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

[Thomas Huth <thuth@redhat.com>]

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.

  Thomas