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From:   Kees Cook <keescook@chromium.org>
Date:   Thu, 8 Mar 2018 17:46:42 -0800
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Subject: Re: [PATCH] kernel.h: Skip single-eval logic on literals in min()/max()
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>,
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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