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

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Date:	Thu, 5 May 2016 15:22:27 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	tytso@....edu, Sandy Harris <sandyinchina@...il.com>,
	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 05/05/2016 03:18 PM, 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....
> 

In theory, theory and practice should agree, but in practice, practice
is what counts.  I fully agree we should get rid of UD behavior where
doing so is practical, but not at the cost of breaking real-life
compilers, expecially not gcc, and to a lesser but still very real
extent icc and clang.

I would also agree that we should push the gcc developers to add to the
manual C-standard-UD behavior which are well-defined under the
gnu89/gnu99/gnu11 C dialects.

	-hpa