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Date: Thu, 8 Mar 2018 12:39:58 -0800
From: Kees Cook <keescook@...omium.org>
To: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	LKML <linux-kernel@...r.kernel.org>, Jonathan Corbet <corbet@....net>, 
	"Gustavo A. R. Silva" <gustavo@...eddedor.com>, Steven Rostedt <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 <linux-btrfs@...r.kernel.org>, 
	Network Development <netdev@...r.kernel.org>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 0/3] Remove accidental VLA usage

On Thu, Mar 8, 2018 at 11:57 AM, Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
> 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.

Eek, yes, we can't do the double-eval. The proposed change breaks
things badly. :)

a:   20
b:   40
max(a++, b++): 40
a:   21
b:   41

a:   20
b:   40
new_max(a++, b++): 41
a:   21
b:   42

However, this works for me:

#define __new_max(t1, t2, max1, max2, x, y)                    \
       __builtin_choose_expr(__builtin_constant_p(x) && \
                             __builtin_constant_p(y) && \
                             __builtin_types_compatible_p(t1, t2),     \
                             (t1)(x) > (t2)(y) ? (t1)(x) : (t2)(y),    \
                             __max(t1, t2, max1, max2, x, y))

#define new_max(x, y) \
        __new_max(typeof(x), typeof(y),                 \
              __UNIQUE_ID(max1_), __UNIQUE_ID(max2_),   \
              x, y)

(pardon the whitespace damage...)

Let me spin a sane patch and test it...

-Kees

-- 
Kees Cook
Pixel Security

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