selftests: seg6: Add test cases for End.X with link-local nexthop

selftests: seg6: Add test cases for End.X with link-local nexthop

In the current test topology, all the routers are connected to each
other via dedicated links with addresses of the form fcf0:0:x:y::/64.

The test configures rt-3 with an adjacency with rt-4 and rt-4 with an
adjacency with rt-1:

 # ip -n rt_3-IgWSBJ -6 route show tab 90 fcbb:0:300::/48
 fcbb:0:300::/48  encap seg6local action End.X nh6 fcf0:0:3:4::4 flavors next-csid lblen 32 nflen 16 dev dum0 metric 1024 pref medium
 # ip -n rt_4-JdCunK -6 route show tab 90 fcbb:0:400::/48
 fcbb:0:400::/48  encap seg6local action End.X nh6 fcf0:0:1:4::1 flavors next-csid lblen 32 nflen 16 dev dum0 metric 1024 pref medium

The routes are used when pinging hs-2 from hs-1 and vice-versa.

Extend the test to also cover End.X behavior with an IPv6 link-local
nexthop address and an output interface. Configure every router
interface with an IPv6 link-local address of the form fe80::x:y/64 and
before re-running the ping tests, replace the previous End.X routes with
routes that use the new IPv6 link-local addresses:

 # ip -n rt_3-IgWSBJ -6 route show tab 90 fcbb:0:300::/48
 fcbb:0:300::/48  encap seg6local action End.X nh6 fe80::4:3 oif veth-rt-3-4 flavors next-csid lblen 32 nflen 16 dev dum0 metric 1024 pref medium
 # ip -n rt_4-JdCunK -6 route show tab 90 fcbb:0:400::/48
 fcbb:0:400::/48  encap seg6local action End.X nh6 fe80::1:4 oif veth-rt-4-1 flavors next-csid lblen 32 nflen 16 dev dum0 metric 1024 pref medium

The new test cases fail without the previous patch ("seg6: Allow End.X
behavior to accept an oif"):

 # ./srv6_end_x_next_csid_l3vpn_test.sh
 [...]
 ################################################################################
 TEST SECTION: SRv6 VPN connectivity test hosts (h1 <-> h2, IPv6), link-local
 ################################################################################

     TEST: IPv6 Hosts connectivity: hs-1 -> hs-2                         [FAIL]

     TEST: IPv6 Hosts connectivity: hs-2 -> hs-1                         [FAIL]

 ################################################################################
 TEST SECTION: SRv6 VPN connectivity test hosts (h1 <-> h2, IPv4), link-local
 ################################################################################

     TEST: IPv4 Hosts connectivity: hs-1 -> hs-2                         [FAIL]

     TEST: IPv4 Hosts connectivity: hs-2 -> hs-1                         [FAIL]

 Tests passed:  40
 Tests failed:   4

And pass with it:

 # ./srv6_end_x_next_csid_l3vpn_test.sh
 [...]
 ################################################################################
 TEST SECTION: SRv6 VPN connectivity test hosts (h1 <-> h2, IPv6), link-local
 ################################################################################

     TEST: IPv6 Hosts connectivity: hs-1 -> hs-2                         [ OK ]

     TEST: IPv6 Hosts connectivity: hs-2 -> hs-1                         [ OK ]

 ################################################################################
 TEST SECTION: SRv6 VPN connectivity test hosts (h1 <-> h2, IPv4), link-local
 ################################################################################

     TEST: IPv4 Hosts connectivity: hs-1 -> hs-2                         [ OK ]

     TEST: IPv4 Hosts connectivity: hs-2 -> hs-1                         [ OK ]

 Tests passed:  44
 Tests failed:   0

Without the previous patch, rt-3 and rt-4 resolve the wrong routes for
the link-local nexthops, with the output interface being the input
interface:

 # perf script
 [...]
 ping    1067 [001]    37.554486: fib6:fib6_table_lookup: table 254 oif 0 iif 11 proto 41 cafe::254/0 -> fe80::4:3/0 flowlabel 0xb7973 tos 0 scope 0 flags 2 ==> dev veth-rt-3-1 gw :: err 0
 [...]
 ping    1069 [002]    41.573360: fib6:fib6_table_lookup: table 254 oif 0 iif 12 proto 41 cafe::254/0 -> fe80::1:4/0 flowlabel 0xb7973 tos 0 scope 0 flags 2 ==> dev veth-rt-4-2 gw :: err 0

But the correct routes are resolved with the patch:

 # perf script
 [...]
 ping    1066 [006]    30.672355: fib6:fib6_table_lookup: table 254 oif 13 iif 1 proto 41 cafe::254/0 -> fe80::4:3/0 flowlabel 0x85941 tos 0 scope 0 flags 6 ==> dev veth-rt-3-4 gw :: err 0
 [...]
 ping    1066 [006]    30.672411: fib6:fib6_table_lookup: table 254 oif 11 iif 1 proto 41 cafe::254/0 -> fe80::1:4/0 flowlabel 0x91de0 tos 0 scope 0 flags 6 ==> dev veth-rt-4-1 gw :: err 0

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Link: https://patch.msgid.link/20250612122323.584113-5-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>