[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>

Author: Pablo Neira Ayuso

lib/Kconfig

@@ -72,6 +72,16 @@ config TEXTSEARCH
 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
 

lib/Makefile

@@ -38,6 +38,7 @@ obj-$(CONFIG_REED_SOLOMON) += reed_solomon/
 
 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

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);