Machinery,
13 *       20(10), 1977, pp. 762-772.
14 *       https://www.cs.utexas.edu/users/moore/publications/fstrpos.pdf
15 *
16 *   [2] Handbook of Exact String Matching Algorithms, Thierry Lecroq, 2004
17 *       http://www-igm.univ-mlv.fr/~lecroq/string/string.pdf
18 *
19 *   Note: Since Boyer-Moore (BM) performs searches for matchings from right
20 *   to left, it's still possible that a matching could be spread over
21 *   multiple blocks, in that case this algorithm won't find any coincidence.
22 *
23 *   If you're willing to ensure that such thing won't ever happen, use the
24 *   Knuth-Pratt-Morris (KMP) implementation instead. In conclusion, choose
25 *   the proper string search algorithm depending on your setting.
26 *
27 *   Say you're using the textsearch infrastructure for filtering, NIDS or
28 *   any similar security focused purpose, then go KMP. Otherwise, if you
29 *   really care about performance, say you're classifying packets to apply
30 *   Quality of Service (QoS) policies, and you don't mind about possible
31 *   matchings spread over multiple fragments, then go BM.
32 */
33
34#include <linux/kernel.h>
35#include <linux/module.h>
36#include <linux/types.h>
37#include <linux/string.h>
38#include <linux/ctype.h>
39#include <linux/textsearch.h>
40
41/* Alphabet size, use ASCII */
42#define ASIZE 256
43
44#if 0
45#define DEBUGP printk
46#else
47#define DEBUGP(args, format...)
48#endif
49
50struct ts_bm
51{
52	u8 *		pattern;
53	unsigned int	patlen;
54	unsigned int 	bad_shift[ASIZE];
55	unsigned int	good_shift[];
56};
57
58static unsigned int matchpat(const u8 *pattern, unsigned int patlen,
59			     const u8 *text, bool icase)
60{
61	unsigned int i;
62
63	for (i = 0; i < patlen; i++) {
64		u8 t = *(text-i);
65
66		if (icase)
67			t = toupper(t);
68
69		if (t != *(pattern-i))
70			break;
71	}
72
73	return i;
74}
75
76static unsigned int bm_find(struct ts_config *conf, struct ts_state *state)
77{
78	struct ts_bm *bm = ts_config_priv(conf);
79	unsigned int i, text_len, consumed = state->offset;
80	const u8 *text;
81	int bs;
82	const u8 icase = conf->flags & TS_IGNORECASE;
83
84	for (;;) {
85		int shift = bm->patlen - 1;
86
87		text_len = conf->get_next_block(consumed, &text, conf, state);
88
89		if (unlikely(text_len == 0))
90			break;
91
92		while (shift < text_len) {
93			DEBUGP("Searching in position %d (%c)\n",
94			       shift, text[shift]);
95
96			i = matchpat(&bm->pattern[bm->patlen-1], bm->patlen,
97				     &text[shift], icase);
98			if (i == bm->patlen) {
99				/* London calling... */
100				DEBUGP("found!\n");
101				return consumed + (shift-(bm->patlen-1));
102			}
103
104			bs = bm->bad_shift[text[shift-i]];
105
106			/* Now jumping to... */
107			shift = max_t(int, shift-i+bs, shift+bm->good_shift[i]);
108		}
109		consumed += text_len;
110	}
111
112	return UINT_MAX;
113}
114
115static int subpattern(u8 *pattern, int i, int j, int g)
116{
117	int x = i+g-1, y = j+g-1, ret = 0;
118
119	while(pattern[x--] == pattern[y--]) {
120		if (y < 0) {
121			ret = 1;
122			break;
123		}
124		if (--g == 0) {
125			ret = pattern[i-1] != pattern[j-1];
126			break;
127		}
128	}
129
130	return ret;
131}
132
133static void compute_prefix_tbl(struct ts_bm *bm, int flags)
134{
135	int i, j, g;
136
137	for (i = 0; i < ASIZE; i++)
138		bm->bad_shift[i] = bm->patlen;
139	for (i = 0; i < bm->patlen - 1; i++) {
140		bm->bad_shift[bm->pattern[i]] = bm->patlen - 1 - i;
141		if (flags & TS_IGNORECASE)
142			bm->bad_shift[tolower(bm->pattern[i])]
143			    = bm->patlen - 1 - i;
144	}
145
146	/* Compute the good shift array, used to match reocurrences
147	 * of a subpattern */
148	bm->good_shift[0] = 1;
149	for (i = 1; i < bm->patlen; i++)
150		bm->good_shift[i] = bm->patlen;
151        for (i = bm->patlen-1, g = 1; i > 0; g++, i--) {
152		for (j = i-1; j >= 1-g ; j--)
153			if (subpattern(bm->pattern, i, j, g)) {
154				bm->good_shift[g] = bm->patlen-j-g;
155				break;
156			}
157	}
158}
159
160static struct ts_config *bm_init(const void *pattern, unsigned int len,
161				 gfp_t gfp_mask, int flags)
162{
163	struct ts_config *conf;
164	struct ts_bm *bm;
165	int i;
166	unsigned int prefix_tbl_len = len * sizeof(unsigned int);
167	size_t priv_size = sizeof(*bm) + len + prefix_tbl_len;
168
169	conf = alloc_ts_config(priv_size, gfp_mask);
170	if (IS_ERR(conf))
171		return conf;
172
173	conf->flags = flags;
174	bm = ts_config_priv(conf);
175	bm->patlen = len;
176	bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len;
177	if (flags & TS_IGNORECASE)
178		for (i = 0; i < len; i++)
179			bm->pattern[i] = toupper(((u8 *)pattern)[i]);
180	else
181		memcpy(bm->pattern, pattern, len);
182	compute_prefix_tbl(bm, flags);
183
184	return conf;
185}
186
187static void *bm_get_pattern(struct ts_config *conf)
188{
189	struct ts_bm *bm = ts_config_priv(conf);
190	return bm->pattern;
191}
192
193static unsigned int bm_get_pattern_len(struct ts_config *conf)
194{
195	struct ts_bm *bm = ts_config_priv(conf);
196	return bm->patlen;
197}
198
199static struct ts_ops bm_ops = {
200	.name		  = "bm",
201	.find		  = bm_find,
202	.init		  = bm_init,
203	.get_pattern	  = bm_get_pattern,
204	.get_pattern_len  = bm_get_pattern_len,
205	.owner		  = THIS_MODULE,
206	.list		  = LIST_HEAD_INIT(bm_ops.list)
207};
208
209static int __init init_bm(void)
210{
211	return textsearch_register(&bm_ops);
212}
213
214static void __exit exit_bm(void)
215{
216	textsearch_unregister(&bm_ops);
217}
218
219MODULE_LICENSE("GPL");
220
221module_init(init_bm);
222module_exit(exit_bm);
223