linux-kernel - Re: drivers: random: Shift out-of-bounds in _mix_pool

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Message-Id: <2E512EF7-577C-43C2-AB95-30DC25AD059D@dilger.ca>
Date:	Fri, 24 Oct 2014 16:09:48 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Theodore Ts'o <tytso@....edu>,
	Daniel Borkmann <dborkman@...hat.com>,
	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Michal Marek <mmarek@...e.cz>,
	"x86@...nel.org" <x86@...nel.org>, linux-kbuild@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Konstantin Khlebnikov <koct9i@...il.com>
Subject: Re: drivers: random: Shift out-of-bounds in _mix_pool_bytes

On Oct 24, 2014, at 9:10 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Fri, Oct 24, 2014 at 7:04 PM, Sasha Levin <sasha.levin@...cle.com> wrote:
>> On 10/24/2014 09:42 AM, Peter Zijlstra wrote:
>>> On Fri, Oct 24, 2014 at 09:23:35AM -0400, Sasha Levin wrote:
>>>> 
>>>> i >> 32 may happen to be "i", but is there anything that prevents the compiler from returning, let's say, 42?
>>> 
>>> Not really, although gcc seems to opt for the 'sane' option and emit
>>> the instruction and let the arch figure out how to deal with it. 
>>> Hence the 'fun' difference between x86 and ARM.
>> 
>> It's interesting how many different views on undefined behaviour there are between kernel folks. 
>> 
>> Everything between Ted Ts'o saying that GCC can launch nethack on oversized shifts, to DaveM saying he will file a GCC bug if the
>> behaviour isn't sane w.r.t to memcpy().
> 
> One of the benefits of fixing such issues (or not letting them into
> code in the first place) is just saving numerous hours of top-notch
> engineers spent on disputes like this.

By the principle of least surprise, I would expect "__u32 >> N", where
N >= 32 to return zero instead of random garbage.  For N < 32 it will
return progressively smaller numbers, until it has shifted away all of
the set bits, at which turn it will return 0.  For it suddenly to jump
up once N = 32 is used, is counter-intuitive.

Cheers, Andreas






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