Re: s390-ccw: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Apr 22, 2021 at 06:47:30AM +0200, Thomas Huth wrote:
> On 22/04/2021 06.18, Philippe Mathieu-Daudé wrote:
> > Hi Thomas, Daniel, Stefano,
> > 
> > Regarding the following warning (GCC 11 on Fedora 34):
> > 
> > In file included from pc-bios/s390-ccw/main.c:11:
> > 
> > In function ‘memset’,
> > 
> >      inlined from ‘boot_setup’ at pc-bios/s390-ccw/main.c:185:5,
> > 
> >      inlined from ‘main’ at pc-bios/s390-ccw/main.c:288:5:
> > 
> > pc-bios/s390-ccw/libc.h:28:14: warning: writing 1 byte into a region of
> > size 0 [-Wstringop-overflow=]
> > 
> >     28 |         p[i] = c;
> > 
> >        |         ~~~~~^~~
> > 
> > Daniel were right on IRC:
> > 
> > danpb: it is from a call  memset((char *)S390EP, 0, 6)     where  S390EP
> > is just a constant address 0x10008
> > danpb: the compiler doesn't now how big that is, so it seems to assume
> > it is zero length
> > 
> > This is a known GCC issue:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578
> > "gcc-11 -Warray-bounds or -Wstringop-overread warning when accessing a
> > pointer from integer literal"
> 
>  Hi Philippe,
> 
> thanks for following up with the gcc bugzilla!
> 
> ... so the problem is that GCC thinks we're in fact dereferencing a NULL
> pointer at offset 0x10008 here? Wow, that's ... crazy.
> 
> Not sure what to do now - wait for the bug to get resolved? Compile the
> s390-ccw bios with -Wno-stringop-overread ? Add "volatiles" here and there
> to hope that these silence the compiler warnings? ... I tend to wait for the
> bug ticket to see whether the GCC folks change the behavior of the compiler
> again, but I'm open for other suggestions.

Assuming it is just this one place in the code ,then we should just
use "pragma" to temporarily disable/re-enable that single warning flag
either side of the problem.

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|