linux-kernel - Re: [PATCH net] ipv6: add complete rcu protection around np->opt

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Date:	Tue, 01 Dec 2015 14:13:15 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	Vlad Yasevich <vyasevich@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	William Dauchy <wdauchy@...il.com>,
	Rainer Weikusat <rweikusat@...ileactivedefense.com>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [PATCH net] ipv6: add complete rcu protection around np->opt

On Tue, Dec 1, 2015, at 14:05, Eric Dumazet wrote:
> On Tue, 2015-12-01 at 12:11 +0100, Hannes Frederic Sowa wrote:
> > Hi Eric,
> > 
> > On Mon, Nov 30, 2015, at 04:37, Eric Dumazet wrote:
> > > -                       opt = xchg(&np->opt, NULL);
> > > -                       if (opt)
> > > -                               sock_kfree_s(sk, opt, opt->tot_len);
> > > +                       opt = xchg((__force struct ipv6_txoptions
> > > **)&np->opt,
> > > +                                  NULL);
> > > +                       if (opt) {
> > > +                               atomic_sub(opt->tot_len,
> > > &sk->sk_omem_alloc);
> > > +                               txopt_put(opt);
> > > +                       }
> > >  			pktopt = xchg(&np->pktoptions, NULL);
> > >  			kfree_skb(pktopt);
> > 
> > Is here something special going on (because of the xchg). I don't see
> > why you cannot simply use a RCU_INIT_POINTER?
> > 
> > Thanks,
> > Hannes
> 
> Yes, I mentioned this earlier, and will be addressed in net-next tree
> later.
> 
> (Same for np->pktoptions)
> 
> The xchg() here does not bring additional protection as we are the last
> user of np.
> 
> Thanks.

Ah, sorry, maybe I missed it. Just reviewed the patches after a longer
weekend here.

The rest looked fine to me:

Acked-by: Hannes Frederic Sowa <hannes@...essinduktion.org>

Thanks!
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