netdev - Re: [PATCH 2/6] xen-netfront: reduce gso_max_size to account for ethernet header

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Date:	Mon, 25 Mar 2013 19:50:44 +0400
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	xen-devel@...ts.xen.org, netdev@...r.kernel.org,
	ian.campbell@...rix.com, annie.li@...cle.com,
	konrad.wilk@...cle.com, david.vrabel@...rix.com
Subject: Re: [PATCH 2/6] xen-netfront: reduce gso_max_size to account for
 ethernet header

Hello.

On 25-03-2013 15:08, Wei Liu wrote:

> The maximum packet including ethernet 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@...rix.com>
> ---
>   drivers/net/xen-netfront.c |   14 ++++++++++++++
>   1 file changed, 14 insertions(+)

> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 5527663..3ae9dc1 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -547,6 +547,18 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
>   	unsigned int len = skb_headlen(skb);
>   	unsigned long flags;
>
> +	/* Wire format of xen_netif_tx_request only supports skb->len
> +	 * < 64K, because size field in xen_netif_tx_request is
> +	 * uint16_t. If skb->len is too big, drop it and alert user about
> +	 * misconfiguration.
> +	 */
> +	if (unlikely(skb->len >= (uint16_t)(~0))) {

    Such types as 'uint16_t' are intended for userland -- use 'u16' instead. 
Better still, just use 0xffffu.

WBR, Sergei

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