Linus Torvalds [off-list ref] wrote:

Because if people think splice is complicated, you haven't looked at
the skb rules. They are completely arbitrary and complex and spread
all over the tree.

Yeah - I fell foul of the net loopback driver just reflecting the outgoing
packet back, complete with all the original spliced bufferage.  I was
wondering if the loopback driver needs to look at the skbuff, see if it has
zerocopy elements of some sort and, if so, copy it (or drop it if ENOMEM).

David