Messages in this thread | | | From | Kees Cook <> | Date | Sat, 17 Mar 2018 13:07:32 -0700 | Subject | Re: [PATCH v5 0/2] Remove false-positive VLAs when using max() |
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On Sat, Mar 17, 2018 at 11:52 AM, Linus Torvalds <torvalds@linux-foundation.org> 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 ‘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!
-Kees
-- Kees Cook Pixel Security
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