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Date:   Fri, 9 Mar 2018 17:31:42 -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 5:30 PM, Kees Cook <keescook@...omium.org> wrote:
> --
> Kees Cook
> Pixel Security<div class="gmail_extra"><br><div class="gmail_quote">On
> [...]

WTF, gmail just blasted HTML into my explicitly plain-text email?! Apologies...

-- 
Kees Cook
Pixel Security<div class="gmail_extra"><br><div class="gmail_quote">On
Fri, Mar 9, 2018 at 5:30 PM, Kees Cook <span dir="ltr">&lt;<a
href="mailto:keescook@...omium.org"
target="_blank">keescook@...omium.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 4:38 PM, Linus Torvalds<br>
&lt;<a href="mailto:torvalds@...ux-foundation.org">torvalds@...ux-foundation.org</a><wbr>&gt;
wrote:<br>
&gt; 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;&gt;<br>
&gt;&gt; I wonder which gcc versions actually accept Kees's addition.<br>
<br>
</span>Ah, my old nemesis, gcc 4.4.4. *sob*<br>
<span class=""><br>
&gt; Note that we already do have this pattern, as seen by:<br>
&gt;<br>
&gt;&nbsp; &nbsp;git grep -2&nbsp; __builtin_choose_expr | grep -2
__builtin_constant_p<br>
&gt;<br>
&gt; which show drivers/pinctrl/intel/pinctrl-<wbr>intel.h, so the pattern<br>
&gt; already exists current kernels and _works_ - it apparently just<br>
&gt; doesn't work in slightly more complicated cases.<br>
<br>
</span>Fun. Yeah, so all the PIN_GROUP() callers have either a literal or an<br>
array name for that argument, so the argument to<br>
__builtin_constant_p() isn't complex.<br>
<br>
git grep '\bPIN_GROUP\b' | egrep -v '(1|2|3|true|false)\)'<br>
<span class=""><br>
&gt; It's one reason why I wondered if simplifying the expression to have<br>
&gt; just that single __builtin_constant_p() might not end up working..<br>
<br>
</span>Yeah, it seems like it doesn't bail out as "false" for complex<br>
expressions given to __builtin_constant_p().<br>
<br>
If no magic solution, then which of these?<br>
<br>
- go back to my original "multi-eval max only for constants" macro (meh)<br>
- add gcc version checks around this and similarly for -Wvla in the
future (eww)<br>
- raise gcc version (yikes)<br>
<br>
-Kees<br>
<br>
--<br>
Kees Cook<br>
Pixel Security&lt;div class="gmail_extra"&gt;&lt;br&gt;&lt;div
class="gmail_quote"&gt;On<br>
Fri, Mar 9, 2018 at 4:38 PM, Linus Torvalds &lt;span
dir="ltr"&gt;&amp;lt;&lt;a<br>
<span class="">href="mailto:<a
href="mailto:torvalds@...ux-foundation.org">torvalds@...ux-<wbr>foundation.org</a>"<br>
target="_blank"&gt;<a
href="mailto:torvalds@...ux-foundation.org">torvalds@<wbr>linux-foundation.org</a>&lt;/a&gt;&amp;gt;&lt;/<wbr>span&gt;<br>
wrote:&lt;br&gt;&lt;blockquote class="gmail_quote" style="margin:0 0 0<br>
</span>.8ex;border-left:1px #ccc solid;padding-left:1ex"&gt;&lt;span
class=""&gt;On<br>
Fri, Mar 9, 2018 at 4:32 PM, Andrew Morton &amp;lt;&lt;a<br>
href="mailto:<a
href="mailto:akpm@...ux-foundation.org">akpm@...ux-<wbr>foundation.org</a>"&gt;<a
href="mailto:akpm@...ux-foundation.org">akpm@...ux-<wbr>foundation.org</a>&lt;/a&gt;&amp;gt;<br>
wrote:&lt;br&gt;<br>
&amp;gt;&lt;br&gt;<br>
&amp;gt; I wonder which gcc versions actually accept Kees's
addition.&lt;br&gt;<br>
&lt;br&gt;<br>
&lt;/span&gt;Note that we already do have this pattern, as seen
by:&lt;br&gt;<br>
&lt;br&gt;<br>
&amp;nbsp; git grep -2&amp;nbsp; __builtin_choose_expr | grep -2<br>
__builtin_constant_p&lt;br&gt;<br>
&lt;br&gt;<br>
which show drivers/pinctrl/intel/pinctrl-<wbr>&lt;wbr&gt;intel.h, so
the pattern&lt;br&gt;<br>
already exists current kernels and _works_ - it apparently just&lt;br&gt;<br>
doesn't work in slightly more complicated cases.&lt;br&gt;<br>
&lt;br&gt;<br>
It's one reason why I wondered if simplifying the expression to
have&lt;br&gt;<br>
just that single __builtin_constant_p() might not end up working..&lt;br&gt;<br>
<div class="HOEnZb"><div class="h5">&lt;span
class="HOEnZb"&gt;&lt;font color="#888888"&gt;&lt;br&gt;<br>
&amp;nbsp; &amp;nbsp; &amp;nbsp; &amp;nbsp; &amp;nbsp; &amp;nbsp;
&amp;nbsp; Linus&lt;br&gt;<br>
&lt;/font&gt;&lt;/span&gt;&lt;/blockquote&gt;&lt;/<wbr>div&gt;&lt;br&gt;&lt;br
clear="all"&gt;&lt;div&gt;&lt;br&gt;&lt;/div&gt;--<br>
&lt;br&gt;&lt;div class="gmail_signature"
data-smartmail="gmail_<wbr>signature"&gt;Kees<br>
Cook&lt;br&gt;Pixel Security&lt;/div&gt;<br>
&lt;/div&gt;<br>
</div></div></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>

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