linux-kernel - Re: [PATCH RESEND] uapi/linux/const.h: Prefer ISO-friendly __typeof_

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Date:   Wed, 12 Apr 2023 15:01:18 +0200
From:   Kevin Brodsky <kevin.brodsky@....com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Petr Vorel <pvorel@...e.cz>
Cc:     linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Petr Vorel <petr.vorel@...il.com>,
        Ruben Ayrapetyan <ruben.ayrapetyan@....com>
Subject: Re: [PATCH RESEND] uapi/linux/const.h: Prefer ISO-friendly __typeof__

On 12/04/2023 02:24, Andrew Morton wrote:
> On Tue, 11 Apr 2023 23:39:46 +0200 Petr Vorel <pvorel@...e.cz> wrote:
>
>>> Hi Kevin,
>>>> typeof is (still) a GNU extension, which means that it cannot be
>>>> used when building ISO C (e.g. -std=c99). It should therefore be
>>>> avoided in uapi headers in favour of the ISO-friendly __typeof__.
>>> IMHO UAPI are built with -std=c90 -Wall -Werror=implicit-function-declaration
>>> (see usr/include/Makefile).
>>> But one or the other, you're right both require __typeof__.
>>> "If you are writing a header file that must work when included in ISO C
>>> programs, write __typeof__ instead of typeof."
>>> https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/Typeof.html
>>> Reviewed-by: Petr Vorel <pvorel@...e.cz>
>>> Tested-by: Petr Vorel <pvorel@...e.cz>
>> IMHO problem was introduced when -std=c90 was added (back then the code was in
>> include/uapi/linux/kernel.h).
> Well...  what actually _is_ the problem?  Presumably build issues under
> some circumstances.  Could we please see an instance of those issues
> and a description of the circumstances under which they occur?

This matters from a userspace perspective, not a kernel one. uapi
headers and their contents are expected to be usable in a variety of
situations, and in particular when building ISO C applications (with
-std=c99 or similar).

This particular problem can be reproduced by trying to use the
__ALIGN_KERNEL macro directly in application code, say:

  #include <linux/const.h>

  int align(int x, int a)
  {
      return __KERNEL_ALIGN(x, a);
  }

and trying to build that with -std=c99.

I do not believe this patch really fixes any other patch:
__ALIGN_KERNEL() has always used typeof, so it has never been
ISO-friendly. d6fc9fcbaa65 ("kbuild: compile-test exported headers to
ensure they are self-contained") could not have spotted this issue,
because the macro is never expanded when simply including uapi headers.
One could consider that 607ca46e97a1 ("UAPI: (Scripted) Disintegrate
include/linux") is the culprit because it moved the macro to
include/uapi as-is, but that's arguable.

>
>> Fixes: d6fc9fcbaa65 ("kbuild: compile-test exported headers to ensure they are self-contained")
> Might need a cc:stable, depending on the answers to the above.

Considering this issue has always been present, I do not believe it is
the case.

Thanks,
Kevin
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