linux-kernel - Re: better patch for linux/bitops.h

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Date:	Thu, 05 May 2016 15:38:15 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	tytso@....edu, Sandy Harris <sandyinchina@...il.com>
CC:	Jeffrey Walton <noloader@...il.com>, John Denker <jsd@...n.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Stephan Mueller <smueller@...onox.de>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Andi Kleen <andi@...stfloor.org>,
	Jason Cooper <cryptography@...edaemon.net>,
	linux-crypto@...r.kernel.org
Subject: Re: better patch for linux/bitops.h

On May 5, 2016 3:18:09 PM PDT, tytso@....edu wrote:
>On Thu, May 05, 2016 at 05:34:50PM -0400, Sandy Harris wrote:
>> 
>> I completely fail to see why tests or compiler versions should be
>> part of the discussion. The C standard says the behaviour in
>> certain cases is undefined, so a standard-compliant compiler
>> can generate more-or-less any code there.
>> 
>
>> As long as any of portability, reliability or security are among our
>> goals, any code that can give undefined behaviour should be
>> considered problematic.
>
>Because compilers have been known not necessarily to obey the specs,
>and/or interpret the specs in way that not everyone agrees with.  It's
>also the case that we are *already* disabling certain C optimizations
>which are technically allowed by the spec, but which kernel
>programmers consider insane (e.g., strict aliasing).
>
>And of course, memzero_explicit() which crypto people understand is
>necessary, is something that technically compilers are allowed to
>optimize according to the spec.  So trying to write secure kernel code
>which will work on arbitrary compilers may well be impossible.
>
>And which is also why people have said (mostly in jest), "A
>sufficiently advanced compiler is indistinguishable from an
>adversary."  (I assume people will agree that optimizing away a memset
>needed to clear secrets from memory would be considered adversarial,
>at the very least!)
>
>So this is why I tend to take a much more pragmatic viewpoint on
>things.  Sure, it makes sense to pay attention to what the C standard
>writers are trying to do to us; but if we need to suppress certain
>optimizations to write sane kernel code --- I'm ok with that.  And
>this is why using a trust-but-verify on a specific set of compilers
>and ranges of compiler versions is a really good idea....
>
>    	      	       		     - Ted

I have filed a gcc bug to have the preexisting rotate idiom officially documented as a GNU C extension.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70967
-- 
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