linux-kernel - Re: [PATCH v2 4/4] minmax: remove useless cast in __is

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Message-ID: <20251220100201.26d9b0db@pumpkin>
Date: Sat, 20 Dec 2025 10:02:01 +0000
From: David Laight <david.laight.linux@...il.com>
To: Vincent Mailhol <mailhol@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nsc@...nel.org>,
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling
 <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, Maarten
 Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
 <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie
 <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, Chris Mason
 <clm@...com>, David Sterba <dsterba@...e.com>, Kees Cook <kees@...nel.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>, Linus Torvalds
 <torvalds@...ux-foundation.org>, linux-kbuild@...r.kernel.org,
 linux-sparse@...r.kernel.org, linux-kernel@...r.kernel.org,
 llvm@...ts.linux.dev, dri-devel@...ts.freedesktop.org,
 linux-btrfs@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 4/4] minmax: remove useless cast in __is_nonneg()

On Fri, 19 Dec 2025 23:39:48 +0100
Vincent Mailhol <mailhol@...nel.org> wrote:

> The function like macro __is_nonneg() casts its argument to (long long)
> in an attempt to silence -Wtype-limits warnings on unsigned values.

nak.

The cast is needed for pointer types, not for -Wtype-limits.
which is why the '#if __SIZEOF_POINTER__ == __SIZEOF_LONG_LONG__'
test is there.

	David

> 
> But this workaround is incomplete as proven here:
> 
>   $ cat foo.c
>   #include <linux/minmax.h>
> 
>   int foo(unsigned int a)
>   {
>   	return __is_nonneg(a);
>   }
>   $ make CFLAGS_KERNEL="-Wtype-limits" foo.o
>     CALL    scripts/checksyscalls.sh
>     DESCEND objtool
>     INSTALL libsubcmd_headers
>     CC      foo.o
>   foo.c: In function 'foo':
>   ./include/linux/minmax.h:68:57: warning: comparison is always true due to limited range of data type [-Wtype-limits]
>      68 | #define __is_nonneg(ux) statically_true((long long)(ux) >= 0)
>         |                                                         ^~
>   ./include/linux/compiler.h:350:50: note: in definition of macro 'statically_true'
>     350 | #define statically_true(x) (__builtin_constant_p(x) && (x))
>         |                                                  ^
>   foo.c:5:16: note: in expansion of macro '__is_nonneg'
>       5 |         return __is_nonneg(a);
>         |                ^~~~~~~~~~~
>   ./include/linux/minmax.h:68:57: warning: comparison is always true due to limited range of data type [-Wtype-limits]
>      68 | #define __is_nonneg(ux) statically_true((long long)(ux) >= 0)
>         |                                                         ^~
>   ./include/linux/compiler.h:350:57: note: in definition of macro 'statically_true'
>     350 | #define statically_true(x) (__builtin_constant_p(x) && (x))
>         |                                                         ^
>   foo.c:5:16: note: in expansion of macro '__is_nonneg'
>       5 |         return __is_nonneg(a);
>         |                ^~~~~~~~~~~
> 
> And because -Wtype-limits is now globally disabled, such a workaround
> now becomes useless. Remove the __is_nonneg()'s cast and its related
> comment.
> 
> Signed-off-by: Vincent Mailhol <mailhol@...nel.org>
> ---
> Changelog:
> 
>   v1 -> v2: new patch
> ---
>  include/linux/minmax.h | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/include/linux/minmax.h b/include/linux/minmax.h
> index a0158db54a04..3e2e3e539ba1 100644
> --- a/include/linux/minmax.h
> +++ b/include/linux/minmax.h
> @@ -52,9 +52,6 @@
>  /*
>   * Check whether a signed value is always non-negative.
>   *
> - * A cast is needed to avoid any warnings from values that aren't signed
> - * integer types (in which case the result doesn't matter).
> - *
>   * On 64-bit any integer or pointer type can safely be cast to 'long long'.
>   * But on 32-bit we need to avoid warnings about casting pointers to integers
>   * of different sizes without truncating 64-bit values so 'long' or 'long long'
> @@ -65,7 +62,7 @@
>   * but they are handled by the !is_signed_type() case).
>   */
>  #if __SIZEOF_POINTER__ == __SIZEOF_LONG_LONG__
> -#define __is_nonneg(ux) statically_true((long long)(ux) >= 0)
> +#define __is_nonneg(ux) statically_true((ux) >= 0)
>  #else
>  #define __is_nonneg(ux) statically_true( \
>  	(typeof(__builtin_choose_expr(sizeof(ux) > 4, 1LL, 1L)))(ux) >= 0)
>