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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 8 Mar 2018 15:48:10 -0800
Message-ID: <CA+55aFyf-ajRzW2sUwFrrMOAu5UyuOk5L8g9xfkr4VuJeWjV9w@mail.gmail.com>
Subject: Re: [PATCH] kernel.h: Skip single-eval logic on literals in min()/max()
To:     Kees Cook <keescook@chromium.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>,
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        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>,
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        Thomas Gleixner <tglx@linutronix.de>,
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On Thu, Mar 8, 2018 at 1:40 PM, Kees Cook <keescook@chromium.org> wrote:
> +#define __min(t1, t2, x, y)                                            \
> +       __builtin_choose_expr(__builtin_constant_p(x) &&                \
> +                             __builtin_constant_p(y) &&                \
> +                             __builtin_types_compatible_p(t1, t2),     \
> +                             (t1)(x) < (t2)(y) ? (t1)(x) : (t2)(y),    \

I understand why you use __builtin_types_compatible_p(), but please don't.

It will mean that trivial constants like "5" and "sizeof(x)" won't
simplify, because they have different types.

The ?: will give the right combined type anyway, and if you want the
type comparison warning, just add a comma-expression with something
like like

   (t1 *)1 == (t2 *)1

to get the type compatibility warning.

Yeah, yeah, maybe none of the VLA cases triggered that, but it seems
silly to not just get that obvious constant case right.

Hmm?

              Linus