Re: [PATCH 5.10.y 0/3] v5.10: fix build with GCC 15

[Matthieu Baerts <matttbe@kernel.org>]

Hi Greg,

Thank you for your reply!

On 20/10/2025 15:32, Greg Kroah-Hartman wrote:
> On Fri, Oct 17, 2025 at 06:53:24PM +0200, Matthieu Baerts (NGI0) wrote:
>> This kernel version doesn't build with GCC 15:
>>
>>   In file included from include/uapi/linux/posix_types.h:5,
>>                    from include/uapi/linux/types.h:14,
>>                    from include/linux/types.h:6,
>>                    from arch/x86/realmode/rm/wakeup.h:11,
>>                    from arch/x86/realmode/rm/wakemain.c:2:
>>   include/linux/stddef.h:11:9: error: cannot use keyword 'false' as enumeration constant
>>      11 |         false   = 0,
>>         |         ^~~~~
>>   include/linux/stddef.h:11:9: note: 'false' is a keyword with '-std=c23' onwards
>>   include/linux/types.h:30:33: error: 'bool' cannot be defined via 'typedef'
>>      30 | typedef _Bool                   bool;
>>         |                                 ^~~~
>>   include/linux/types.h:30:33: note: 'bool' is a keyword with '-std=c23' onwards
>>   include/linux/types.h:30:1: warning: useless type name in empty declaration
>>      30 | typedef _Bool                   bool;
>>         | ^~~~~~~
>>
>> I initially fixed this by adding -std=gnu11 in arch/x86/Makefile, then I
>> realised this fix was already done in an upstream commit, created before
>> the GCC 15 release and not mentioning the error I had. This is the first
>> patch.
>>
>> When I was investigating my error, I noticed other commits were already
>> backported to stable versions. They were all adding -std=gnu11 in
>> different Makefiles. In their commit message, they were mentioning
>> 'gnu11' was picked to use the same as the one from the main Makefile.
>> But this is not the case in this kernel version. Patch 2 fixes that.
>>
>> Finally, I noticed an extra warning that I didn't have in v5.15. Patch 3
>> fixes that.
> 
> As with 5.4.y, this kernel isn't going to be around all that much
> longer, and I strongly doubt anyone that relies on it is also using
> gcc15 with it.  Normally near the end of the 6 year window of kernels,
> we are "stuck" with using older gcc releases with that, and not newer
> ones, and that's fine, as that's what the users of those kernels are
> also doing.
> 
> So I don't think the effort is worth it.  gcc14 will be around in
> distros for the next year so all should be ok here.  Just like gcc13 was
> around long enough for us to keep 4.19.y working just fine with that
> maximum compiler release as well.

Fine by me, but just to avoid confusions, I would like to add:

- Only the first patch fixes the GCC 15 support, and it only adds
'-std=gnu11' in x86/boot. All the other GCC 15 specific patches have
already been backported, only that one didn't have a Fixes tag and was
missing in <= v5.15.

- Patch 2/3 fixes the (accidental) switch to C11 standard for some arch.
It should not be a problem in v5.10, because GCC minimal version is 4.9
which supports C11 if I'm not mistaken. It is just to be consistent with
what is done in the main Makefile, but it can be drop if preferred.

- Patch 3/3 is not linked to GCC 15 support, it fixes an ISO C90 warning.

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.