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

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

On Thu, Mar 8, 2018 at 5:35 PM, Linus Torvalds
<torvalds@...ux-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