Older kernels like 6.6 and lower may not compile with GCC15 due to -std=gnu23

Older kernels like 6.6 and lower may not compile with GCC15 due to -std=gnu23

[Philip Müller]

Hi all,

GCC 15 changed the default C standard dialect from gnu17 to gnu23,
which should not have impacted the kernel because it explicitly requests
the gnu11 standard in the main Makefile. However I see these errors in 
older kernels series and some realtime kernels:

5.4, 5.10, 5.15, 6.1, 6.1-rt, 6.6, 6.13-rt

2025-04-30T19:26:10.5226566Z                  from 
arch/x86/realmode/rm/wakemain.c:2:
2025-04-30T19:26:10.5228293Z ./include/linux/stddef.h:11:9: error: 
cannot use keyword ‘false’ as enumeration constant
2025-04-30T19:26:10.5229525Z    11 |         false   = 0,
2025-04-30T19:26:10.5230691Z       |         ^~~~~
2025-04-30T19:26:10.5232337Z ./include/linux/stddef.h:11:9: note: 
‘false’ is a keyword with ‘-std=c23’ onwards
2025-04-30T19:26:10.5234141Z ./include/linux/types.h:30:33: error: 
‘bool’ cannot be defined via ‘typedef’
2025-04-30T19:26:10.5235478Z    30 | typedef _Bool                   bool;
2025-04-30T19:26:10.5236557Z       |                                 ^~~~
2025-04-30T19:26:10.5238016Z ./include/linux/types.h:30:33: note: ‘bool’ 
is a keyword with ‘-std=c23’ onwards
2025-04-30T19:26:10.5239544Z ./include/linux/types.h:30:1: warning: 
useless type name in empty declaration
2025-04-30T19:26:10.5241119Z    30 | typedef _Bool                   bool;
2025-04-30T19:26:10.5242189Z       | ^~~~~~~
2025-04-30T19:26:10.5351835Z make[4]: *** [scripts/Makefile.build:250: 
arch/x86/realmode/rm/wakemain.o] Error 1
2025-04-30T19:26:10.5362380Z make[3]: *** 
[arch/x86/realmode/Makefile:22: arch/x86/realmode/rm/realmode.bin] Error 2

I found this with mips: 
https://lkml.iu.edu/hypermail/linux/kernel/2503.2/09285.html

Do we miss some patch not backported?

-- 
Best, Philip

Re: Older kernels like 6.6 and lower may not compile with GCC15 due to -std=gnu23

[Greg Kroah-Hartman]

On Thu, May 01, 2025 at 08:34:36AM +0200, Philip Müller wrote:
> Hi all,
> 
> GCC 15 changed the default C standard dialect from gnu17 to gnu23,
> which should not have impacted the kernel because it explicitly requests
> the gnu11 standard in the main Makefile. However I see these errors in older
> kernels series and some realtime kernels:
> 
> 5.4, 5.10, 5.15, 6.1, 6.1-rt, 6.6, 6.13-rt

That's to be expected, no one has backported the fixes for this yet.

Someone on the list started to, but then said they would wait for
someone else to do it, see this thread:
	https://lore.kernel.org/r/fb4cce81-1e36-4887-a1e0-0cfd1a26693e@googlemail.com

So, until that happens, or one of my build-boxes accidentally gets
upgraded to gcc-15 and I'm forced to do it myself, this is going to stay
as-is.

thanks,

greg k-h