Re: [PATCH 5.15.y 2/3] arch: back to -std=gnu89 in < v5.18

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Oct 20, 2025 at 05:58:12PM +0200, Matthieu Baerts wrote:
> Hi Greg,
> 
> On 20/10/2025 15:30, Greg Kroah-Hartman wrote:
> > On Fri, Oct 17, 2025 at 06:24:01PM +0200, Matthieu Baerts (NGI0) wrote:
> >> Recent fixes have been backported to < v5.18 to fix build issues with
> >> GCC 5.15. They all force -std=gnu11 in the CFLAGS, "because [the kernel]
> >> requests the gnu11 standard via '-std=' in the main Makefile".
> >>
> >> This is true for >= 5.18 versions, but not before. This switch to
> >> -std=gnu11 has been done in commit e8c07082a810 ("Kbuild: move to
> >> -std=gnu11").
> >>
> >> For a question of uniformity, force -std=gnu89, similar to what is done
> >> in the main Makefile.
> >>
> >> Note: the fixes tags below refers to upstream commits, but this fix is
> >> only for kernels not having commit e8c07082a810 ("Kbuild: move to
> >> -std=gnu11").
> >>
> >> Fixes: 7cbb015e2d3d ("parisc: fix building with gcc-15")
> >> Fixes: 3b8b80e99376 ("s390: Add '-std=gnu11' to decompressor and purgatory CFLAGS")
> >> Fixes: b3bee1e7c3f2 ("x86/boot: Compile boot code with -std=gnu11 too")
> >> Fixes: ee2ab467bddf ("x86/boot: Use '-std=gnu11' to fix build with GCC 15")
> >> Fixes: 8ba14d9f490a ("efi: libstub: Use '-std=gnu11' to fix build with GCC 15")
> >> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
> >> ---
> >> Note:
> >>   An alternative is to backport commit e8c07082a810 ("Kbuild: move to
> >>   -std=gnu11"), but I guess we might not want to do that for stable, as
> >>   it might introduce new warnings.
> > 
> > I would rather do that, as that would allow us to make things align up
> > and be easier to support over the next two years that this kernel needs
> > to be alive for.  How much work would that entail?
> 
> Good question. I'm not an expert in this area, but I just did a quick
> test: I backported commit e8c07082a810 ("Kbuild: move to -std=gnu11")
> and its parent commit 4d94f910e79a ("Kbuild: use
> -Wdeclaration-after-statement"). A build with 'make allyesconfig' and
> GCC 5.15 looks OK to me, no warnings.
> 
> But when looking a bit around, I noticed these patches have already been
> suggested to be backported to v5.15 in 2023, but they got removed --
> except the doc update, see patch 3/3 -- because they were causing build
> issues with GCC 8 and 12, see:
> 
>  https://lore.kernel.org/a2fbbaa2-51d2-4a8c-b032-5331e72cd116@linaro.org
> 
> I didn't try to reproduce the issues, but maybe we can re-add them to
> the v5.15 queue, and ask the CIs to test that?
> 
> Note that even if we do that, the first patch will still be needed to
> have GCC 15 support in v5.15.

Ok, 5.15.y and 5.10.y will be alive for a while, so I'll go queue these
up now as odds are, I'll end up hitting this issue myself :)

thanks for the patches!

greg k-h