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

[Christian Borntraeger]

On 22.04.21 06:47, 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.

I tend to follow the Linux kernel. If the compiler tends to produce stupid 
errors,
silence them with Wno-*.
I think it makes no sense to have warnings with a high tendency of false 
positives in a
project with Werror.