Avoid possible bug with strict aliasing

Tell code we are breaking strict aliasing.
Most of the time works but with some strong optimisation
compiler could produce not working code because of the
aliasing breakage.

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>

1 files changed, 11 insertions, 4 deletions

diff --git a/tun.c b/tun.c
index 4ab93b3..5d0a30e 100644
--- a/tun.c
+++ b/tun.c
@@ -43,6 +43,12 @@
 #define uh_sum check
 #endif
 
+#if defined(__GNUC__)
+#  define MAY_ALIAS __attribute__((__may_alias__))
+#else
+#  define MAY_ALIAS
+#endif
+
 static int tun_fd = -1;
 static int tun_fd_back = -1;
 static int remote_sock = -1;
@@ -345,13 +351,14 @@ reduce_cksum(unsigned sum)
 static unsigned
 cksum(const void *pkt, size_t len, unsigned int start)
 {
-	const uint16_t *data = (const uint16_t *) pkt;
+	typedef union MAY_ALIAS { uint16_t full; uint8_t part; } u16;
+	const u16 *data = (const u16 *) pkt;
 	unsigned sum = start;
 
-	for (; len >= 2; len -= 2)
-		sum += *data++;
+	for (; len >= 2; len -= 2, ++data)
+		sum += data->full;
 	if (len)
-		sum += ntohs(*((const uint8_t *)data) << 8);
+		sum += ntohs(data->part << 8);
 	sum = reduce_cksum(sum);
 	sum = reduce_cksum(sum);
 	return sum;