netdev - Re: [ovs-dev] [PATCH net-next] net: remove abuse of VLAN DEI/CFI bit

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Date:   Mon, 5 Dec 2016 10:55:45 -0800
From:   Ben Pfaff <blp@....org>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     netdev@...r.kernel.org, dev@...nvswitch.org,
        bridge@...ts.linux-foundation.org
Subject: Re: [ovs-dev] [PATCH net-next] net: remove abuse of VLAN DEI/CFI bit

On Mon, Dec 05, 2016 at 06:24:36PM +0100, Michał Mirosław wrote:
> On Sat, Dec 03, 2016 at 03:27:30PM -0800, Ben Pfaff wrote:
> > On Sat, Dec 03, 2016 at 10:22:28AM +0100, Michał Mirosław wrote:
> > > This All-in-one patch removes abuse of VLAN CFI bit, so it can be passed
> > > intact through linux networking stack.
> > This appears to change the established Open vSwitch userspace API.  You
> > can see that simply from the way that it changes the documentation for
> > the userspace API.  If I'm right about that, then this change will break
> > all userspace programs that use the Open vSwitch kernel module,
> > including Open vSwitch itself.
> 
> If I understood the code correctly, it does change expected meaning for
> the (unlikely?) case of header truncated just before the VLAN TCI - it will
> be impossible to differentiate this case from the VLAN TCI == 0.
> 
> I guess this is a problem with OVS API, because it doesn't directly show
> the "missing" state of elements, but relies on an "invalid" value.

That particular corner case should not be a huge problem in any case.

The real problem is that this appears to break the common case use of
VLANs in Open vSwitch.  After this patch, parse_vlan() in
net/openvswitch/flow.c copies the tpid and tci from sk_buff (either the
accelerated version of them or the version in the skb data) into
sw_flow_key members.  OK, that's fine on it's own.  However, I don't see
any corresponding change to the code in flow_netlink.c to compensate for
the fact that, until now, the VLAN CFI bit (formerly VLAN_TAG_PRESENT)
was always required to be set to 1 in flow matches inside Netlink
messages sent from userspace, and the kernel always set it to 1 in
corresponding messages sent to userspace.

In other words, if I'm reading this change correctly:

    * With a kernel before this change, userspace always had to set
      VLAN_TAG_PRESENT to 1 to match on a VLAN, or the kernel would
      reject the flow match.

    * With a kernel after this change, userspace must not set
      VLAN_TAG_PRESENT to 1, otherwise the kernel will accept the flow
      match but nothing will ever match because packets do not actually
      have the CFI bit set.

Take a look at this code that the patch deletes from
validate_vlan_from_nlattrs(), for example, and see how it insisted that
VLAN_TAG_PRESENT was set:

	if (!(tci & htons(VLAN_TAG_PRESENT))) {
		if (tci) {
			OVS_NLERR(log, "%s TCI does not have VLAN_TAG_PRESENT bit set.",
				  (inner) ? "C-VLAN" : "VLAN");
			return -EINVAL;
		} else if (nla_len(a[OVS_KEY_ATTR_ENCAP])) {
			/* Corner case for truncated VLAN header. */
			OVS_NLERR(log, "Truncated %s header has non-zero encap attribute.",
				  (inner) ? "C-VLAN" : "VLAN");
			return -EINVAL;
		}
	}

Please let me know if I'm overlooking something.

Thanks,

Ben.