udp_tap_handler() currently skips outbound packets if they have a payload
length of zero.  This is not correct, since in a datagram protocol zero
length packets still have meaning.

Adjust this to correctly forward the zero-length packets by using a msghdr
with msg_iovlen == 0.

Bugzilla: https://bugs.passt.top/show_bug.cgi?id=19

Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au>
---
 udp.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/udp.c b/udp.c
index c4ebecc..caa852a 100644
--- a/udp.c
+++ b/udp.c
@@ -1075,19 +1075,19 @@ int udp_tap_handler(struct ctx *c, int af, const void *addr,
 		uh_send = packet_get(p, i, 0, sizeof(*uh), &len);
 		if (!uh_send)
 			return p->count;
+
+		mm[i].msg_hdr.msg_name = sa;
+		mm[i].msg_hdr.msg_namelen = sl;
+		count++;
+
 		if (!len)
 			continue;
 
 		m[i].iov_base = (char *)(uh_send + 1);
 		m[i].iov_len = len;
 
-		mm[i].msg_hdr.msg_name = sa;
-		mm[i].msg_hdr.msg_namelen = sl;
-
 		mm[i].msg_hdr.msg_iov = m + i;
 		mm[i].msg_hdr.msg_iovlen = 1;
-
-		count++;
 	}
 
 	count = sendmmsg(s, mm, count, MSG_NOSIGNAL);
-- 
2.37.3