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@xxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > > From: Markus Elfring <elfring@xxxxxxxxxxxxxxxxxxxxx>
> > > 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@xxxxxxxxxxxxxxxxxxxxx>
> > > ---
> > > 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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