From: Andrey Ryabinin <ryabinin.a.a@gmail.com>
To: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Cc: Sasha Levin <sasha.levin@oracle.com>,
"H. Peter Anvin" <hpa@zytor.com>,
Andreas Dilger <adilger@dilger.ca>,
Dmitry Vyukov <dvyukov@google.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: Sun, 26 Oct 2014 00:49:32 +0400 [thread overview]
Message-ID: <CAPAsAGw52-L=fGOS6LfgYLJBBzQu-3MnW552wErWJM51eXLNbA@mail.gmail.com> (raw)
In-Reply-To: <20141025213017.2da2f819@alan.etchedpixels.co.uk>
2014-10-25 23:30 GMT+03:00 One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>:
> On Fri, 24 Oct 2014 20:50:46 -0400
> Sasha Levin <sasha.levin@oracle.com> wrote:
>
>> On 10/24/2014 06:22 PM, H. Peter Anvin wrote:
>> >> 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.
>>
>> Now I'm curious about things like "memcpy(ptr, NULL, 0)". According to the
>> standard they're undefined, and since we're using gcc's implementation for
>> memcpy() we are doing "undefined memcpy" in quite a few places in the kernel.
>>
>> Is it an issue, or would you expect memcpy() to not deref the "from" ptr
>> since length is 0?
>
> No. Furthermore gcc 4.9 actually has optimiser magic around this. See the
> "Porting to gcc 4.9" notes.
>
> --------
>
> GCC might now optimize away the null pointer check in code like:
>
>
> int copy (int* dest, int* src, size_t nbytes) {
> memmove (dest, src, nbytes);
> if (src != NULL)
> return *src;
> return 0;
> }
>
> The pointers passed to memmove (and similar functions in <string.h>) must
> be non-null even when nbytes==0, so GCC can use that information to
> remove the check after the memmove call. Calling copy(p, NULL, 0) can
> therefore deference a null pointer and crash.
>
> -------------
>
> Which is unfortunate because an operating system has a lot of legitimate
> reasons to copy data to address 0 (on many processors its the exception
> vectors for example)
That is why kernel builds with -fno-delete-null-pointer-checks.
>
> Alan
--
Best regards,
Andrey Ryabinin
next prev parent reply other threads:[~2014-10-25 20:49 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
2014-10-25 0:50 ` Sasha Levin
2014-10-25 20:30 ` One Thousand Gnomes
2014-10-25 20:49 ` Andrey Ryabinin [this message]
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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