Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()

[Josh Poimboeuf]

On Sat, Mar 17, 2018 at 01:07:32PM -0700, Kees Cook wrote:
> On Sat, Mar 17, 2018 at 11:52 AM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > 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 âget_mm_hiwater_rssâ:
> ./include/linux/mm.h:1567: warning: variable length array is used
> ./include/linux/mm.h:1567: error: first argument to
> â__builtin_choose_exprâ 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