[ovs-dev] Extending ovs_action_attr to add a new action

[Matteo Croce]

Hi,

I need to add a field to enum ovs_action_attr, but I see that the
definition between the upstream header[1] and the one in compat[2]
differs.
Upstream enum stops at OVS_ACTION_ATTR_CHECK_PKT_LEN, with an extra
"hidden" element after __OVS_ACTION_ATTR_MAX (22)
Our compat version instead, has OVS_ACTION_ATTR_TUNNEL_{PUSH,POP}
defined only #ifndef __KERNEL__, with __OVS_ACTION_ATTR_MAX being 22
for the kernel and 24 for userspace.

If I add a field OVS_ACTION_ATTR_WHATEVER just before
__OVS_ACTION_ATTR_MAX in the kernel, older userspace will incorrectly
see the new action as OVS_ACTION_ATTR_TUNNEL_PUSH.

How can we extend this enum without breaking compatibility?
If OVS_ACTION_ATTR_TUNNEL_* really is unused in the kernel datapath,
what if we pad the kernel header with two padding fields?

-----------------------------%<-----------------------------
--- a/include/uapi/linux/openvswitch.h
+++ b/include/uapi/linux/openvswitch.h
@@ -925,6 +926,8 @@ enum ovs_action_attr {
  OVS_ACTION_ATTR_METER,        /* u32 meter ID. */
  OVS_ACTION_ATTR_CLONE,        /* Nested OVS_CLONE_ATTR_*.  */
  OVS_ACTION_ATTR_CHECK_PKT_LEN, /* Nested OVS_CHECK_PKT_LEN_ATTR_*. */
+ _OVS_ACTION_ATTR_TUNNEL_PUSH, /* unused in kernel datapath. */
+ _OVS_ACTION_ATTR_TUNNEL_POP,  /* unused in kernel datapath. */

  __OVS_ACTION_ATTR_MAX,       /* Nothing past this will be accepted
----------------------------->%-----------------------------

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/openvswitch.h?h=v5.3#n899
[2] https://github.com/openvswitch/ovs/blob/v2.12.0/datapath/linux/compat/include/linux/openvswitch.h#L962

Regards,
-- 
Matteo Croce
per aspera ad upstream