Re: [PATCH] kernel.h: Skip single-eval logic on literals in min()/max()

[Kees Cook <keescook@chromium.org>]

On Thu, Mar 8, 2018 at 3:48 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Mar 8, 2018 at 1:40 PM, Kees Cook <keescook@chromium.org> wrote:
>> +#define __min(t1, t2, 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),    \
>
> I understand why you use __builtin_types_compatible_p(), but please don't.
>
> It will mean that trivial constants like "5" and "sizeof(x)" won't
> simplify, because they have different types.

Rasmus mentioned this too. What I said there was that I was shy to
make that change, since we already can't mix that kind of thing with
the existing min()/max() implementation. The existing min()/max() is
already extremely strict, so there are no instances of this in the
tree. If I explicitly add one, I see this with or without the patch:

In file included from drivers/misc/lkdtm.h:7:0,
                 from drivers/misc/lkdtm_core.c:33:
drivers/misc/lkdtm_core.c: In function ‘lkdtm_module_exit’:
./include/linux/kernel.h:809:16: warning: comparison of distinct
pointer types lacks a cast
  (void) (&max1 == &max2);   \
                ^
./include/linux/kernel.h:818:2: note: in expansion of macro ‘__max’
  __max(typeof(x), typeof(y),   \
  ^~~~~
./include/linux/printk.h:308:34: note: in expansion of macro ‘max’
  printk(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__)
                                  ^~~~~~~~~~~
drivers/misc/lkdtm_core.c:500:2: note: in expansion of macro ‘pr_info’
  pr_info("%lu\n", max(16, sizeof(unsigned long)));
  ^~~~~~~

> The ?: will give the right combined type anyway, and if you want the
> type comparison warning, just add a comma-expression with something
> like like
>
>    (t1 *)1 == (t2 *)1
>
> to get the type compatibility warning.

When I tried removing __builtin_types_compatible_p(), I still got the
type-check warning because I think the preprocessor still sees the
"(void) (&min1 == &min2)" before optimizing? So, I technically _can_
drop the __builtin_types_compatible_p(), and still keep the type
warning. :P

> Yeah, yeah, maybe none of the VLA cases triggered that, but it seems
> silly to not just get that obvious constant case right.
>
> Hmm?

So are you saying you _want_ the type enforcement weakened here, or
that I should just not use __builtin_types_compatible_p()?

Thanks!

-Kees

-- 
Kees Cook
Pixel Security