Re: [PATCH 3/3] netfilter: ctnetlink: allow userspace to set labels

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Mon, Dec 03, 2012 at 12:13:32PM +0100, Florian Westphal wrote:
> Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> > > No matter what xchg tricks you do in the kernel: if 1<<31 was set
> > > after the dump completed, it will be un-set again via
> > > send_change_to_kernel(), i.e. we clear bit 1<<31, even though we didn't
> > > want to.  I don't see how this can be solved; kernel has no idea that
> > > userspace doesn't wish to alter 1<<31.
> > 
> > We can return -EAGAIN to userspace with cmpxchg. From kernel-space:
> > 
> > old = word;
> > word |= flags & mask; /* to set/unset a bunch of bits */
> > if (xchgcmp(&word, new, old) != old)
> >         return -EAGAIN;
> 
> Ah.  you're pulling a mask parameter out of your hat :-)

I did :-)

BTW, -EAGAIN already has a meaning for nfnetlink, so some other error
should be returned for the approach above.

Not sure if you checked the other approach I mentioned:

do {
        old = word;
        word |= flags & mask;
} while (xchgcmp(&word, new, old) != old);

So ctnetlink would keep trying until no interference happen.