net: ipv6: Fix IPv6 prefix comparision

If the prefix length % 8 is not 0, then the remaining
bit length was calculated incorrectly and the prefixes
were claimed to match even though they might not be the
same.

Adding a test cases for testing this properly.

Coverity-CID: 157591

Change-Id: I9cb5a73d5cc211ec183176400fa5e2dfd209e2da
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>

1 files changed, 15 insertions, 6 deletions

diff --git a/include/net/net_ip.h b/include/net/net_ip.h
index f058bc552..e055d71c9 100644
--- a/include/net/net_ip.h
+++ b/include/net/net_ip.h
@@ -360,27 +360,36 @@ static inline bool net_is_my_ipv6_maddr(struct in6_addr *maddr)
  * @param addr2 Second IPv6 address.
  * @param length Prefix length (max length is 128).
  *
- * @return True if addresses are the same, False otherwise.
+ * @return True if IPv6 prefixes are the same, False otherwise.
  */
 static inline bool net_is_ipv6_prefix(const uint8_t *addr1,
 				      const uint8_t *addr2,
 				      uint8_t length)
 {
 	uint8_t bits = 128 - length;
-	uint8_t bytes = bits / 8;
+	uint8_t bytes = length / 8;
 	uint8_t remain = bits % 8;
+	uint8_t mask;
 
 	if (length > 128) {
 		return false;
 	}
 
-	if (memcmp(addr1, addr2, 16 - bytes)) {
+	if (memcmp(addr1, addr2, bytes)) {
 		return false;
 	}
 
-	return ((addr1[16 - bytes] & ((8 - remain) << 8))
-		==
-		(addr2[16 - bytes] & ((8 - remain) << 8)));
+	if (!remain) {
+		/* No remaining bits, the prefixes are the same as first
+		 * bytes are the same.
+		 */
+		return true;
+	}
+
+	/* Create a mask that has remaining most significant bits set */
+	mask = ((0xff << (8 - remain)) ^ 0xff) << remain;
+
+	return (addr1[bytes] & mask) == (addr2[bytes] & mask);
 }
 
 /**