sock: make cookie generation global instead of per netns

Generating and retrieving socket cookies are a useful feature that is
exposed to BPF for various program types through bpf_get_socket_cookie()
helper.

The fact that the cookie counter is per netns is quite a limitation
for BPF in practice in particular for programs in host namespace that
use socket cookies as part of a map lookup key since they will be
causing socket cookie collisions e.g. when attached to BPF cgroup hooks
or cls_bpf on tc egress in host namespace handling container traffic
from veth or ipvlan devices with peer in different netns. Change the
counter to be global instead.

Socket cookie consumers must assume the value as opqaue in any case.
Not every socket must have a cookie generated and knowledge of the
counter value itself does not provide much value either way hence
conversion to global is fine.

Signed-off-by: Daniel Borkmann <[e-mail]>
Cc: Eric Dumazet <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Cc: Willem de Bruijn <[e-mail]>
Cc: Martynas Pumputis <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/net_namespace.h

···
 	spinlock_t		rules_mod_lock;
 
 	u32			hash_mix;
-	atomic64_t		cookie_gen;
 
 	struct list_head	list;		/* list of network namespaces */
 	struct list_head	exit_list;	/* To linked to call pernet exit

include/uapi/linux/bpf.h

···
  * 		If no cookie has been set yet, generate a new cookie. Once
  * 		generated, the socket cookie remains stable for the life of the
  * 		socket. This helper can be useful for monitoring per socket
- * 		networking traffic statistics as it provides a unique socket
- * 		identifier per namespace.
+ * 		networking traffic statistics as it provides a global socket
+ * 		identifier that can be assumed unique.
  * 	Return
  * 		A 8-byte long non-decreasing number on success, or 0 if the
  * 		socket field is missing inside *skb*.

net/core/sock_diag.c

···
 static int (*inet_rcv_compat)(struct sk_buff *skb, struct nlmsghdr *nlh);
 static DEFINE_MUTEX(sock_diag_table_mutex);
 static struct workqueue_struct *broadcast_wq;
+static atomic64_t cookie_gen;
 
 u64 sock_gen_cookie(struct sock *sk)
 {
···
 
 		if (res)
 			return res;
-		res = atomic64_inc_return(&sock_net(sk)->cookie_gen);
+		res = atomic64_inc_return(&cookie_gen);
 		atomic64_cmpxchg(&sk->sk_cookie, 0, res);
 	}
 }