Re: [PATCH] kernel.h: Skip single-eval logic on literals in min()/max()

From: Kees Cook <keescook@chromium.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.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>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH] kernel.h: Skip single-eval logic on literals in min()/max()
Date: Thu, 8 Mar 2018 17:46:42 -0800	[thread overview]
Message-ID: <CAGXu5jJG9JL_3=rgqS+_B-FEumwa4qZkL6YA=KKxaMe-ugz0Rg@mail.gmail.com> (raw)
In-Reply-To: <CA+55aFxXb8JhpELWcFJnQMYy6sTyndwdNpCLFUznA9ML7-oL_Q@mail.gmail.com>

On Thu, Mar 8, 2018 at 5:35 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> I don't want to weaken the type enforcement, and I _thought_ you had
> done that __builtin_types_compatible_p() to keep it in place.

I thought so too (that originally came from Josh), but on removal, I
was surprised that the checking was retained. :)

> But if that's not why you did it, then why was it there at all? If the
> type warning shows through even if it's in the other expression, then
> just a
>
>
> #define __min(t1, t2, x, y)                             \
>         __builtin_choose_expr(                          \
>                 __builtin_constant_p(x) &               \
>                 __builtin_constant_p(y),                \
>                 (t1)(x) < (t2)(y) ? (t1)(x) : (t2)(y),  \
>                 __single_eval_min(t1, t2,               \
>    ...
>
> would seem to be sufficient?
>
> Because logically, the only thing that matters is that x and y don't
> have any side effects and can be evaluated twice, and
> "__builtin_constant_p()" is already a much stronger version of that.
>
> Hmm? The __builtin_types_compatible_p() just doesn't seem to matter
> for the only thing I thought it was there for.

Yup, agreed. I'll drop it.

-Kees

-- 
Kees Cook
Pixel Security