Subject: Re: [PATCH] openvswitch: Orphan frags before sending to userspace via Netlink to avoid guest stall

From: Thomas Graf <[email-masked]>
Date: Fri, 07 Mar 2014 16:58:06 +0100

On 03/07/2014 05:46 AM, Pravin Shelar wrote:
> But I found bug in datapath user-space queue code. I am not sure how
> this can work with skb fragments and MMAP-netlink socket.
> Here is what happens, OVS allocates netlink skb and adds fragments to
> skb using skb_zero_copy(), then calls genlmsg_unicast().
> But if netlink sock is mmped then netlink-send queues netlink
> allocated skb->head (linear data of skb) and ignore skb frags.
>
> Currently this is not problem with OVS vswitchd since it does not use
> netlink MMAP sockets. But if vswitchd stats using MMAP-netlink socket,
> it can break it.

The secret is out ;-)

I was very surprised too when I noticed that it worked. It's not just
OVS, it's nfqueue as well. The reason is that an netlink mmaped skb is
setup with a giant tailroom in netlink_ring_setup_skb():

	skb->end	= skb->tail + size;

and skb_zerocopy() will consume whatever tailroom is available first:

	/* dont bother with small payloads */
	if (len <= skb_tailroom(to)) {
		skb_copy_bits(from, 0, skb_put(to, len), len);
		return;
	}

I was planning to fix this while adding GSO support to the upcall as
that is the moment when this bug would really surface.