Hello!

The iptables ULOG target passes the skb receive timestamp to userspace,

No differences of packet socket.

one netlink message sent to userspace.  If userspace would make a timestamp,

Nobody proposed to do this in userspace. This would be even not "inaccuracy",
the userspace time of read() is not correlated to real one at all
f.e. if userspace is going to do some dns, times will differ inpredictably.

Alexey