On Tue, Apr 09, 2024 at 02:51:38AM -0400, Michael S. Tsirkin wrote:
> On Fri, Apr 05, 2024 at 02:20:15PM +0300, Alexey Dobriyan wrote:
> > 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.
>
> won't the same issue trigger with a 1 byte packet
> as opposed to a 0 byte packet though?
I don't think so. Only "return 0" is problematic from qemu_sendv_packet_async().
->deliver hooks are supposed to return total length, so 1 should be fine.
> > 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 <adobri...@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
>
