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Date: Mon, 12 Mar 2018 21:28:57 -0700
From: Kees Cook <keescook@...omium.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...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, 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];
  ^~~~~~~~


-Kees

-- 
Kees Cook
Pixel Security

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