netdev - Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()

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Date:   Fri, 9 Mar 2018 17:30:15 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.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>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()

On Fri, Mar 9, 2018 at 4:38 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Mar 9, 2018 at 4:32 PM, Andrew Morton <akpm@...ux-foundation.org> wrote:
>>
>> I wonder which gcc versions actually accept Kees's addition.

Ah, my old nemesis, gcc 4.4.4. *sob*

> Note that we already do have this pattern, as seen by:
>
>   git grep -2  __builtin_choose_expr | grep -2 __builtin_constant_p
>
> which show drivers/pinctrl/intel/pinctrl-intel.h, so the pattern
> already exists current kernels and _works_ - it apparently just
> doesn't work in slightly more complicated cases.

Fun. Yeah, so all the PIN_GROUP() callers have either a literal or an
array name for that argument, so the argument to
__builtin_constant_p() isn't complex.

git grep '\bPIN_GROUP\b' | egrep -v '(1|2|3|true|false)\)'

> It's one reason why I wondered if simplifying the expression to have
> just that single __builtin_constant_p() might not end up working..

Yeah, it seems like it doesn't bail out as "false" for complex
expressions given to __builtin_constant_p().

If no magic solution, then which of these?

- go back to my original "multi-eval max only for constants" macro (meh)
- add gcc version checks around this and similarly for -Wvla in the future (eww)
- raise gcc version (yikes)

-Kees

-- 
Kees Cook
Pixel Security<div class="gmail_extra"><br><div class="gmail_quote">On
Fri, Mar 9, 2018 at 4:38 PM, Linus Torvalds <span dir="ltr">&lt;<a
href="mailto:torvalds@...ux-foundation.org"
target="_blank">torvalds@...ux-foundation.org</a>&gt;</span>
wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex"><span class="">On
Fri, Mar 9, 2018 at 4:32 PM, Andrew Morton &lt;<a
href="mailto:akpm@...ux-foundation.org">akpm@...ux-foundation.org</a>&gt;
wrote:<br>
&gt;<br>
&gt; I wonder which gcc versions actually accept Kees's addition.<br>
<br>
</span>Note that we already do have this pattern, as seen by:<br>
<br>
&nbsp; git grep -2&nbsp; __builtin_choose_expr | grep -2
__builtin_constant_p<br>
<br>
which show drivers/pinctrl/intel/pinctrl-<wbr>intel.h, so the pattern<br>
already exists current kernels and _works_ - it apparently just<br>
doesn't work in slightly more complicated cases.<br>
<br>
It's one reason why I wondered if simplifying the expression to have<br>
just that single __builtin_constant_p() might not end up working..<br>
<span class="HOEnZb"><font color="#888888"><br>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Linus<br>
</font></span></blockquote></div><br><br clear="all"><div><br></div>--
<br><div class="gmail_signature" data-smartmail="gmail_signature">Kees
Cook<br>Pixel Security</div>
</div>