From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ian Campbell <Ian.Campbell@citrix.com>
Subject: Re: [Xen-devel] [PATCH] xen/netfront: handle compound page
 fragments on transmit
Date: Tue, 20 Nov 2012 15:06:54 +0000
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Cc: Stefan Bader <stefan.bader@canonical.com>,
	Sander Eikelenboom <linux@eikelenboom.it>,
	Eric Dumazet <edumazet@google.com>,
	KonradRzeszutekWilk <konrad@kernel.org>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>,
	ANNIE LI <annie.li@oracle.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>
To: Jan Beulich <JBeulich@suse.com>
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On Tue, 2012-11-20 at 14:32 +0000, Jan Beulich wrote:
> > @@ -517,15 +540,16 @@ static int xennet_start_xmit(struct sk_buff *skb, 
> > struct net_device *dev)
> >  	grant_ref_t ref;
> >  	unsigned long mfn;
> >  	int notify;
> > -	int frags = skb_shinfo(skb)->nr_frags;
> > +	int slots;
> >  	unsigned int offset = offset_in_page(data);
> >  	unsigned int len = skb_headlen(skb);
> >  	unsigned long flags;
> >  
> > -	frags += DIV_ROUND_UP(offset + len, PAGE_SIZE);
> > -	if (unlikely(frags > MAX_SKB_FRAGS + 1)) {
> > -		printk(KERN_ALERT "xennet: skb rides the rocket: %d frags\n",
> > -		       frags);
> > +	slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
> > +		xennet_count_skb_frag_slots(skb);
> > +	if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
> 
> But still - isn't this wrong now (i.e. can't it now validly exceed the
> boundary checked for)?

In practice no because of the property that the number of pages backing
the frags is <= MAX_SKB_FRAGS even if you are using compound pages as
the frags.

Ian.