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

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Date:   Fri, 30 Mar 2018 21:57:01 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Ingo Molnar <mingo@...nel.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>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v6] kernel.h: Retain constant expression output for max()/min()

On Mon, Mar 26, 2018 at 10:47 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * 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?

For -Wvla? Out of the original 112 files with VLAs, 42 haven't had a
patch applied yet. Doing a linux-next allmodconfig build with the
max() patch and my latest ecc patch, we've gone from 316 warning
instances to 205. More than half of those are in
include/crypto/skcipher.h and include/crypto/hash.h.

-Kees

-- 
Kees Cook
Pixel Security