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Date: Fri, 24 Oct 2014 11:04:11 -0400
From: Sasha Levin <sasha.levin@oracle.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: "Theodore Ts'o" <tytso@mit.edu>, Daniel Borkmann <dborkman@redhat.com>,
        Andrey Ryabinin <a.ryabinin@samsung.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Michal Marek <mmarek@suse.cz>,
        x86@kernel.org, linux-kbuild@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Dmitry Vyukov <dvyukov@google.com>,
        Konstantin Khlebnikov <koct9i@gmail.com>
Subject: Re: drivers: random: Shift out-of-bounds in _mix_pool_bytes
References: <1413802499-17928-1-git-send-email-a.ryabinin@samsung.com> <5444EBFA.5030103@samsung.com> <20141020124929.GA23177@thunk.org> <54451501.2070700@samsung.com> <5445179A.4080804@redhat.com> <20141020141635.GA4499@thunk.org> <20141024100108.GF12706@worktop.programming.kicks-ass.net> <544A52D7.6000202@oracle.com> <20141024134205.GB21513@worktop.programming.kicks-ass.net>
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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().


Thanks,
Sasha