linux-kernel - Re: [PATCH 3/5] powerpc-pseries: Delete an unnecessary variable initialisation in iommu_pseries_alloc

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Date:   Fri, 20 Oct 2017 12:06:24 +1100
From:   David Gibson <david@...son.dropbear.id.au>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Michal Suchánek <msuchanek@...e.de>,
        SF Markus Elfring <elfring@...rs.sourceforge.net>,
        linuxppc-dev@...ts.ozlabs.org,
        Alex Williamson <alex.williamson@...hat.com>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Doug Ledford <dledford@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Johan Hovold <johan@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        Paul Mackerras <paulus@...ba.org>,
        Rob Herring <robh@...nel.org>,
        Sahil Mehta <sahilmehta17@...il.com>,
        Tyrel Datwyler <tyreld@...ux.vnet.ibm.com>,
        kernel-janitors@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] powerpc-pseries: Delete an unnecessary variable
 initialisation in iommu_pseries_alloc_group()

On Thu, Oct 19, 2017 at 03:55:59PM +0300, Dan Carpenter wrote:
> On Thu, Oct 19, 2017 at 01:37:18PM +0200, Michal Suchánek wrote:
> > Hello,
> > 
> > On Wed, 18 Oct 2017 21:24:25 +0200
> > SF Markus Elfring <elfring@...rs.sourceforge.net> wrote:
> > 
> > > From: Markus Elfring <elfring@...rs.sourceforge.net>
> > > Date: Wed, 18 Oct 2017 19:14:39 +0200
> > > 
> > > The variable "table_group" will be set to an appropriate pointer.
> > > Thus omit the explicit initialisation at the beginning.
> > > 
> > > Signed-off-by: Markus Elfring <elfring@...rs.sourceforge.net>
> > > ---
> > >  arch/powerpc/platforms/pseries/iommu.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/powerpc/platforms/pseries/iommu.c
> > > b/arch/powerpc/platforms/pseries/iommu.c index
> > > b37d4fb20d1c..b6c12b8e3ace 100644 ---
> > > a/arch/powerpc/platforms/pseries/iommu.c +++
> > > b/arch/powerpc/platforms/pseries/iommu.c @@ -55,7 +55,7 @@
> > >  
> > >  static struct iommu_table_group *iommu_pseries_alloc_group(int node)
> > >  {
> > > -	struct iommu_table_group *table_group = NULL;
> > > +	struct iommu_table_group *table_group;
> > >  	struct iommu_table *tbl = NULL;
> > >  	struct iommu_table_group_link *tgl = NULL;
> > >  
> > 
> > I think initializing pointers to NULL is generally a good idea.
> > 
> > If there is no use of the variable before it is reinitialized by
> > allocation gcc is free to optimize out the variable and its initial
> > value.
> > 
> > On the other hand, if the code is changed later and use of the variable
> > becomes possible you may crash (and get a gcc warning, too).
> 
> No, it's the opposite. GCC doesn't warn about potential NULL
> dereferences, it warns about uninitialized variables.  By initializing
> it to a bogus value, you're deliberately disabling static analysis.
> We do see bugs where, if only people didn't initialize stuff to bogus
> values, then the bug would have been caught before it was merged.

Seconded, I've seen this a number of times.  I think this alone is a
reason not to initiaize locals if they don't require it.
 
> You might imagine that static analysis tools would catch NULL
> dereferences but it's actually really really hard.  We used to have
> an __uninitialized_var() macro which was used to silence GCC false
> positives, but now we initialize the pointers to NULL instead.  So
> most of the code that you're dealing with is stuff that was marked as
> too hard for GCC to understand.  It's tricky.
> 
> regards,
> dan carpenter
> 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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