virtio-net: truncating packet

virtio net attempts to peek into virtio queue to
determine that we have enough space for the complete
packet to fit. However, it fails to account for space
consumed by virtio net header when it does this,
under stress this results in a failure
with the message 'truncating packet'.

redhat bz 591494.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio-net.c

@@ -532,16 +532,17 @@ static ssize_t virtio_net_receive(VLANClientState *nc, const uint8_t *buf, size_
     if (!virtio_net_can_receive(&n->nic->nc))
         return -1;
 
-    if (!virtio_net_has_buffers(n, size))
+    /* hdr_len refers to the header we supply to the guest */
+    hdr_len = n->mergeable_rx_bufs ?
+        sizeof(struct virtio_net_hdr_mrg_rxbuf) : sizeof(struct virtio_net_hdr);
+
+
+    if (!virtio_net_has_buffers(n, size + hdr_len))
         return 0;
 
     if (!receive_filter(n, buf, size))
         return size;
 
-    /* hdr_len refers to the header we supply to the guest */
-    hdr_len = n->mergeable_rx_bufs ?
-        sizeof(struct virtio_net_hdr_mrg_rxbuf) : sizeof(struct virtio_net_hdr);
-
     offset = i = 0;
 
     while (offset < size) {
@@ -555,7 +556,9 @@ static ssize_t virtio_net_receive(VLANClientState *nc, const uint8_t *buf, size_
             virtqueue_pop(n->rx_vq, &elem) == 0) {
             if (i == 0)
                 return -1;
-            fprintf(stderr, "virtio-net truncating packet\n");
+            fprintf(stderr, "virtio-net truncating packet: "
+                    "offset %zd, size %zd, hdr_len %zd\n",
+                    offset, size, hdr_len);
             exit(1);
         }