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

[David Laight <David.Laight@xxxxxxxxxx>]

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

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