linux-hardening - RE: [PATCH 01/10] compiler.h: add statically

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Message-ID: <e115a4245e5342a994a7e596cc6357fa@AcuMS.aculab.com>
Date: Wed, 4 Dec 2024 18:30:40 +0000
From: David Laight <David.Laight@...LAB.COM>
To: Vincent Mailhol <mailhol.vincent@...adoo.fr>, Linus Torvalds
	<torvalds@...ux-foundation.org>, Luc Van Oostenryck
	<luc.vanoostenryck@...il.com>, Nathan Chancellor <nathan@...nel.org>, "Nick
 Desaulniers" <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, Yury Norov <yury.norov@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>, Kees Cook <kees@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>, Jani Nikula
	<jani.nikula@...ux.intel.com>, Joonas Lahtinen
	<joonas.lahtinen@...ux.intel.com>, Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Tvrtko Ursulin <tursulin@...ulin.net>, David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>, Suzuki K Poulose <suzuki.poulose@....com>,
	Mike Leach <mike.leach@...aro.org>, James Clark <james.clark@...aro.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Rikard Falkeborn
	<rikard.falkeborn@...il.com>, Martin Uecker
	<Martin.Uecker@....uni-goettingen.de>
CC: "linux-sparse@...r.kernel.org" <linux-sparse@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
	"intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"coresight@...ts.linaro.org" <coresight@...ts.linaro.org>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>
Subject: RE: [PATCH 01/10] compiler.h: add statically_false()

From: Vincent Mailhol
> Sent: 02 December 2024 17:33
> 
> From: Vincent Mailhol <mailhol.vincent@...adoo.fr>
> 
> For completion, add statically_false() which is the equivalent of
> statically_true() except that it will return true only if the input is
> known to be false at compile time.

This is pretty much pointless.
It is just as easy to invert the condition at the call site.

	David

> 
> The == operator is used instead of the ! negation to prevent a
> -Wint-in-bool-context compiler warning when the argument is not a
> boolean. For example:
> 
>   statically_false(var * 0)
> 
> Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
> ---
>  include/linux/compiler.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 469a64dd6495fefab2c85ffc279568a657b72660..a2a56a50dd85227a4fdc62236a2710ca37c5ba52 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -314,6 +314,7 @@ static inline void *offset_to_ptr(const int *off)
>   * values to determine that the condition is statically true.
>   */
>  #define statically_true(x) (__builtin_constant_p(x) && (x))
> +#define statically_false(x) (__builtin_constant_p(x) && (x) == 0)
> 
>  /*
>   * This is needed in functions which generate the stack canary, see
> 
> --
> 2.45.2
> 
> 

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