Commit 3f330317 authored by Pablo Neira Ayuso's avatar Pablo Neira Ayuso Committed by David S. Miller
Browse files

[TEXTSEARCH]: Fix broken good shift array calculation in Boyer-Moore 



The current logic does not calculate correctly the good shift array:
Let x be the pattern that is being searched. Let y be the block of data. 
The good shift array aligns the segment:

x[i+1 ... m-1] = y[i+j+1 ... j+m-1]

with its rightmost occurrence in x that fulfils x[i] neq y[i+j].

In previous version, the good shift array for the pattern ANPANMAN is:
[1, 8, 3, 8, 8, 8, 8, 8]
and should be:
[1, 8, 3, 6, 6, 6, 6, 6]

Signed-off-by: default avatarPablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent f00c401b

lib/ts_bm.c

+25 −15
Original line number Diff line number Diff line
@@ -94,10 +94,28 @@ next: bs = bm->bad_shift[text[shift-i]]; 
	return UINT_MAX;

}



static int subpattern(u8 *pattern, int i, int j, int g)

{

	int x = i+g-1, y = j+g-1, ret = 0;



	while(pattern[x--] == pattern[y--]) {

		if (y < 0) {

			ret = 1;

			break;

		}

		if (--g == 0) {

			ret = pattern[i-1] != pattern[j-1];

			break;

		}

	}



	return ret;

}



static void compute_prefix_tbl(struct ts_bm *bm, const u8 *pattern,

			       unsigned int len)

{

	int i, j, ended, l[ASIZE];

	int i, j, g;



	for (i = 0; i < ASIZE; i++)

		bm->bad_shift[i] = len;
@@ -106,23 +124,15 @@ static void compute_prefix_tbl(struct ts_bm *bm, const u8 *pattern, 


	/* 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;

        for (i = bm->patlen-1, g = 1; i > 0; g++, i--) {

		for (j = i-1; j >= 1-g ; j--)

			if (subpattern(bm->pattern, i, j, g)) {

				bm->good_shift[g] = bm->patlen-j-g;

				break;

			}

	}

}

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