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SubjectRe: [PATCH 0/3] Remove accidental VLA usage
On Thu, Mar 8, 2018 at 11:57 AM, Rasmus Villemoes
<linux@rasmusvillemoes.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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