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
