Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date: Sat, 31 Mar 2018 07:34:34 +0200
From: Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH v6] kernel.h: Retain constant expression output for
 max()/min()
Message-ID: <20180331053434.akno723avvcri654@gmail.com>
References: <20180326221554.GA45166@beast>
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In-Reply-To: <CAGXu5jK74cVQPigf2Q23E2pjonfzq+MMxhbqTNvzbWFxP4aGig@mail.gmail.com>
To: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, Linus Torvalds <torvalds@linux-foundation.org>, Martin Uecker <Martin.Uecker@med.uni-goettingen.de>, Josh Poimboeuf <jpoimboe@redhat.com>, Rasmus Villemoes <linux@rasmusvillemoes.dk>, Randy Dunlap <rdunlap@infradead.org>, Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>, David Laight <David.Laight@aculab.com>, Ian Abbott <abbotti@mev.co.uk>, LKML <linux-kernel@vger.kernel.org>, Kernel Hardening <kernel-hardening@lists.openwall.com>
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* Kees Cook <keescook@chromium.org> wrote:

> On Mon, Mar 26, 2018 at 10:47 PM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Kees Cook <keescook@chromium.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@kernel.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.

Great - once the number of warnings is zero, is the plan to enable the warning 
unconditionally?

Thanks,

	Ingo