linux-kernel - Re: [PATCH next] compiler.h: Move __is

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Message-ID: <202310020947.CAF558C6@keescook>
Date:   Mon, 2 Oct 2023 09:47:54 -0700
From:   Kees Cook <keescook@...omium.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "'linux@...musvillemoes.dk'" <linux@...musvillemoes.dk>,
        'Steven Rostedt' <rostedt@...dmis.org>,
        "'bvanassche@....org'" <bvanassche@....org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH next] compiler.h: Move __is_constexpr() to compiler.h.

On Mon, Oct 02, 2023 at 02:34:05PM +0000, David Laight wrote:
> Prior to f747e6667ebb2 __is_constexpr() was in its only user minmax.h.
> That commit moved it to const.h - but that file just defined ULL(x) and
>   UL(x) so that constants can be defined for .S and .c files.
> So apart from the word 'const' it wasn't really a good location.
> Instead move the definition to compiler.h just before the similar
>   is_signed_type() and is_unsigned_type().
> (Which were moved there by dcf8e5633e2e6)
> This may not be a good long-term home, but the three definitions
>   belong together.
> 
> Signed-off-by: David Laight <david.laight@...lab.com>
> ---
> This makes it possible to use __is_constexpr() inside is_signed_type()
> so that the result is constant integer expression for pointer types.
> In particular (void *)1 isn't constant enough.
> 
>  include/linux/compiler.h    | 8 ++++++++
>  include/linux/const.h       | 8 --------
>  tools/include/linux/const.h | 8 --------
>  3 files changed, 8 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index d7779a18b24f..2efec9bfcc40 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -230,6 +230,14 @@ static inline void *offset_to_ptr(const int *off)
>  /* &a[0] degrades to a pointer: a different type from an array */
>  #define __must_be_array(a)	BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
>  
> +/*
> + * This returns a constant expression while determining if an argument is
> + * a constant expression, most importantly without evaluating the argument.
> + * Glory to Martin Uecker <Martin.Uecker@....uni-goettingen.de>
> + */
> +#define __is_constexpr(x) \
> +	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
> +
>  /*
>   * Whether 'type' is a signed type or an unsigned type. Supports scalar types,
>   * bool and also pointer types.
> diff --git a/include/linux/const.h b/include/linux/const.h
> index 435ddd72d2c4..81b8aae5a855 100644
> --- a/include/linux/const.h
> +++ b/include/linux/const.h
> @@ -3,12 +3,4 @@
>  
>  #include <vdso/const.h>
>  
> -/*
> - * This returns a constant expression while determining if an argument is
> - * a constant expression, most importantly without evaluating the argument.
> - * Glory to Martin Uecker <Martin.Uecker@....uni-goettingen.de>
> - */
> -#define __is_constexpr(x) \
> -	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
> -
>  #endif /* _LINUX_CONST_H */
> diff --git a/tools/include/linux/const.h b/tools/include/linux/const.h
> index 435ddd72d2c4..81b8aae5a855 100644
> --- a/tools/include/linux/const.h
> +++ b/tools/include/linux/const.h
> @@ -3,12 +3,4 @@
>  
>  #include <vdso/const.h>
>  
> -/*
> - * This returns a constant expression while determining if an argument is
> - * a constant expression, most importantly without evaluating the argument.
> - * Glory to Martin Uecker <Martin.Uecker@....uni-goettingen.de>
> - */
> -#define __is_constexpr(x) \
> -	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))

Doesn't tools/ still need its own copy somewhere?

-Kees

> -
>  #endif /* _LINUX_CONST_H */
> -- 
> 2.17.1
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
> 

-- 
Kees Cook