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

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Message-ID: <20230411213946.GA1803920@pevik>
Date:   Tue, 11 Apr 2023 23:39:46 +0200
From:   Petr Vorel <pvorel@...e.cz>
To:     Kevin Brodsky <kevin.brodsky@....com>,
        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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ruben Ayrapetyan <ruben.ayrapetyan@....com>
Subject: Re: [PATCH RESEND] uapi/linux/const.h: Prefer ISO-friendly __typeof__

> 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).

Fixes: d6fc9fcbaa65 ("kbuild: compile-test exported headers to ensure they are self-contained")

Kind regards,
Petr

> Kind regards,
> Petr

> > Unfortunately this issue could not be detected by
> > CONFIG_UAPI_HEADER_TEST=y as the __ALIGN_KERNEL() macro is not
> > expanded in any uapi header.

> > Reported-by: Ruben Ayrapetyan <ruben.ayrapetyan@....com>
> > Tested-by: Ruben Ayrapetyan <ruben.ayrapetyan@....com>
> > Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
> > ---
> >  include/uapi/linux/const.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)

> > diff --git a/include/uapi/linux/const.h b/include/uapi/linux/const.h
> > index af2a44c08683..a429381e7ca5 100644
> > --- a/include/uapi/linux/const.h
> > +++ b/include/uapi/linux/const.h
> > @@ -28,7 +28,7 @@
> >  #define _BITUL(x)	(_UL(1) << (x))
> >  #define _BITULL(x)	(_ULL(1) << (x))

> > -#define __ALIGN_KERNEL(x, a)		__ALIGN_KERNEL_MASK(x, (typeof(x))(a) - 1)
> > +#define __ALIGN_KERNEL(x, a)		__ALIGN_KERNEL_MASK(x, (__typeof__(x))(a) - 1)
> >  #define __ALIGN_KERNEL_MASK(x, mask)	(((x) + (mask)) & ~(mask))

> >  #define __KERNEL_DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))