[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: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 25 insertions, 15 deletions

diff --git a/lib/ts_bm.c b/lib/ts_bm.c
index 8a8b3a16133e..c4c1ac5fbd1a 100644
--- a/lib/ts_bm.c
+++ b/lib/ts_bm.c
@@ -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;
+                        }
         }
 }