linux-kernel - Re: [PATCH v4 1/2] kernel.h: Introduce const

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Date:   Thu, 15 Mar 2018 16:31:59 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Randy Dunlap <rdunlap@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        David Laight <David.Laight@...lab.com>,
        Ian Abbott <abbotti@....co.uk>,
        linux-input <linux-input@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v4 1/2] kernel.h: Introduce const_max() for VLA removal

On Thu, Mar 15, 2018 at 4:17 PM, Miguel Ojeda
<miguel.ojeda.sandonis@...il.com> wrote:
>> The full one, using your naming convention:
>>
>> #define const_max(x, y)                                          \
>>     ({                                                           \
>>         if (!__builtin_constant_p(x))                            \
>>             __error_not_const_arg();                             \
>>         if (!__builtin_constant_p(y))                            \
>>             __error_not_const_arg();                             \
>>         if (!__builtin_types_compatible_p(typeof(x), typeof(y))) \
>>             __error_incompatible_types();                        \
>>         if ((x) < 0)                                             \
>>             __error_not_positive_arg();                          \
>>         if ((y) < 0)                                             \
>>             __error_not_positive_arg();                          \
>>         __builtin_choose_expr((x) > (y), (x), (y));              \
>>     })
>>
>
> Nevermind... gcc doesn't take that as a constant expr, even if it
> compiles as one at -O0.

Yeah, unfortunately. :(

-Kees

-- 
Kees Cook
Pixel Security