Message-ID: <1448975100.25582.9.camel@edumazet-glaptop2.roam.corp.google.com>
Subject: Re: [PATCH net] ipv6: add complete rcu protection around np->opt
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Dmitry Vyukov <dvyukov@google.com>, Vlad Yasevich <vyasevich@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        James Morris <jmorris@namei.org>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Patrick McHardy <kaber@trash.net>, netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Eric Dumazet <edumazet@google.com>, William Dauchy <wdauchy@gmail.com>,
        Rainer Weikusat <rweikusat@mobileactivedefense.com>,
        syzkaller <syzkaller@googlegroups.com>,
        Kostya Serebryany <kcc@google.com>,
        Alexander Potapenko <glider@google.com>,
        Sasha Levin <sasha.levin@oracle.com>
Date: Tue, 01 Dec 2015 05:05:00 -0800
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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.


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