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
50 struct ts_bm
51 {
52 u8 * pattern;
53 unsigned int patlen;
54 unsigned int bad_shift[ASIZE];
55 unsigned int good_shift[];
56 };
57
58 static 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
76 static 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
115 static 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
133 static 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
160 static 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
187 static void *bm_get_pattern(struct ts_config *conf)
188 {
189 struct ts_bm *bm = ts_config_priv(conf);
190 return bm->pattern;
191 }
192
193 static 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
199 static 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
209 static int __init init_bm(void)
210 {
211 return textsearch_register(&bm_ops);
212 }
213
214 static void __exit exit_bm(void)
215 {
216 textsearch_unregister(&bm_ops);
217 }
218
219 MODULE_DESCRIPTION("Boyer-Moore text search implementation");
220 MODULE_LICENSE("GPL");
221
222 module_init(init_bm);
223 module_exit(exit_bm);
224
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