[LIB]: Boyer-Moore extension for textsearch infrastructure strike #2
Attached the implementation of the Boyer-Moore string search
algorithm for the new textsearch infrastructure.
I've added as well a note about the limitations that this approach
presents, as Thomas has remarked.
Signed-off-by: Pablo Neira Ayuso <pablo@eurodev.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/lib/Kconfig b/lib/Kconfig
index eeb429a..b62f685 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -72,6 +72,16 @@
config TEXTSEARCH_KMP
tristate
+config TEXTSEARCH_BM
+ depends on TEXTSEARCH
+ tristate "Boyer-Moore"
+ help
+ Say Y here if you want to be able to search text using the
+ Boyer-Moore textsearch algorithm.
+
+ To compile this code as a module, choose M here: the
+ module will be called ts_bm.
+
config TEXTSEARCH_FSM
tristate
diff --git a/lib/Makefile b/lib/Makefile
index f28d903..52f8338 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -38,6 +38,7 @@
obj-$(CONFIG_TEXTSEARCH) += textsearch.o
obj-$(CONFIG_TEXTSEARCH_KMP) += ts_kmp.o
+obj-$(CONFIG_TEXTSEARCH_BM) += ts_bm.o
obj-$(CONFIG_TEXTSEARCH_FSM) += ts_fsm.o
hostprogs-y := gen_crc32table
diff --git a/lib/ts_bm.c b/lib/ts_bm.c
new file mode 100644
index 0000000..2cc79112
--- /dev/null
+++ b/lib/ts_bm.c
@@ -0,0 +1,185 @@
+/*
+ * lib/ts_bm.c Boyer-Moore text search implementation
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ *
+ * Authors: Pablo Neira Ayuso <pablo@eurodev.net>
+ *
+ * ==========================================================================
+ *
+ * Implements Boyer-Moore string matching algorithm:
+ *
+ * [1] A Fast String Searching Algorithm, R.S. Boyer and Moore.
+ * Communications of the Association for Computing Machinery,
+ * 20(10), 1977, pp. 762-772.
+ * http://www.cs.utexas.edu/users/moore/publications/fstrpos.pdf
+ *
+ * [2] Handbook of Exact String Matching Algorithms, Thierry Lecroq, 2004
+ * http://www-igm.univ-mlv.fr/~lecroq/string/string.pdf
+ *
+ * Note: Since Boyer-Moore (BM) performs searches for matchings from right
+ * to left, it's still possible that a matching could be spread over
+ * multiple blocks, in that case this algorithm won't find any coincidence.
+ *
+ * If you're willing to ensure that such thing won't ever happen, use the
+ * Knuth-Pratt-Morris (KMP) implementation instead. In conclusion, choose
+ * the proper string search algorithm depending on your setting.
+ *
+ * Say you're using the textsearch infrastructure for filtering, NIDS or
+ * any similar security focused purpose, then go KMP. Otherwise, if you
+ * really care about performance, say you're classifying packets to apply
+ * Quality of Service (QoS) policies, and you don't mind about possible
+ * matchings spread over multiple fragments, then go BM.
+ */
+
+#include <linux/config.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/types.h>
+#include <linux/string.h>
+#include <linux/textsearch.h>
+
+/* Alphabet size, use ASCII */
+#define ASIZE 256
+
+#if 0
+#define DEBUGP printk
+#else
+#define DEBUGP(args, format...)
+#endif
+
+struct ts_bm
+{
+ u8 * pattern;
+ unsigned int patlen;
+ unsigned int bad_shift[ASIZE];
+ unsigned int good_shift[0];
+};
+
+static unsigned int bm_find(struct ts_config *conf, struct ts_state *state)
+{
+ struct ts_bm *bm = ts_config_priv(conf);
+ unsigned int i, text_len, consumed = state->offset;
+ const u8 *text;
+ int shift = bm->patlen, bs;
+
+ for (;;) {
+ text_len = conf->get_next_block(consumed, &text, conf, state);
+
+ if (unlikely(text_len == 0))
+ break;
+
+ while (shift < text_len) {
+ DEBUGP("Searching in position %d (%c)\n",
+ shift, text[shift]);
+ for (i = 0; i < bm->patlen; i++)
+ if (text[shift-i] != bm->pattern[bm->patlen-1-i])
+ goto next;
+
+ /* London calling... */
+ DEBUGP("found!\n");
+ return consumed += (shift-(bm->patlen-1));
+
+next: bs = bm->bad_shift[text[shift-i]];
+
+ /* Now jumping to... */
+ shift = max_t(int, shift-i+bs, shift+bm->good_shift[i]);
+ }
+ consumed += text_len;
+ }
+
+ return UINT_MAX;
+}
+
+static void compute_prefix_tbl(struct ts_bm *bm, const u8 *pattern,
+ unsigned int len)
+{
+ int i, j, ended, l[ASIZE];
+
+ for (i = 0; i < ASIZE; i++)
+ bm->bad_shift[i] = len;
+ for (i = 0; i < len - 1; i++)
+ bm->bad_shift[pattern[i]] = len - 1 - i;
+
+ /* Compute the good shift array, used to match reocurrences
+ * of a subpattern */
+ for (i = 1; i < bm->patlen; i++) {
+ for (j = 0; j < bm->patlen && bm->pattern[bm->patlen - 1 - j]
+ == bm->pattern[bm->patlen - 1 - i - j]; j++);
+ l[i] = j;
+ }
+
+ bm->good_shift[0] = 1;
+ for (i = 1; i < bm->patlen; i++)
+ bm->good_shift[i] = bm->patlen;
+ for (i = bm->patlen - 1; i > 0; i--)
+ bm->good_shift[l[i]] = i;
+ ended = 0;
+ for (i = 0; i < bm->patlen; i++) {
+ if (l[i] == bm->patlen - 1 - i)
+ ended = i;
+ if (ended)
+ bm->good_shift[i] = ended;
+ }
+}
+
+static struct ts_config *bm_init(const void *pattern, unsigned int len,
+ int gfp_mask)
+{
+ struct ts_config *conf;
+ struct ts_bm *bm;
+ unsigned int prefix_tbl_len = len * sizeof(unsigned int);
+ size_t priv_size = sizeof(*bm) + len + prefix_tbl_len;
+
+ conf = alloc_ts_config(priv_size, gfp_mask);
+ if (IS_ERR(conf))
+ return conf;
+
+ bm = ts_config_priv(conf);
+ bm->patlen = len;
+ bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len;
+ compute_prefix_tbl(bm, pattern, len);
+ memcpy(bm->pattern, pattern, len);
+
+ return conf;
+}
+
+static void *bm_get_pattern(struct ts_config *conf)
+{
+ struct ts_bm *bm = ts_config_priv(conf);
+ return bm->pattern;
+}
+
+static unsigned int bm_get_pattern_len(struct ts_config *conf)
+{
+ struct ts_bm *bm = ts_config_priv(conf);
+ return bm->patlen;
+}
+
+static struct ts_ops bm_ops = {
+ .name = "bm",
+ .find = bm_find,
+ .init = bm_init,
+ .get_pattern = bm_get_pattern,
+ .get_pattern_len = bm_get_pattern_len,
+ .owner = THIS_MODULE,
+ .list = LIST_HEAD_INIT(bm_ops.list)
+};
+
+static int __init init_bm(void)
+{
+ return textsearch_register(&bm_ops);
+}
+
+static void __exit exit_bm(void)
+{
+ textsearch_unregister(&bm_ops);
+}
+
+MODULE_LICENSE("GPL");
+
+module_init(init_bm);
+module_exit(exit_bm);