Subject: Re: [PATCH net-next 21/33] virtio_net: add XDP frame size in two code paths

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 30 Apr 2020 12:14:58 +0200

On Tue, 28 Apr 2020 17:50:11 +0800
Jason Wang <[email-masked]> wrote:

> We tried to reserve space for vnet header before
> xdp.data_hard_start. But this is useless since the packet could be
> modified by XDP which may invalidate the information stored in the
> header and there's no way for XDP to know the existence of the vnet
> header currently.

I think this is wrong.  We can reserve space for vnet header before
xdp.data_hard_start, and it will be safe and cannot be modified by XDP
as BPF program cannot access data before xdp.data_hard_start.
 
> So let's just not reserve space for vnet header in this case.
> 
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/net/virtio_net.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 2fe7a3188282..9bdaf2425e6e 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -681,8 +681,8 @@ static struct sk_buff *receive_small(struct net_device *dev,
>  			page = xdp_page;
>  		}
>  
> -		xdp.data_hard_start = buf + VIRTNET_RX_PAD + vi->hdr_len;
> -		xdp.data = xdp.data_hard_start + xdp_headroom;
> +		xdp.data_hard_start = buf + VIRTNET_RX_PAD;
> +		xdp.data = xdp.data_hard_start + xdp_headroom + vi->hdr_len;;

I think this is wrong.  You are exposing the vi header info, which will
be overwritten when code creates an xdp_frame.  I find it very fragile,
as later in the code the vi header info is copied, but only if xdp_prog
is not loaded, so in principle it's okay, but when someone later
figures out that we want to look at this area, we will be in trouble
(and I expect this will be needed when we work towards multi-buffer
XDP).

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer