Re: [PATCH v3] virtio: Work around frames incorrectly marked as gso

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Feb 24, 2020 at 01:25:50PM +0000, anton.ivanov@cambridgegreys.com wrote:
> From: Anton Ivanov <anton.ivanov@cambridgegreys.com>
> 
> Some of the locally generated frames marked as GSO which
> arrive at virtio_net_hdr_from_skb() have no GSO_TYPE, no
> fragments (data_len = 0) and length significantly shorter
> than the MTU (752 in my experiments).
> 
> This is observed on raw sockets reading off vEth interfaces
> in all 4.x and 5.x kernels. The frames are reported as
> invalid, while they are in fact gso-less frames.
> 
> The easiest way to reproduce is to connect a User Mode
> Linux instance to the host using the vector raw transport
> and a vEth interface. Vector raw uses recvmmsg/sendmmsg
> with virtio headers on af_packet sockets. When running iperf
> between the UML and the host, UML regularly complains about
> EINVAL return from recvmmsg.
> 
> This patch marks the vnet header as non-GSO instead of
> reporting it as invalid.
> 
> Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  include/linux/virtio_net.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index 0d1fe9297ac6..2c99c752cb20 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -98,10 +98,11 @@ static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
>  					  bool has_data_valid,
>  					  int vlan_hlen)
>  {
> +	struct skb_shared_info *sinfo = skb_shinfo(skb);
> +
>  	memset(hdr, 0, sizeof(*hdr));   /* no info leak */
>  
> -	if (skb_is_gso(skb)) {
> -		struct skb_shared_info *sinfo = skb_shinfo(skb);
> +	if (skb_is_gso(skb) && sinfo->gso_type) {
>  
>  		/* This is a hint as to how much should be linear. */
>  		hdr->hdr_len = __cpu_to_virtio16(little_endian,
> -- 
> 2.20.1