linux-kernel - RE: [PATCH] netfilter: ip_vs_sync: fix bogus maybe-uninitialized warning

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Date:   Tue, 25 Oct 2016 16:15:16 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...db.de>, Julian Anastasov <ja@....bg>
CC:     Wensong Zhang <wensong@...ux-vs.org>,
        Simon Horman <horms@...ge.net.au>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Patrick McHardy <kaber@...sh.net>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        "David S. Miller" <davem@...emloft.net>,
        Quentin Armitage <quentin@...itage.org.uk>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "lvs-devel@...r.kernel.org" <lvs-devel@...r.kernel.org>,
        "netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
        "coreteam@...filter.org" <coreteam@...filter.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] netfilter: ip_vs_sync: fix bogus maybe-uninitialized
 warning

From: Arnd Bergmann
> Sent: 24 October 2016 21:22
> On Monday, October 24, 2016 10:47:54 PM CEST Julian Anastasov wrote:
> > > diff --git a/net/netfilter/ipvs/ip_vs_sync.c b/net/netfilter/ipvs/ip_vs_sync.c
> > > index 1b07578bedf3..9350530c16c1 100644
> > > --- a/net/netfilter/ipvs/ip_vs_sync.c
> > > +++ b/net/netfilter/ipvs/ip_vs_sync.c
> > > @@ -283,6 +283,7 @@ struct ip_vs_sync_buff {
> > >   */
> > >  static void ntoh_seq(struct ip_vs_seq *no, struct ip_vs_seq *ho)
> > >  {
> > > +     memset(ho, 0, sizeof(*ho));
> > >       ho->init_seq       = get_unaligned_be32(&no->init_seq);
> > >       ho->delta          = get_unaligned_be32(&no->delta);
> > >       ho->previous_delta = get_unaligned_be32(&no->previous_delta);
> >
> >         So, now there is a double write here?
> 
> Correct. I would hope that a sane version of gcc would just not
> perform the first write. What happens instead is that the version
> that produces the warning here moves the initialization to the
> top of the calling function.

Maybe doing the 3 get_unaligned_be32() before the memset will stop
the double-writes.
The problem is that the compiler doesn't know that the two structures
don't alias each other.

	David