[PATCH v2 2/3] udp: Don't drop zero-length outbound UDP packets

[David Gibson <david@gibson.dropbear.id.au>]

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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 | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/udp.c b/udp.c
index 864e7b2..0b4e134 100644
--- a/udp.c
+++ b/udp.c
@@ -1075,17 +1075,20 @@ 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;
-		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;
+		if (len) {
+			m[i].iov_base = (char *)(uh_send + 1);
+			m[i].iov_len = len;
+
+			mm[i].msg_hdr.msg_iov = m + i;
+			mm[i].msg_hdr.msg_iovlen = 1;
+		} else {
+			mm[i].msg_hdr.msg_iov = NULL;
+			mm[i].msg_hdr.msg_iovlen = 0;
+		}
 
 		mm[i].msg_hdr.msg_control = NULL;
 		mm[i].msg_hdr.msg_controllen = 0;
-- 
@@ -1075,17 +1075,20 @@ 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;
-		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;
+		if (len) {
+			m[i].iov_base = (char *)(uh_send + 1);
+			m[i].iov_len = len;
+
+			mm[i].msg_hdr.msg_iov = m + i;
+			mm[i].msg_hdr.msg_iovlen = 1;
+		} else {
+			mm[i].msg_hdr.msg_iov = NULL;
+			mm[i].msg_hdr.msg_iovlen = 0;
+		}
 
 		mm[i].msg_hdr.msg_control = NULL;
 		mm[i].msg_hdr.msg_controllen = 0;
-- 
2.37.3