inet: add RCU protection to inet->opt

We lack proper synchronization to manipulate inet->opt ip_options

Problem is ip_make_skb() calls ip_setup_cork() and
ip_setup_cork() possibly makes a copy of ipc->opt (struct ip_options),
without any protection against another thread manipulating inet->opt.

Another thread can change inet->opt pointer and free old one under us.

Use RCU to protect inet->opt (changed to inet->inet_opt).

Instead of handling atomic refcounts, just copy ip_options when
necessary, to avoid cache line dirtying.

We cant insert an rcu_head in struct ip_options since its included in
skb->cb[], so this patch is large because I had to introduce a new
ip_options_rcu structure.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
int compat_ip_getsockopt(struct sock *sk, int level, int optname,
			 char __user *optval, int __user *optlen)
{
	int err;

	if (optname == MCAST_MSFILTER)
		return compat_mc_getsockopt(sk, level, optname, optval, optlen,
			ip_getsockopt);

	err = do_ip_getsockopt(sk, level, optname, optval, optlen);

#ifdef CONFIG_NETFILTER
	/* we need to exclude all possible ENOPROTOOPTs except default case */
	if (err == -ENOPROTOOPT && optname != IP_PKTOPTIONS &&
			!ip_mroute_opt(optname)) {
		int len;

		if (get_user(len, optlen))
			return -EFAULT;

		lock_sock(sk);
		err = compat_nf_getsockopt(sk, PF_INET, optname, optval, &len);
		release_sock(sk);
		if (err >= 0)
			err = put_user(len, optlen);
		return err;
	}
#endif
	return err;
}

int compat_ip_getsockopt(struct sock *sk, int level, int optname,
			 char __user *optval, int __user *optlen)
{
	int err;

	if (optname == MCAST_MSFILTER)
		return compat_mc_getsockopt(sk, level, optname, optval, optlen,
			ip_getsockopt);

	err = do_ip_getsockopt(sk, level, optname, optval, optlen);

#ifdef CONFIG_NETFILTER
	/* we need to exclude all possible ENOPROTOOPTs except default case */
	if (err == -ENOPROTOOPT && optname != IP_PKTOPTIONS &&
			!ip_mroute_opt(optname)) {
		int len;

		if (get_user(len, optlen))
			return -EFAULT;

		lock_sock(sk);
		err = compat_nf_getsockopt(sk, PF_INET, optname, optval, &len);
		release_sock(sk);
		if (err >= 0)
			err = put_user(len, optlen);
		return err;
	}
#endif
	return err;
}
