From: Andrey Ryabinin <a.ryabinin@samsung.com>
To: Sasha Levin <sasha.levin@oracle.com>, David Miller <davem@davemloft.net>
Cc: 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: Mon, 27 Oct 2014 19:46:33 +0300 [thread overview]
Message-ID: <544E76E9.3090909@samsung.com> (raw)
In-Reply-To: <544E59DC.3060906@oracle.com>
On 10/27/2014 05:42 PM, Sasha Levin wrote:
> On 10/26/2014 10:03 PM, David Miller wrote:
>> From: Sasha Levin <sasha.levin@oracle.com>
>> Date: Sun, 26 Oct 2014 19:32:42 -0400
>>
>>> 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.
>>
>> Show me a legal way in which one could legally dereference the pointer
>> when length is zero, and I'll entertain this patch.
>
> The moment you've triggered an undefined behaviour you have GCC license to
> dereference anything it wants. GCC would be well within it's rights
> dereferencing a NULL "from".
>
> They even state it clearly in that GCC 4.9 porting guide I've linked above:
>
> """
> Calling copy(p, NULL, 0) can therefore deference a null pointer and crash.
>
> The example above needs to be fixed to avoid the invalid memmove call, for example:
>
>
> if (nbytes != 0)
> memmove (dest, src, nbytes);
> """
>
In example from link null ptr deref could happen because GCC will optimize away null pointer check after
memmove():
int copy (int* dest, int* src, size_t nbytes) {
memmove (dest, src, nbytes);
if (src != NULL) <---- GCC will eliminate this check because src can't be null.
return *src; <-- NULL ptr deref
return 0;
}
Even though GCC and C standard treats such code ( memmove(dest, NULL, 0); ) as invalid, it probably will not crash in linux kernel case,
because that kind of optimization disabled via -fno-delete-null-pointer-checks option.
>
> Thanks,
> Sasha
>
next prev parent reply other threads:[~2014-10-27 16:46 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
2014-10-27 2:03 ` David Miller
2014-10-27 14:42 ` Sasha Levin
2014-10-27 16:46 ` Andrey Ryabinin [this message]
2014-10-22 8:55 ` David Laight
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