From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sasha Levin Subject: Re: [PATCH] netlink: don't copy over empty attribute data Date: Sun, 26 Oct 2014 19:32:42 -0400 Message-ID: <544D849A.4040304@oracle.com> References: <1413924669-26732-1-git-send-email-sasha.levin@oracle.com> <20141021.213908.1088381802543942481.davem@davemloft.net> <54471438.1040907@oracle.com> <20141022.021508.2011745433893496421.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Cc: a.ryabinin@samsung.com, pablo@netfilter.org, mschmidt@redhat.com, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org To: David Miller Return-path: In-Reply-To: <20141022.021508.2011745433893496421.davem@davemloft.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 10/22/2014 02:15 AM, David Miller wrote: > From: Sasha Levin > Date: Tue, 21 Oct 2014 22:19:36 -0400 > >> On 10/21/2014 09:39 PM, David Miller wrote: >>> From: Sasha Levin >>> 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 >>> 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 ) 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