On Sat, Mar 17, 2018 at 01:07:32PM -0700, Kees Cook wrote:

On Sat, Mar 17, 2018 at 11:52 AM, Linus Torvalds
[off-list ref] wrote:

quoted

So the above is completely insane, bit there is actually a chance that
using that completely crazy "x -> sizeof(char[x])" conversion actually
helps, because it really does have a (very odd) evaluation-time
change.  sizeof() has to be evaluated as part of the constant
expression evaluation, in ways that "__builtin_constant_p()" isn't
specified to be done.

But it is also definitely me grasping at straws. If that doesn't work
for 4.4, there's nothing else I can possibly see.

No luck! :( gcc 4.4 refuses to play along. And, hilariously, not only
does it not change the complaint about __builtin_choose_expr(), it
also thinks that's a VLA now.

./include/linux/mm.h: In function ‘get_mm_hiwater_rss’:
./include/linux/mm.h:1567: warning: variable length array is used
./include/linux/mm.h:1567: error: first argument to
‘__builtin_choose_expr’ not a constant

6.8 is happy with it (of course).

I do think the earlier version (without the
sizeof-hiding-builting_constant_p) provides a template for a
const_max() that both you and Rasmus would be happy with, though!

I thought we were dropping support for 4.4 (for other reasons).  Isn't
it 4.6 we should be looking at?

-- 
Josh