From: Sasha Levin <sasha.levin@oracle.com>
To: David Miller <davem@davemloft.net>
Cc: a.ryabinin@samsung.com, pablo@netfilter.org, mschmidt@redhat.com,
akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
netdev@vger.kernel.org
Subject: Re: [PATCH] netlink: don't copy over empty attribute data
Date: Sun, 26 Oct 2014 19:32:42 -0400 [thread overview]
Message-ID: <544D849A.4040304@oracle.com> (raw)
In-Reply-To: <20141022.021508.2011745433893496421.davem@davemloft.net>
On 10/22/2014 02:15 AM, David Miller wrote:
> From: Sasha Levin <sasha.levin@oracle.com>
> Date: Tue, 21 Oct 2014 22:19:36 -0400
>
>> On 10/21/2014 09:39 PM, David Miller wrote:
>>> From: Sasha Levin <sasha.levin@oracle.com>
>>> Date: Tue, 21 Oct 2014 16:51:09 -0400
>>>
>>>>> netlink uses empty data to seperate different levels. However, we still
>>>>> try to copy that data from a NULL ptr using memcpy, which is an undefined
>>>>> behaviour.
>>>>>
>>>>> Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
>>> This isn't a POSIX C library, this it the Linux kernel, and as such
>>> we can make sure none of our memcpy() implementations try to access
>>> any bytes if the given length is NULL.
>>
>> We can make *our* implementations work around that undefined behaviour if we
>> want, but right now our implementations is to call GCC's builtin memcpy(),
>> which follows the standards and doesn't allow you to call it with NULL 'from'
>> ptr.
>>
>> The fact that it doesn't die and behaves properly is just "luck".
>
> If GCC's internal memcpy() starts accessing past 'len', I'm going
> to report the bug rather than code around it.
How so? GCC states clearly that you should *never* pass a NULL pointer there:
"The pointers passed to memmove (and similar functions in <string.h>) must
be non-null even when nbytes==0" (https://gcc.gnu.org/gcc-4.9/porting_to.html).
Even if it doesn't dereference it, it can break somehow in a subtle way. Leaving
the kernel code assuming that gcc (or any other compiler) would always behave
the same in a situation that shouldn't occur.
Thanks,
Sasha
next prev parent reply other threads:[~2014-10-26 23:32 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-21 20:51 [PATCH] netlink: don't copy over empty attribute data Sasha Levin
2014-10-22 1:39 ` David Miller
2014-10-22 2:19 ` Sasha Levin
2014-10-22 6:15 ` David Miller
2014-10-26 23:32 ` Sasha Levin [this message]
2014-10-27 2:03 ` David Miller
2014-10-27 14:42 ` Sasha Levin
2014-10-27 16:46 ` Andrey Ryabinin
2014-10-22 8:55 ` David Laight
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