Re: [PATCH v4 1/2] kernel.h: Introduce const_max() for VLA removal

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, Mar 15, 2018 at 3:16 PM, Kees Cook <keescook@chromium.org> wrote:
>
> size_t __error_not_const_arg(void) \
> __compiletime_error("const_max() used with non-compile-time constant arg");
> #define const_max(x, y)                                         \
>         __builtin_choose_expr(__builtin_constant_p(x) &&        \
>                               __builtin_constant_p(y),          \
>                               (typeof(x))(x) > (typeof(y))(y) ? \
>                                         (x) : (y),              \
>                               __error_not_const_arg())
>
> Is typeof() forcing enums to int? Regardless, I'll put this through
> larger testing. How does that look?

Ok, that alleviates my worry about one class of insane behavior, but
it does raise a few other questions:

 - what drugs is gcc on where (typeof(x)(x)) makes a difference? Funky.

 - this does have the usual "what happen if you do

     const_max(-1,sizeof(x))

where the comparison will now be done in 'size_t', and -1 ends up
being a very very big unsigned integer.

Is there no way to get that type checking inserted? Maybe now is a
good point for that __builtin_types_compatible(), and add it to the
constness checking (and change the name of that error case function)?

          Linus