From: "H. Peter Anvin" <hpa@zytor.com>
To: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
Dmitry Vyukov <dvyukov@google.com>
Cc: Sasha Levin <sasha.levin@oracle.com>,
Peter Zijlstra <peterz@infradead.org>,
"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>, Michal Marek <mmarek@suse.cz>,
"x86@kernel.org" <x86@kernel.org>,
linux-kbuild@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
Andreas Dilger <adilger.kernel@dilger.ca>,
Konstantin Khlebnikov <koct9i@gmail.com>
Subject: Re: drivers: random: Shift out-of-bounds in _mix_pool_bytes
Date: Fri, 24 Oct 2014 15:23:40 -0700 [thread overview]
Message-ID: <544AD16C.40904@zytor.com> (raw)
In-Reply-To: <20141024220502.54cac82d@alan.etchedpixels.co.uk>
On 10/24/2014 02:05 PM, One Thousand Gnomes wrote:
> On Fri, 24 Oct 2014 19:10:49 +0400
> Dmitry Vyukov <dvyukov@google.com> wrote:
>
>> On Fri, Oct 24, 2014 at 7:04 PM, Sasha Levin <sasha.levin@oracle.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.
>
> Also it means when someone quietly changes the default behaviour next
> year in the compiler they won't spend months trying to work out why it
> broke.
>
> gcc has one behaviour but people also try and build the kernel with icc
> and with llvm. In addition in some cases you risk the compiler simply
> generating an undefined in hardware operation and the hardware behaviour
> changing. If x >> 32 is undefined then generating "load Y with the
> shift, shift X left by Y" is fine. What happens in future silicon - who
> knows.
>
> Most of the kernel is already very careful about the >> 32 problem.
>
The real question is if we can rely on the gcc-ism:
(x >> (S-y)) | (x << y)
... where S is the number of bits to indicate a rotate.
This is technically a gcc extension to the C language.
-hpa
next prev parent reply other threads:[~2014-10-24 22:24 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-20 10:54 [RFC] UBSan: run-time undefined behavior sanity checker Andrey Ryabinin
2014-10-20 10:54 ` [RFC PATCH] " Andrey Ryabinin
2014-10-20 19:35 ` Sasha Levin
2014-10-21 8:03 ` Andrey Ryabinin
2014-10-24 8:31 ` y.gribov
2014-10-24 10:36 ` Andrey Ryabinin
2014-10-21 9:47 ` Peter Zijlstra
2014-10-21 10:09 ` Andrey Ryabinin
2014-10-24 10:30 ` Peter Zijlstra
2014-10-21 17:06 ` Randy Dunlap
2014-10-22 9:58 ` Rasmus Villemoes
2014-10-22 11:16 ` Andrey Ryabinin
2014-10-20 11:03 ` drivers: random: Shift out-of-bounds in _mix_pool_bytes Andrey Ryabinin
2014-10-20 12:49 ` Theodore Ts'o
2014-10-20 13:58 ` Andrey Ryabinin
2014-10-20 14:08 ` Theodore Ts'o
2014-10-20 14:09 ` Daniel Borkmann
2014-10-20 14:13 ` Sasha Levin
2014-10-20 14:16 ` Theodore Ts'o
2014-10-20 14:42 ` Andrey Ryabinin
2014-10-24 10:01 ` Peter Zijlstra
2014-10-24 10:16 ` Andrey Ryabinin
2014-10-24 13:23 ` Sasha Levin
2014-10-24 13:42 ` Peter Zijlstra
2014-10-24 15:04 ` Sasha Levin
2014-10-24 15:10 ` Dmitry Vyukov
2014-10-24 21:05 ` One Thousand Gnomes
2014-10-24 22:23 ` H. Peter Anvin [this message]
2014-10-24 22:09 ` Andreas Dilger
2014-10-24 22:22 ` H. Peter Anvin
2014-10-25 0:50 ` Sasha Levin
2014-10-25 20:30 ` One Thousand Gnomes
2014-10-25 20:49 ` Andrey Ryabinin
2014-10-20 11:07 ` kernel: clockevents: shift out-of-bounds Andrey Ryabinin
2014-10-24 10:25 ` Peter Zijlstra
2014-10-20 11:16 ` fs: ext4: mballoc: negative shift exponent Andrey Ryabinin
2014-10-20 11:23 ` jbd2: revoke: negative shift exponent in hash() Andrey Ryabinin
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