seg6: Allow End.X behavior to accept an oif

Extend the End.X behavior to accept an output interface as an optional
attribute and make use of it when resolving a route. This is needed when
user space wants to use a link-local address as the nexthop address.

Before:

 # ip route add 2001:db8:1::/64 encap seg6local action End.X nh6 fe80::1 oif eth0 dev sr6
 # ip route add 2001:db8:2::/64 encap seg6local action End.X nh6 2001:db8:10::1 dev sr6
 $ ip -6 route show
 2001:db8:1::/64  encap seg6local action End.X nh6 fe80::1 dev sr6 metric 1024 pref medium
 2001:db8:2::/64  encap seg6local action End.X nh6 2001:db8:10::1 dev sr6 metric 1024 pref medium

After:

 # ip route add 2001:db8:1::/64 encap seg6local action End.X nh6 fe80::1 oif eth0 dev sr6
 # ip route add 2001:db8:2::/64 encap seg6local action End.X nh6 2001:db8:10::1 dev sr6
 $ ip -6 route show
 2001:db8:1::/64  encap seg6local action End.X nh6 fe80::1 oif eth0 dev sr6 metric 1024 pref medium
 2001:db8:2::/64  encap seg6local action End.X nh6 2001:db8:10::1 dev sr6 metric 1024 pref medium

Note that the oif attribute is not dumped to user space when it was not
specified (as an oif of 0) since each entry keeps track of the optional
attributes that it parsed during configuration (see struct
seg6_local_lwt::parsed_optattrs).

Signed-off-by: Ido Schimmel <[email protected]>
Reviewed-by: Andrea Mayer <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: idosch

net/ipv6/seg6_local.c

@@ -421,7 +421,7 @@ static int end_next_csid_core(struct sk_buff *skb, struct seg6_local_lwt *slwt)
 static int input_action_end_x_finish(struct sk_buff *skb,
 				     struct seg6_local_lwt *slwt)
 {
-	seg6_lookup_any_nexthop(skb, &slwt->nh6, 0, false, 0);
+	seg6_lookup_any_nexthop(skb, &slwt->nh6, 0, false, slwt->oif);
 
 	return dst_input(skb);
 }
@@ -1480,7 +1480,8 @@ static struct seg6_action_desc seg6_action_table[] = {
 		.action		= SEG6_LOCAL_ACTION_END_X,
 		.attrs		= SEG6_F_ATTR(SEG6_LOCAL_NH6),
 		.optattrs	= SEG6_F_LOCAL_COUNTERS |
-				  SEG6_F_LOCAL_FLAVORS,
+				  SEG6_F_LOCAL_FLAVORS |
+				  SEG6_F_ATTR(SEG6_LOCAL_OIF),
 		.input		= input_action_end_x,
 	},
 	{