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SubjectRe: [PATCH 0/3] Remove accidental VLA usage
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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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