netdev - Re: [PATCH 2/2] tbf scheduler: TSO support (update 2)

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Date:	Fri, 18 May 2007 11:35:07 +0900 (JST)
From:	Hirokazu Takahashi <taka@...inux.co.jp>
To:	herbert@...dor.apana.org.au
Cc:	shemminger@...ux-foundation.org, netdev@...r.kernel.org,
	kaber@...sh.net, davem@...emloft.net, linux-net@...r.kernel.org
Subject: Re: [PATCH 2/2] tbf scheduler: TSO support (update 2)

Hi,

> > @@ -924,7 +926,9 @@ cbq_dequeue_prio(struct Qdisc *sch, int 
> >  				cl->xstats.borrows += skb->len;
> >  #endif
> >  			}
> > -			q->tx_len = skb->len;
> > +			q->tx_segs = skb_shinfo(skb)->gso_segs ? :
> > +			  skb_shinfo(skb)->gso_size ? skb->len/skb_shinfo(skb)->gso_size + 1 : 1;
> > +			q->tx_len = (skb->len - 1)/q->tx_segs + 1;
> 
> This isn't safe for Xen (and potentially other virtualisation
> environments) since qdisc code runs before dev_hard_start_xmit
> which is where we verify the sanity of gso_segs.  So you could
> be using some arbitrary value from an untrusted source.
> 
> If you really want to use it, you should test for SKB_GSO_DODGY
> on the packet which will be set if gso_segs can't be trusted.

Yep, you have a point that some sanity check should be added.
I think a simple check would be enough not to crash CBQ
as the accurate checking will be done in dev_hard_start_xmit or
device drivers.

Thanks,
Hirokazu Takahashi.

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