netdev - Re: race in skb_splice

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Message-ID: <20080528201659.GA2696@ami.dom.local>
Date:	Wed, 28 May 2008 22:16:59 +0200
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Octavian Purdila <opurdila@...acom.com>
Cc:	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Ben Hutchings <bhutchings@...arflare.com>,
	netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: race in skb_splice_bits?

On Wed, May 28, 2008 at 11:09:13PM +0300, Octavian Purdila wrote:
> On Wednesday 28 May 2008, Jarek Poplawski wrote:
> 
> > >> We queue data under the lock and clone appropriate skb (and then grab it
> > >> multiple times), so even it will be dropped, its data will not freed,
> > >> and thus we will be able to read it. Or you are talking about different
> > >> skbs?
> > >
> > > You are right, I forgot about the clone.
> >
> > Probably I miss something, but how does it help when tcp_collapse()
> > uses __kfree_skb()?
> >
> 
> __kfree_skb() -> skb_release_all() -> skb_release_data():
> 
> static void skb_release_data(struct sk_buff *skb)
> {
>         if (!skb->cloned ||
> <snip>
>                 kfree(skb->head);
> 
> Since we clone the skb in skb_splice_bits() the skb's data will only be freed 
> when the last clone is deleted.

Right!

Thanks for explanation,
Jarek P.
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