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>
static int __udp4_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
				    struct udphdr  *uh,
				    __be32 saddr, __be32 daddr,
				    struct udp_table *udptable)
{
	struct sock *sk, *stack[256 / sizeof(struct sock *)];
	struct udp_hslot *hslot = udp_hashslot(udptable, net, ntohs(uh->dest));
	int dif;
	unsigned int i, count = 0;

	spin_lock(&hslot->lock);
	sk = sk_nulls_head(&hslot->head);
	dif = skb->dev->ifindex;
	sk = udp_v4_mcast_next(net, sk, uh->dest, daddr, uh->source, saddr, dif);
	while (sk) {
		stack[count++] = sk;
		sk = udp_v4_mcast_next(net, sk_nulls_next(sk), uh->dest,
				       daddr, uh->source, saddr, dif);
		if (unlikely(count == ARRAY_SIZE(stack))) {
			if (!sk)
				break;
			flush_stack(stack, count, skb, ~0);
			count = 0;
		}
	}
	/*
	 * before releasing chain lock, we must take a reference on sockets
	 */
	for (i = 0; i < count; i++)
		sock_hold(stack[i]);

	spin_unlock(&hslot->lock);

	/*
	 * do the slow work with no lock held
	 */
	if (count) {
		flush_stack(stack, count, skb, count - 1);

		for (i = 0; i < count; i++)
			sock_put(stack[i]);
	} else {
		kfree_skb(skb);
	}
	return 0;
}

static int __udp4_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
				    struct udphdr  *uh,
				    __be32 saddr, __be32 daddr,
				    struct udp_table *udptable)
{
	struct sock *sk, *stack[256 / sizeof(struct sock *)];
	struct udp_hslot *hslot = udp_hashslot(udptable, net, ntohs(uh->dest));
	int dif;
	unsigned int i, count = 0;

	spin_lock(&hslot->lock);
	sk = sk_nulls_head(&hslot->head);
	dif = skb->dev->ifindex;
	sk = udp_v4_mcast_next(net, sk, uh->dest, daddr, uh->source, saddr, dif);
	while (sk) {
		stack[count++] = sk;
		sk = udp_v4_mcast_next(net, sk_nulls_next(sk), uh->dest,
				       daddr, uh->source, saddr, dif);
		if (unlikely(count == ARRAY_SIZE(stack))) {
			if (!sk)
				break;
			flush_stack(stack, count, skb, ~0);
			count = 0;
		}
	}
	/*
	 * before releasing chain lock, we must take a reference on sockets
	 */
	for (i = 0; i < count; i++)
		sock_hold(stack[i]);

	spin_unlock(&hslot->lock);

	/*
	 * do the slow work with no lock held
	 */
	if (count) {
		flush_stack(stack, count, skb, count - 1);

		for (i = 0; i < count; i++)
			sock_put(stack[i]);
	} else {
		kfree_skb(skb);
	}
	return 0;
}
