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

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Message-ID: <20161028093422.GA29559@salvia>
Date:   Fri, 28 Oct 2016 11:34:22 +0200
From:   Pablo Neira Ayuso <pablo@...filter.org>
To:     Simon Horman <horms@...ge.net.au>, Julian Anastasov <ja@....bg>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Wensong Zhang <wensong@...ux-vs.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, lvs-devel@...r.kernel.org,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] netfilter: ip_vs_sync: fix bogus maybe-uninitialized
 warning

On Mon, Oct 24, 2016 at 10:47:54PM +0300, Julian Anastasov wrote:
> 
> 	Hello,
> 
> On Mon, 24 Oct 2016, Arnd Bergmann wrote:
> 
> > Building the ip_vs_sync code with CONFIG_OPTIMIZE_INLINING on x86
> > confuses the compiler to the point where it produces a rather
> > dubious warning message:
> > 
> > net/netfilter/ipvs/ip_vs_sync.c:1073:33: error: ‘opt.init_seq’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
> >   struct ip_vs_sync_conn_options opt;
> >                                  ^~~
> > net/netfilter/ipvs/ip_vs_sync.c:1073:33: error: ‘opt.delta’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > net/netfilter/ipvs/ip_vs_sync.c:1073:33: error: ‘opt.previous_delta’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > net/netfilter/ipvs/ip_vs_sync.c:1073:33: error: ‘*((void *)&opt+12).init_seq’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > net/netfilter/ipvs/ip_vs_sync.c:1073:33: error: ‘*((void *)&opt+12).delta’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > net/netfilter/ipvs/ip_vs_sync.c:1073:33: error: ‘*((void *)&opt+12).previous_delta’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > 
> > The problem appears to be a combination of a number of factors, including
> > the __builtin_bswap32 compiler builtin being slightly odd, having a large
> > amount of code inlined into a single function, and the way that some
> > functions only get partially inlined here.
> > 
> > I've spent way too much time trying to work out a way to improve the
> > code, but the best I've come up with is to add an explicit memset
> > right before the ip_vs_seq structure is first initialized here. When
> > the compiler works correctly, this has absolutely no effect, but in the
> > case that produces the warning, the warning disappears.
> > 
> > In the process of analysing this warning, I also noticed that
> > we use memcpy to copy the larger ip_vs_sync_conn_options structure
> > over two members of the ip_vs_conn structure. This works because
> > the layout is identical, but seems error-prone, so I'm changing
> > this in the process to directly copy the two members. This change
> > seemed to have no effect on the object code or the warning, but
> > it deals with the same data, so I kept the two changes together.
> > 
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> 
> 	OK,
> 
> Acked-by: Julian Anastasov <ja@....bg>
> 
> 	I guess, Simon will take the patch for ipvs-next.

@Simon: If you have no more pending updates, I can save you one pull
request for this small fix by placing this.

Thanks!