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Date: Mon, 12 Mar 2018 21:28:57 -0700
From: Kees Cook <>
To: Linus Torvalds <>
Cc: Andrew Morton <>, 
	Linux Kernel Mailing List <>, Josh Poimboeuf <>, 
	Rasmus Villemoes <>, "Gustavo A. R. Silva" <>, 
	"Tobin C. Harding" <>, Steven Rostedt <>, Jonathan Corbet <>, 
	Chris Mason <>, Josef Bacik <>, David Sterba <>, 
	"David S. Miller" <>, Alexey Kuznetsov <>, 
	Hideaki YOSHIFUJI <>, Ingo Molnar <>, 
	Peter Zijlstra <>, Thomas Gleixner <>, 
	Masahiro Yamada <>, Borislav Petkov <>, 
	Randy Dunlap <>, Ian Abbott <>, 
	Sergey Senozhatsky <>, Petr Mladek <>, 
	Andy Shevchenko <>, 
	Pantelis Antoniou <>, Linux Btrfs <>, 
	Network Development <>, 
	Kernel Hardening <>
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
<> wrote:
> On Mon, Mar 12, 2018 at 3:55 PM, Andrew Morton
> <> 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 Cook
Pixel Security

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