linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: "H. Peter Anvin" <hpa@zytor.com>
To: Andreas Dilger <adilger@dilger.ca>, 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 15:22:27 -0700	[thread overview]
Message-ID: <544AD123.1010304@zytor.com> (raw)
In-Reply-To: <2E512EF7-577C-43C2-AB95-30DC25AD059D@dilger.ca>

On 10/24/2014 03:09 PM, Andreas Dilger wrote:
> On Oct 24, 2014, at 9:10 AM, 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.
> 
> 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.
> 

That's why it is undefined.

	-hpa



  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
2014-10-24 22:09                     ` Andreas Dilger
2014-10-24 22:22                       ` H. Peter Anvin [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=544AD123.1010304@zytor.com \
    --to=hpa@zytor.com \
    --cc=a.ryabinin@samsung.com \
    --cc=adilger.kernel@dilger.ca \
    --cc=adilger@dilger.ca \
    --cc=akpm@linux-foundation.org \
    --cc=dborkman@redhat.com \
    --cc=dvyukov@google.com \
    --cc=koct9i@gmail.com \
    --cc=linux-kbuild@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=mmarek@suse.cz \
    --cc=peterz@infradead.org \
    --cc=sasha.levin@oracle.com \
    --cc=tglx@linutronix.de \
    --cc=tytso@mit.edu \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).