linux-kernel - RE: [PATCH 02/10] compiler.h: add is_const() as a replacement of __is

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <8f4b7d1bfbd84885bddf4cc1dfb9ce43@AcuMS.aculab.com>
Date: Fri, 6 Dec 2024 19:43:39 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Willy Tarreau' <w@....eu>, Linus Torvalds <torvalds@...ux-foundation.org>
CC: Vincent Mailhol <vincent.mailhol@...il.com>, 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>, "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>, "uecker@...raz.at" <uecker@...raz.at>
Subject: RE: [PATCH 02/10] compiler.h: add is_const() as a replacement of
 __is_constexpr()

From: Willy Tarreau
> Sent: 06 December 2024 19:39
> On Fri, Dec 06, 2024 at 11:15:20AM -0800, Linus Torvalds wrote:
> > On Fri, 6 Dec 2024 at 11:07, David Laight <David.Laight@...lab.com> wrote:
> > >
> > > I'm missing the compiler version and options to generate the error.
> >
> > Just -Wall with most recent gcc versions seems to do it. At least I
> > can repro it with gcc-14.2.1 and something silly like this:
> >
> >   $ cat t.c
> >   int fn(int a) { return (a<<2)?1:2; }
> >   $ gcc -Wall -S t.c
> >   t.c: In function 'fn':
> >   t.c:1:26: warning: '<<' in boolean context, did you mean '<'?
> > [-Wint-in-bool-context]
> >
> > > Does a '+ 0' help?  "(var << 2) + 0 ? 0 : 0"
> >
> > Yeah, that actually works.
> >
> > And "+0" is nice in that it should work in any context.
> 
> I've already used "+0" to shut certain warnings, I don't really remember
> which one, but also remember it was OK everywhere I needed.

I've often used +0u when -Wsign-compare is enabled.
Much safer than a cast.

> 
> Another trick I've been using to shut up the compiler is a cast via typeof
> and an intermediary variable:
> 
>   #define shut_up(expr)                           \
>           ({                                      \
>                   typeof(expr) _expr_ = expr;     \
>                   _expr_;                         \
>                   })

That is like OPTIMISER_HIDE_VAR() and can't be used in a 'constant integer expression'.

I suspect it also has the same nasty habit of adding an extra register move.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)