cfg80211: fix cmp_ies

When comparing two items by IE, the sort order
wasn't stable, which could lead to issues in the
rbtree. Make it stable by making a missing IE
sort before a present IE.

Also sort by length first if it differs and then
by contents.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/wireless/scan.c

@@ -259,17 +259,20 @@ static int cmp_ies(u8 num, u8 *ies1, size_t len1, u8 *ies2, size_t len2)
 {
 	const u8 *ie1 = cfg80211_find_ie(num, ies1, len1);
 	const u8 *ie2 = cfg80211_find_ie(num, ies2, len2);
-	int r;
 
+	/* equal if both missing */
 	if (!ie1 && !ie2)
 		return 0;
-	if (!ie1 || !ie2)
+	/* sort missing IE before (left of) present IE */
+	if (!ie1)
 		return -1;
+	if (!ie2)
+		return 1;
 
-	r = memcmp(ie1 + 2, ie2 + 2, min(ie1[1], ie2[1]));
-	if (r == 0 && ie1[1] != ie2[1])
+	/* sort by length first, then by contents */
+	if (ie1[1] != ie2[1])
 		return ie2[1] - ie1[1];
-	return r;
+	return memcmp(ie1 + 2, ie2 + 2, ie1[1]);
 }
 
 static bool is_bss(struct cfg80211_bss *a,