[V5,5/7] xen-netfront: reduce gso_max_size to account for max TCP header

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@citrix.com>
---
 drivers/net/xen-netfront.c       |   16 ++++++++++++++--
 include/xen/interface/io/netif.h |    1 +
 2 files changed, 15 insertions(+), 2 deletions(-)

On Mon, 2013-04-15 at 18:06 +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@citrix.com>

Acked-by: Ian Campbell <ian.campbell@citrix.com>

(one commentlet is that you've used 65535 a few times (or left existing
uses) but you've also just defined XEN_NETIF_MAX_TX_SIZE which would be
a better thing to use)

> ---
>  drivers/net/xen-netfront.c       |   16 ++++++++++++++--
>  include/xen/interface/io/netif.h |    1 +
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 1bb2e20..7c53d0c 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -36,7 +36,7 @@
>  #include <linux/skbuff.h>
>  #include <linux/ethtool.h>
>  #include <linux/if_ether.h>
> -#include <linux/tcp.h>
> +#include <net/tcp.h>
>  #include <linux/udp.h>
>  #include <linux/moduleparam.h>
>  #include <linux/mm.h>
> @@ -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 > XEN_NETIF_MAX_TX_SIZE)) {
> +		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);
> diff --git a/include/xen/interface/io/netif.h b/include/xen/interface/io/netif.h
> index 9dfc120..58fadca 100644
> --- a/include/xen/interface/io/netif.h
> +++ b/include/xen/interface/io/netif.h
> @@ -47,6 +47,7 @@
>  #define _XEN_NETTXF_extra_info		(3)
>  #define  XEN_NETTXF_extra_info		(1U<<_XEN_NETTXF_extra_info)
>  
> +#define XEN_NETIF_MAX_TX_SIZE 0xFFFF
>  struct xen_netif_tx_request {
>      grant_ref_t gref;      /* Reference to buffer page */
>      uint16_t offset;       /* Offset within buffer page */


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