Re: [Xen-devel] [PATCH V4 5/7] xen-netfront: reduce gso_max_size to account for max TCP header

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2013-04-12 at 15:24 +0100, Wei Liu wrote:
> The maximum packet including header that can be handled by netfront / netback
> wire format is 65535. Reduce gso_max_size accordingly.
> 
> Drop skb and print warning when skb->len > 65535. This can 1) save the effort
> to send malformed packet to netback, 2) help spotting misconfiguration of
> netfront in the future.
> 
> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> ---
>  drivers/net/xen-netfront.c |   14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 1bb2e20..9c2eca1 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -547,6 +547,16 @@ static int xennet_start_xmit(struct sk_buff *skb, struct 
> net_device *dev)
>       unsigned int len = skb_headlen(skb);
>       unsigned long flags;
>  
> +     /* If skb->len is too big for wire format, drop skb and alert
> +      * user about misconfiguration.
> +      */
> +     if (unlikely(skb->len > (typeof(tx->size))(~0))) {

This seems rather a complicated way of saying... er, something!

Perhaps we could have a #define in netif.h to compare against?

I'm not 100% convinced that the case and the ~ are in the right order to
have the behaviour you want unless perhaps sizeof(tx->size) <
sizeof(int). In any case it's too obfuscated for me ;-)

> +             net_alert_ratelimited(
> +                     "xennet: skb->len = %u, too big for wire format\n",
> +                     skb->len);
> +             goto drop;
> +     }
> +
>       slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
>               xennet_count_skb_frag_slots(skb);
>       if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
> @@ -1058,7 +1068,7 @@ err:
>  
>  static int xennet_change_mtu(struct net_device *dev, int mtu)
>  {
> -     int max = xennet_can_sg(dev) ? 65535 - ETH_HLEN : ETH_DATA_LEN;
> +     int max = xennet_can_sg(dev) ? 65535 - MAX_TCP_HEADER : ETH_DATA_LEN;
>  
>       if (mtu > max)
>               return -EINVAL;
> @@ -1362,6 +1372,8 @@ static struct net_device *xennet_create_dev(struct 
> xenbus_device *dev)
>       SET_ETHTOOL_OPS(netdev, &xennet_ethtool_ops);
>       SET_NETDEV_DEV(netdev, &dev->dev);
>  
> +     netif_set_gso_max_size(netdev, 65535 - MAX_TCP_HEADER);
> +
>       np->netdev = netdev;
>  
>       netif_carrier_off(netdev);



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