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

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Message-ID: <20180317225524.vy7vpopgiwjcp2sa@treble>
Date:   Sat, 17 Mar 2018 17:55:24 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Florian Weimer <fweimer@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Randy Dunlap <rdunlap@...radead.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        David Laight <David.Laight@...lab.com>,
        Ian Abbott <abbotti@....co.uk>,
        linux-input <linux-input@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()

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@...ux-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!

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

-- 
Josh