linux-kernel - Re: [PATCH v6] kernel.h: Retain constant expression output for max()/min()

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Date:   Tue, 27 Mar 2018 07:47:48 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Martin Uecker <Martin.Uecker@....uni-goettingen.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Randy Dunlap <rdunlap@...radead.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Ian Abbott <abbotti@....co.uk>, linux-kernel@...r.kernel.org,
        kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v6] kernel.h: Retain constant expression output for
 max()/min()


* Kees Cook <keescook@...omium.org> wrote:

> In the effort to remove all VLAs from the kernel[1], it is desirable to
> build with -Wvla. However, this warning is overly pessimistic, in that
> it is only happy with stack array sizes that are declared as constant
> expressions, and not constant values. One case of this is the evaluation
> of the max() macro which, due to its construction, ends up converting
> constant expression arguments into a constant value result.
> 
> All attempts to rewrite this macro with __builtin_constant_p() failed with
> older compilers (e.g. gcc 4.4)[2]. However, Martin Uecker constructed[3] a
> mind-shattering solution that works everywhere. Cthulhu fhtagn!
> 
> This patch updates the min()/max() macros to evaluate to a constant
> expression when called on constant expression arguments. This removes
> several false-positive stack VLA warnings from an x86 allmodconfig
> build when -Wvla is added:

Cool!

Acked-by: Ingo Molnar <mingo@...nel.org>

How many warnings are left in an allmodconfig build?

Thanks,

	Ingo