On 10/22/2014 02:15 AM, David Miller wrote: > From: Sasha Levin <sasha.le...@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.le...@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.le...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
