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Date: Fri, 9 Mar 2018 16:32:41 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>, Linux Kernel Mailing List
 <linux-kernel@vger.kernel.org>, Josh Poimboeuf <jpoimboe@redhat.com>,
 Rasmus Villemoes <linux@rasmusvillemoes.dk>, "Gustavo A. R. Silva"
 <gustavo@embeddedor.com>, "Tobin C. Harding" <me@tobin.cc>, Steven Rostedt
 <rostedt@goodmis.org>, Jonathan Corbet <corbet@lwn.net>, Chris Mason
 <clm@fb.com>, Josef Bacik <jbacik@fb.com>, David Sterba <dsterba@suse.com>,
 "David S. Miller" <davem@davemloft.net>, Alexey Kuznetsov
 <kuznet@ms2.inr.ac.ru>, Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>, Ingo
 Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>, Thomas
 Gleixner <tglx@linutronix.de>, Masahiro Yamada
 <yamada.masahiro@socionext.com>, Borislav Petkov <bp@suse.de>, Randy Dunlap
 <rdunlap@infradead.org>, Ian Abbott <abbotti@mev.co.uk>, Sergey Senozhatsky
 <sergey.senozhatsky.work@gmail.com>, Petr Mladek <pmladek@suse.com>, Andy
 Shevchenko <andriy.shevchenko@linux.intel.com>, Pantelis Antoniou
 <pantelis.antoniou@konsulko.com>, Linux Btrfs
 <linux-btrfs@vger.kernel.org>, Network Development
 <netdev@vger.kernel.org>, Kernel Hardening
 <kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in
 min()/max()
Message-Id: <20180309163241.a421e216999bd0b1f43a64c2@linux-foundation.org>
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References: <20180309200536.GA5670@beast>
	<20180309160719.154a3158e2d8ee56e43a918f@linux-foundation.org>
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On Fri, 9 Mar 2018 16:28:51 -0800 Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Fri, Mar 9, 2018 at 4:07 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > A brief poke failed to reveal a workaround - gcc-4.4.4 doesn't appear
> > to know that __builtin_constant_p(x) is a constant.  Or something.
> 
> LOL.
> 
> I suspect it might be that it wants to evaluate
> __builtin_choose_expr() at an earlier stage than it evaluates
> __builtin_constant_p(), so it's not that it doesn't know that
> __builtin_constant_p() is a constant, it just might not know it *yet*.
> 
> Maybe.
> 
> Side note, if it's not that, but just the "complex" expression that
> has the logical 'and' etc, maybe the code could just use
> 
>   __builtin_constant_p((x)+(y))
> 
> or something.

I'll do a bit more poking at it.

> But yeah:
> 
> > Sigh.  Wasn't there some talk about modernizing our toolchain
> > requirements?
> 
> Maybe it's just time to give up on 4.4.  We wanted 4.5 for "asm goto",
> and once we upgrade to 4.5 I think Arnd said that no distro actually
> ships it, so we might as well go to 4.6.
> 
> So maybe this is just the excuse to finally make that official, if
> there is no clever workaround any more.

I wonder which gcc versions actually accept Kees's addition.