Subject: Re: [PATCH] xen: netfront: Drop GSO SKBs which do not have csum_blank.

From: Ian Campbell <[email-masked]>
Date: Sat, 22 Jan 2011 09:43:12 +0000

On Sat, 2011-01-22 at 00:58 +0000, Jeremy Fitzhardinge wrote: 
> On 01/05/2011 05:23 AM, Ian Campbell wrote:
> > The Linux network stack expects all GSO SKBs to have ip_summed ==
> > CHECKSUM_PARTIAL (which implies that the frame contains a partial
> > checksum) and the Xen network ring protocol similarly expects an SKB
> > which has GSO set to also have NETRX_csum_blank (which also implies a
> > partial checksum). Therefore drop such frames on receive otherwise
> > they will trigger the warning in skb_gso_segment.
> >
> > Signed-off-by: Ian Campbell <[email-masked]>
> > Cc: Jeremy Fitzhardinge <[email-masked]>
> > Cc: [email-masked]
> > Cc: [email-masked]
> > ---
> >  drivers/net/xen-netfront.c |    5 +++++
> >  1 files changed, 5 insertions(+), 0 deletions(-)
> >
> > diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> > index cdbeec9..8b8c480 100644
> > --- a/drivers/net/xen-netfront.c
> > +++ b/drivers/net/xen-netfront.c
> > @@ -836,6 +836,11 @@ static int handle_incoming_queue(struct net_device *dev,
> >  				dev->stats.rx_errors++;
> >  				continue;
> >  			}
> > +		} else if (skb_is_gso(skb)) {
> > +			kfree_skb(skb);
> > +			packets_dropped++;
> > +			dev->stats.rx_errors++;
> > +			continue;
> 
> This looks redundant; why not something like:
> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 47e6a71..c1b8f64 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -852,13 +852,12 @@ static int handle_incoming_queue(struct net_device *dev,
>  		/* Ethernet work: Delayed to here as it peeks the header. */
>  		skb->protocol = eth_type_trans(skb, dev);
>  
> -		if (skb->ip_summed == CHECKSUM_PARTIAL) {
> -			if (skb_checksum_setup(skb)) {
> -				kfree_skb(skb);
> -				packets_dropped++;
> -				dev->stats.rx_errors++;
> -				continue;
> -			}
> +		if (skb->ip_summed != CHECKSUM_PARTIAL ||
> +		    skb_checksum_setup(skb)) {

That drops non-partial skbs. However they are fine unless they also
claim to be gso.

Perhaps you meant "skb->ip_summed == CHECKSUM_PARTIAL && !
skb_checksum_setup(skb)" which I think works but doesn't allow us to
correctly chain the gso check onto the else.

Ian.

> +			kfree_skb(skb);
> +			packets_dropped++;
> +			dev->stats.rx_errors++;
> +			continue;
>  		}
>  
>  		dev->stats.rx_packets++;
> 
> Thanks,
> 	J
> 
>