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Date: Thu, 8 Mar 2018 20:57:16 +0100
From: Rasmus Villemoes <linux@...musvillemoes.dk>
To: Josh Poimboeuf <jpoimboe@...hat.com>, Kees Cook <keescook@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
 corbet@....net, gustavo@...eddedor.com, rostedt@...dmis.org,
 Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>,
 David Sterba <dsterba@...e.com>, "David S. Miller" <davem@...emloft.net>,
 Alexey Kuznetsov <kuznet@....inr.ac.ru>,
 Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, Ingo Molnar <mingo@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
 Masahiro Yamada <yamada.masahiro@...ionext.com>, Borislav Petkov
 <bp@...e.de>, Randy Dunlap <rdunlap@...radead.org>,
 Ian Abbott <abbotti@....co.uk>, "Tobin C. Harding" <me@...in.cc>,
 Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
 Petr Mladek <pmladek@...e.com>,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
 linux-btrfs@...r.kernel.org, netdev@...r.kernel.org,
 kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 0/3] Remove accidental VLA usage

On 2018-03-08 16:02, Josh Poimboeuf wrote:
> On Wed, Mar 07, 2018 at 07:30:44PM -0800, Kees Cook wrote:
>> This series adds SIMPLE_MAX() to be used in places where a stack array
>> is actually fixed, but the compiler still warns about VLA usage due to
>> confusion caused by the safety checks in the max() macro.
>>
>> I'm sending these via -mm since that's where I've introduced SIMPLE_MAX(),
>> and they should all have no operational differences.
> 
> What if we instead simplify the max() macro's type checking so that GCC
> can more easily fold the array size constants?  The below patch seems to
> work:
> 

> +extern long __error_incompatible_types_in_min_macro;
> +extern long __error_incompatible_types_in_max_macro;
> +
> +#define __min(t1, t2, x, y)						\
> +	__builtin_choose_expr(__builtin_types_compatible_p(t1, t2),	\
> +			      (t1)(x) < (t2)(y) ? (t1)(x) : (t2)(y),	\
> +			      (t1)__error_incompatible_types_in_min_macro)
>  
>  /**
>   * min - return minimum of two values of the same or compatible types
>   * @x: first value
>   * @y: second value
>   */
> -#define min(x, y)					\
> -	__min(typeof(x), typeof(y),			\
> -	      __UNIQUE_ID(min1_), __UNIQUE_ID(min2_),	\
> -	      x, y)
> +#define min(x, y) __min(typeof(x), typeof(y), x, y)			\
>  

But this introduces the the-chosen-one-of-x-and-y-gets-evaluated-twice
problem. Maybe we don't care? But until we get a
__builtin_assert_this_has_no_side_effects() I think that's a little
dangerous.

Rasmus

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