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: <CAHk-=wjpVXEjX16PP=-hi4CgLqEGJ_U-WvKWq+J3C+FW-hSSfg@mail.gmail.com>
Date: Sat, 7 Dec 2024 10:26:01 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Martin Uecker <muecker@...g.de>
Cc: David Laight <David.Laight@...lab.com>, Vincent Mailhol <mailhol.vincent@...adoo.fr>, 
	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>
Subject: Re: [PATCH 02/10] compiler.h: add is_const() as a replacement of __is_constexpr()

On Sat, 7 Dec 2024 at 05:07, Martin Uecker <muecker@...g.de> wrote:
>
> VLA use *less* stack than a fixed size arrays with fixed bound.

Not really. You end up with tons of problems, not the least of which
is how to actually analyze the stack size. It also gets *very* nasty
to have code that declares the VLA size using an argument that is then
checked afterwards - and if you have a strong preference for
"declarations before code", you end up with *horrific* issues.

And even if you are super-careful, and you solved the analysis
problem, in practice VLAs will cause huge stack issues simply due to
code generation issues.  The compiler will end up doing extra
alignment and extra frame handling and saving, to the point where any
advantages the VLA would bring is completely dwarfed by all the
disadvantages.

We went through this. We are so *much* better off without VLAs that
it's not even funny.

Now when the compiler says "your stack size is big", you just look
"Oh, that struct should be allocated with kmalloc, not on the stack".
Boom. Done.

            Linus