Openwall GNU/*/Linux - a small security-enhanced Linux distro for servers
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Sat, 17 Mar 2018 13:07:32 -0700
From: Kees Cook <>
To: Linus Torvalds <>
Cc: Al Viro <>, Florian Weimer <>, 
	Andrew Morton <>, Josh Poimboeuf <>, 
	Rasmus Villemoes <>, Randy Dunlap <>, 
	Miguel Ojeda <>, Ingo Molnar <>, 
	David Laight <>, Ian Abbott <>, 
	linux-input <>, linux-btrfs <>, 
	Network Development <>, 
	Linux Kernel Mailing List <>, 
	Kernel Hardening <>
Subject: Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()

On Sat, Mar 17, 2018 at 11:52 AM, Linus Torvalds
<> 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 Cook
Pixel Security

Powered by blists - more mailing lists

Your e-mail address:

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.