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

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Date:   Mon, 5 Dec 2016 23:52:47 +0100
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Ben Pfaff <blp@....org>
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 10:55:45AM -0800, Ben Pfaff wrote:
> 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.

Hmm. So the easiest change without disrupting current userspace, would be
to flip the CFI bit on the way to/from OVS userspace. Does this seem
correct?

Best Regards,
Michał Mirosław