[PATCH 1/1] virtio-net: fix bug 1451 aka "assert(!virtio_net_get_subqueue(nc)->async_tx.elem); "

[Alexey Dobriyan]

Don't send zero length packets in virtio_net_flush_tx().

Reproducer from https://gitlab.com/qemu-project/qemu/-/issues/1451
creates small packet (1 segment, len = 10 == n->guest_hdr_len),
destroys queue.

"if (n->host_hdr_len != n->guest_hdr_len)" is triggered, if body creates
zero length/zero segment packet, because there is nothing after guest
header.

qemu_sendv_packet_async() tries to send it.

slirp discards it because it is smaller than Ethernet header,
but returns 0.

0 length is propagated upwards and is interpreted as "packet has been sent"
which is terrible because queue is being destroyed, nothing has been sent,
nobody is waiting for TX to complete and assert it triggered.

Signed-off-by: Alexey Dobriyan <adobriyan@yandex-team.ru>
---
 hw/net/virtio-net.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 58014a92ad..258633f885 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -2765,18 +2765,14 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
         out_sg = elem->out_sg;
         if (out_num < 1) {
             virtio_error(vdev, "virtio-net header not in first element");
-            virtqueue_detach_element(q->tx_vq, elem, 0);
-            g_free(elem);
-            return -EINVAL;
+            goto detach;
         }
 
         if (n->has_vnet_hdr) {
             if (iov_to_buf(out_sg, out_num, 0, &vhdr, n->guest_hdr_len) <
                 n->guest_hdr_len) {
                 virtio_error(vdev, "virtio-net header incorrect");
-                virtqueue_detach_element(q->tx_vq, elem, 0);
-                g_free(elem);
-                return -EINVAL;
+                goto detach;
             }
             if (n->needs_vnet_hdr_swap) {
                 virtio_net_hdr_swap(vdev, (void *) &vhdr);
@@ -2807,6 +2803,11 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
                              n->guest_hdr_len, -1);
             out_num = sg_num;
             out_sg = sg;
+
+            if (iov_size(out_sg, out_num) == 0) {
+                virtio_error(vdev, "virtio-net nothing to send");
+                goto detach;
+            }
         }
 
         ret = qemu_sendv_packet_async(qemu_get_subqueue(n->nic, queue_index),
@@ -2827,6 +2828,11 @@ drop:
         }
     }
     return num_packets;
+
+detach:
+    virtqueue_detach_element(q->tx_vq, elem, 0);
+    g_free(elem);
+    return -EINVAL;
 }
 
 static void virtio_net_tx_timer(void *opaque);
-- 
2.34.1