linux-kernel - Re: [PATCH v3 0/3] Add compile time sanity check of GENMASK inputs

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Date:   Mon, 12 Aug 2019 10:58:11 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Rikard Falkeborn <rikard.falkeborn@...il.com>
Cc:     akpm@...ux-foundation.org, joe@...ches.com,
        johannes@...solutions.net, linux-kernel@...r.kernel.org,
        yamada.masahiro@...ionext.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Subject: Re: [PATCH v3 0/3] Add compile time sanity check of GENMASK inputs

On Sun, Aug 11, 2019 at 08:49:35PM +0200, Rikard Falkeborn wrote:
> Hello,
> 
> A new attempt to try to add build time validity checks of GENMASK (and
> GENMASK_ULL) inputs. There main differences from v2:
> 
> Remove a define of BUILD_BUG_ON in x86/boot to avoid a compiler warning
> about redefining BUILD_BUG_ON. Instead, use the common one from
> include/.
> 
> Drop patch 2 in v2 where GENMASK arguments where made more verbose.
> 
> Add a cast in the BUILD_BUG_ON_ZERO macro change the type to int to
> avoid the somewhat clumpsy casts of BUILD_BUG_ON_ZERO. The second patch
> in this series adds such a cast to BUILD_BUG_ON_ZERO, which makes it
> possible to avoid casts when using BUILD_BUG_ON_ZERO in patch 3.
> 
> I have checked all users of BUILD_BUG_ON_ZERO and I did not find a case
> where adding a cast to int would affect existing users but I'd feel much
> more comfortable if someone else double (or tripple) checked (there are
> ~80 instances plus ~10 copies in tools). Perhaps I should have CC:d
> maintainers of files using BUILD_BUG_ON_ZERO?
> 
> Finally, use __builtin_constant_p instead of __is_constexpr. This avoids
> pulling in kernel.h in bits.h.

Cool; I like this. I spent some time convincing myself that the
side-effects really aren't double-evaluated, and it looks fine to me. :)

For the series:

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees