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
bm_find(struct ts_config * conf,struct ts_state * state)58 static unsigned int bm_find(struct ts_config *conf, struct ts_state *state)
59 {
60 struct ts_bm *bm = ts_config_priv(conf);
61 unsigned int i, text_len, consumed = state->offset;
62 const u8 *text;
63 int shift = bm->patlen - 1, bs;
64 const u8 icase = conf->flags & TS_IGNORECASE;
65
66 for (;;) {
67 text_len = conf->get_next_block(consumed, &text, conf, state);
68
69 if (unlikely(text_len == 0))
70 break;
71
72 while (shift < text_len) {
73 DEBUGP("Searching in position %d (%c)\n",
74 shift, text[shift]);
75 for (i = 0; i < bm->patlen; i++)
76 if ((icase ? toupper(text[shift-i])
77 : text[shift-i])
78 != bm->pattern[bm->patlen-1-i])
79 goto next;
80
81 /* London calling... */
82 DEBUGP("found!\n");
83 return consumed + (shift-(bm->patlen-1));
84
85 next: bs = bm->bad_shift[text[shift-i]];
86
87 /* Now jumping to... */
88 shift = max_t(int, shift-i+bs, shift+bm->good_shift[i]);
89 }
90 consumed += text_len;
91 }
92
93 return UINT_MAX;
94 }
95
subpattern(u8 * pattern,int i,int j,int g)96 static int subpattern(u8 *pattern, int i, int j, int g)
97 {
98 int x = i+g-1, y = j+g-1, ret = 0;
99
100 while(pattern[x--] == pattern[y--]) {
101 if (y < 0) {
102 ret = 1;
103 break;
104 }
105 if (--g == 0) {
106 ret = pattern[i-1] != pattern[j-1];
107 break;
108 }
109 }
110
111 return ret;
112 }
113
compute_prefix_tbl(struct ts_bm * bm,int flags)114 static void compute_prefix_tbl(struct ts_bm *bm, int flags)
115 {
116 int i, j, g;
117
118 for (i = 0; i < ASIZE; i++)
119 bm->bad_shift[i] = bm->patlen;
120 for (i = 0; i < bm->patlen - 1; i++) {
121 bm->bad_shift[bm->pattern[i]] = bm->patlen - 1 - i;
122 if (flags & TS_IGNORECASE)
123 bm->bad_shift[tolower(bm->pattern[i])]
124 = bm->patlen - 1 - i;
125 }
126
127 /* Compute the good shift array, used to match reocurrences
128 * of a subpattern */
129 bm->good_shift[0] = 1;
130 for (i = 1; i < bm->patlen; i++)
131 bm->good_shift[i] = bm->patlen;
132 for (i = bm->patlen-1, g = 1; i > 0; g++, i--) {
133 for (j = i-1; j >= 1-g ; j--)
134 if (subpattern(bm->pattern, i, j, g)) {
135 bm->good_shift[g] = bm->patlen-j-g;
136 break;
137 }
138 }
139 }
140
bm_init(const void * pattern,unsigned int len,gfp_t gfp_mask,int flags)141 static struct ts_config *bm_init(const void *pattern, unsigned int len,
142 gfp_t gfp_mask, int flags)
143 {
144 struct ts_config *conf;
145 struct ts_bm *bm;
146 int i;
147 unsigned int prefix_tbl_len = len * sizeof(unsigned int);
148 size_t priv_size = sizeof(*bm) + len + prefix_tbl_len;
149
150 conf = alloc_ts_config(priv_size, gfp_mask);
151 if (IS_ERR(conf))
152 return conf;
153
154 conf->flags = flags;
155 bm = ts_config_priv(conf);
156 bm->patlen = len;
157 bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len;
158 if (flags & TS_IGNORECASE)
159 for (i = 0; i < len; i++)
160 bm->pattern[i] = toupper(((u8 *)pattern)[i]);
161 else
162 memcpy(bm->pattern, pattern, len);
163 compute_prefix_tbl(bm, flags);
164
165 return conf;
166 }
167
bm_get_pattern(struct ts_config * conf)168 static void *bm_get_pattern(struct ts_config *conf)
169 {
170 struct ts_bm *bm = ts_config_priv(conf);
171 return bm->pattern;
172 }
173
bm_get_pattern_len(struct ts_config * conf)174 static unsigned int bm_get_pattern_len(struct ts_config *conf)
175 {
176 struct ts_bm *bm = ts_config_priv(conf);
177 return bm->patlen;
178 }
179
180 static struct ts_ops bm_ops = {
181 .name = "bm",
182 .find = bm_find,
183 .init = bm_init,
184 .get_pattern = bm_get_pattern,
185 .get_pattern_len = bm_get_pattern_len,
186 .owner = THIS_MODULE,
187 .list = LIST_HEAD_INIT(bm_ops.list)
188 };
189
init_bm(void)190 static int __init init_bm(void)
191 {
192 return textsearch_register(&bm_ops);
193 }
194
exit_bm(void)195 static void __exit exit_bm(void)
196 {
197 textsearch_unregister(&bm_ops);
198 }
199
200 MODULE_LICENSE("GPL");
201
202 module_init(init_bm);
203 module_exit(exit_bm);
204