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Date:   Tue, 13 Mar 2018 15:14:34 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.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 Tue, Mar 13, 2018 at 2:02 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> 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.

For fixing only 6 VLAs, we don't need all this effort. When it looked
like we could get away with just a "better" max(), sure. ;)

I'll send a "const_max()" which will refuse to work on
non-constant-values (so it doesn't get accidentally used on variables
that could be exposed to double-evaluation), and will work for stack
array declarations (to avoid the overly-sensitive -Wvla checks).

-Kees

-- 
Kees Cook
Pixel Security

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