Sending big Netlink messages to userspace

["Julius Volz" <juliusv@google.com>]

Hi,

While adding a Netlink interface to IPVS, I've been wondering how to
properly send very big messages to userspace and found these posts:

http://lists.openwall.net/netdev/2007/03/06/214
http://lists.openwall.net/netdev/2007/03/07/2

Herbert writes in the second one, "Dumps should be done using 4K
(NLMSG_GOODSIZE) skb's, where is the problem?" How is that meant?
Should one manually split up dumps into several NLMSG_GOODSIZE
messages or is there some mechanism for that?

I need to send arbitrarily long lists to userspace and I'm already
choosing a big enough size for nlmsg_new(), so I get no put failures
while constructing the message. However, when receiving the data in
userspace (with libnl), the receive callback is never called. An
strace shows that MSG_TRUNC is set in the oversized message, so the
data is never fully received.

I just call nl_recvmsgs_default(sock) once (which does not return an
error). Am I handling libnl incorrectly or do I need to do this
differently on the kernel side?

Thanks,
Julius

-- 
Google Switzerland GmbH