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Date: Tue, 13 Mar 2018 14:02:48 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Kees Cook <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Linux Kernel Mailing
 List <linux-kernel@...r.kernel.org>, Josh Poimboeuf <jpoimboe@...hat.com>,
 Rasmus Villemoes <linux@...musvillemoes.dk>, "Gustavo A. R. Silva"
 <gustavo@...eddedor.com>, "Tobin C. Harding" <me@...in.cc>, Steven Rostedt
 <rostedt@...dmis.org>, Jonathan Corbet <corbet@....net>, Chris Mason
 <clm@...com>, Josef Bacik <jbacik@...com>, David Sterba <dsterba@...e.com>,
 "David S. Miller" <davem@...emloft.net>, Alexey Kuznetsov
 <kuznet@....inr.ac.ru>, Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, Ingo
 Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Thomas
 Gleixner <tglx@...utronix.de>, Masahiro Yamada
 <yamada.masahiro@...ionext.com>, Borislav Petkov <bp@...e.de>, Randy Dunlap
 <rdunlap@...radead.org>, Ian Abbott <abbotti@....co.uk>, Sergey Senozhatsky
 <sergey.senozhatsky.work@...il.com>, Petr Mladek <pmladek@...e.com>, Andy
 Shevchenko <andriy.shevchenko@...ux.intel.com>, Pantelis Antoniou
 <pantelis.antoniou@...sulko.com>, Linux Btrfs
 <linux-btrfs@...r.kernel.org>, Network Development
 <netdev@...r.kernel.org>, Kernel Hardening
 <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in
 min()/max()

On Mon, 12 Mar 2018 21:28:57 -0700 Kees Cook <keescook@...omium.org> wrote:

> On Mon, Mar 12, 2018 at 4:57 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> > On Mon, Mar 12, 2018 at 3:55 PM, Andrew Morton
> > <akpm@...ux-foundation.org> wrote:
> >>
> >> Replacing the __builtin_choose_expr() with ?: works of course.
> >
> > Hmm. That sounds like the right thing to do. We were so myopically
> > staring at the __builtin_choose_expr() problem that we overlooked the
> > obvious solution.
> >
> > Using __builtin_constant_p() together with a ?: is in fact our common
> > pattern, so that should be fine. The only real reason to use
> > __builtin_choose_expr() is if you want to get the *type* to vary
> > depending on which side you choose, but that's not an issue for
> > min/max.
> 
> This doesn't solve it for -Wvla, unfortunately. That was the point of
> Josh's original suggestion of __builtin_choose_expr().
> 
> Try building with KCFLAGS=-Wval and checking net/ipv6/proc.c:
> 
> net/ipv6/proc.c: In function ‘snmp6_seq_show_item’:
> net/ipv6/proc.c:198:2: warning: ISO C90 forbids array ‘buff’ whose
> size can’t be evaluated [-Wvla]
>   unsigned long buff[SNMP_MIB_MAX];
>   ^~~~~~~~

PITA.  Didn't we once have a different way of detecting VLAs?  Some
post-compilation asm parser, iirc.

I suppose the world wouldn't end if we had a gcc version ifdef in
kernel.h.  We'll get to remove it in, oh, ten years.

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