netdev - Re: [Xen-devel] [PATCH 6/6] xen-netback: don't disconnect frontend when seeing oversize frame

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Date:	Mon, 25 Mar 2013 12:53:57 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Wei Liu" <wei.liu2@...rix.com>
Cc:	<david.vrabel@...rix.com>, <ian.campbell@...rix.com>,
	<xen-devel@...ts.xen.org>, <annie.li@...cle.com>,
	<konrad.wilk@...cle.com>, <netdev@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 6/6] xen-netback: don't disconnect
 frontend when seeing oversize frame

>>> On 25.03.13 at 12:08, Wei Liu <wei.liu2@...rix.com> wrote:
> @@ -947,10 +949,21 @@ static int netbk_count_requests(struct xenvif *vif,
>  
>  		memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + slots),
>  		       sizeof(*txp));
> -		if (txp->size > first->size) {
> -			netdev_err(vif->dev, "Packet is bigger than frame.\n");
> -			netbk_fatal_tx_err(vif);
> -			return -EIO;
> +
> +		/* If the guest submitted a frame >= 64 KiB then
> +		 * first->size overflowed and following slots will
> +		 * appear to be larger than the frame.
> +		 *
> +		 * This cannot be fatal error as there are buggy
> +		 * frontends that do this.
> +		 *
> +		 * Consume all slots and drop the packet.
> +		 */
> +		if (!drop && txp->size > first->size) {
> +			if (net_ratelimit())
> +				netdev_dbg(vif->dev,
> +					   "Packet is bigger than frame.\n");
> +			drop = true;

So this deals with one half of the problem, but shouldn't we also
revert the disconnect when slots would exceed MAX_SKB_FRAGS
(or max_skb_slots after patch 5)? Afaict you could trivially extend
this patch to also cover that case...

Jan

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