netdev - Re: virtio_net: Set correct gso->hdr

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Date:	Fri, 5 Jun 2009 13:51:05 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: virtio_net: Set correct gso->hdr_len

On Thu, 4 Jun 2009 08:29:18 pm Herbert Xu wrote:
> Hi:
>
> virtio_net: Set correct gso->hdr_len
>
> Through a bug in the tun driver, I noticed that virtio_net is
> producing bogus hdr_len values.  In particular, it only includes
> the IP header in the linear area, and excludes the entire TCP
> header.  This causes the TCP header to be copied twice for each
> packet.  (The bug omitted the second copy :)
>
> This patch corrects this.
>
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 4d1d479..1c9cedd 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -470,7 +470,7 @@ static int xmit_skb(struct virtnet_info *vi, struct
> sk_buff *skb) }
>
>  	if (skb_is_gso(skb)) {
> -		hdr->hdr_len = skb_transport_header(skb) - skb->data;
> +		hdr->hdr_len = skb_headlen(skb);

Ouch!

But are we allowed to make the assumption that skb_headlen() == 
skb_transport_header(skb) + sizeof(transport header) ?  Or should we be 
checking that here?

Also how did this ever work?  Why are we getting packets through at all?

Damn, I'm away from my test boxes an I really want to benchmark this fix.
Rusty.
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