Re: + headers-add-check-for-c-standard-version.patch added to mm-hotfixes-unstable branch

[Kees Cook <kees@kernel.org>]

On October 26, 2025 4:56:30 PM PDT, Andrew Morton <akpm@linux-foundation.org> wrote:
>
>The patch titled
>     Subject: headers: add check for C standard version
>has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
>     headers-add-check-for-c-standard-version.patch
>
>This patch will shortly appear at
>     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/headers-add-check-for-c-standard-version.patch
>
>This patch will later appear in the mm-hotfixes-unstable branch at
>    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
>
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>
>*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
>
>The -mm tree is included into linux-next via the mm-everything
>branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
>and is updated there every 2-3 working days
>
>------------------------------------------------------
>From: Hanne-Lotta M��enp���� <hannelotta@gmail.com>
>Subject: headers: add check for C standard version
>Date: Sun, 26 Oct 2025 21:58:46 +0200
>
>Compiling the kernel with GCC 15 results in errors, as with GCC 15 the
>default language version for C compilation has been changed from
>-std=gnu17 to -std=gnu23 - unless the language version has been changed
>using
>
>    KBUILD_CFLAGS += -std=gnu17
>
>or earlier.
>
>C23 includes new keywords 'bool', 'true' and 'false', which cause
>compilation errors in Linux headers:
>
>    ./include/linux/types.h:30:33: error: `bool' cannot be defined
>        via `typedef'
>
>    ./include/linux/stddef.h:11:9: error: cannot use keyword `false'
>        as enumeration constant
>
>Add check for C Standard's version in the header files to be able to
>compile the kernel with C23.

What? These are internal headers, not UAPI. We build Linux with -std=gnu11. Let's not add meaningless version checks.

-Kees

>
>Link: https://lkml.kernel.org/r/20251026195846.69740-1-hannelotta@gmail.com
>Signed-off-by: Hanne-Lotta M��enp���� <hannelotta@gmail.com>
>Cc: David Hunter <david.hunter.linux@gmail.com>
>Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
>Cc: Kees Cook <kees@kernel.org>
>Cc: Arnd Bergmann <arnd@arndb.de>
>Cc: <stable@vger.kernel.org>
>Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>---
>
> include/linux/stddef.h |    2 ++
> include/linux/types.h  |    2 ++
> 2 files changed, 4 insertions(+)
>
>--- a/include/linux/stddef.h~headers-add-check-for-c-standard-version
>+++ a/include/linux/stddef.h
>@@ -7,10 +7,12 @@
> #undef NULL
> #define NULL ((void *)0)
> 
>+#if !defined(__STDC_VERSION__) || __STDC_VERSION__ < 202311L
> enum {
> 	false	= 0,
> 	true	= 1
> };
>+#endif
> 
> #undef offsetof
> #define offsetof(TYPE, MEMBER)	__builtin_offsetof(TYPE, MEMBER)
>--- a/include/linux/types.h~headers-add-check-for-c-standard-version
>+++ a/include/linux/types.h
>@@ -32,7 +32,9 @@ typedef __kernel_timer_t	timer_t;
> typedef __kernel_clockid_t	clockid_t;
> typedef __kernel_mqd_t		mqd_t;
> 
>+#if !defined(__STDC_VERSION__) || __STDC_VERSION__ < 202311L
> typedef _Bool			bool;
>+#endif
> 
> typedef __kernel_uid32_t	uid_t;
> typedef __kernel_gid32_t	gid_t;
>_
>
>Patches currently in -mm which might be from hannelotta@gmail.com are
>
>headers-add-check-for-c-standard-version.patch
>

-- 
Kees Cook