selftests: mptcp: join: check backup support in signal endp

Before the previous commit, 'signal' endpoints with the 'backup' flag
were ignored when sending the MP_JOIN.

The MPTCP Join selftest has then been modified to validate this case:
the "single address, backup" test, is now validating the MP_JOIN with a
backup flag as it is what we expect it to do with such name. The
previous version has been kept, but renamed to "single address, switch
to backup" to avoid confusions.

The "single address with port, backup" test is also now validating the
MPJ with a backup flag, which makes more sense than checking the switch
to backup with an MP_PRIO.

The "mpc backup both sides" test is now validating that the backup flag
is also set in MP_JOIN from and to the addresses used in the initial
subflow, using the special ID 0.

The 'Fixes' tag here below is the same as the one from the previous
commit: this patch here is not fixing anything wrong in the selftests,
but it validates the previous fix for an issue introduced by this commit
ID.

Fixes: 4596a2c1 ("mptcp: allow creating non-backup subflows")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/net/mptcp/mptcp_join.sh

@@ -2639,6 +2639,19 @@ backup_tests()
 
 	# single address, backup
 	if reset "single address, backup" &&
+	   continue_if mptcp_lib_kallsyms_has "subflow_rebuild_header$"; then
+		pm_nl_set_limits $ns1 0 1
+		pm_nl_add_endpoint $ns1 10.0.2.1 flags signal,backup
+		pm_nl_set_limits $ns2 1 1
+		sflags=nobackup speed=slow \
+			run_tests $ns1 $ns2 10.0.1.1
+		chk_join_nr 1 1 1
+		chk_add_nr 1 1
+		chk_prio_nr 1 0 0 1
+	fi
+
+	# single address, switch to backup
+	if reset "single address, switch to backup" &&
 	   continue_if mptcp_lib_kallsyms_has "subflow_rebuild_header$"; then
 		pm_nl_set_limits $ns1 0 1
 		pm_nl_add_endpoint $ns1 10.0.2.1 flags signal
@@ -2654,13 +2667,13 @@ backup_tests()
 	if reset "single address with port, backup" &&
 	   continue_if mptcp_lib_kallsyms_has "subflow_rebuild_header$"; then
 		pm_nl_set_limits $ns1 0 1
-		pm_nl_add_endpoint $ns1 10.0.2.1 flags signal port 10100
+		pm_nl_add_endpoint $ns1 10.0.2.1 flags signal,backup port 10100
 		pm_nl_set_limits $ns2 1 1
-		sflags=backup speed=slow \
+		sflags=nobackup speed=slow \
 			run_tests $ns1 $ns2 10.0.1.1
 		chk_join_nr 1 1 1
 		chk_add_nr 1 1
-		chk_prio_nr 1 1 0 0
+		chk_prio_nr 1 0 0 1
 	fi
 
 	if reset "mpc backup" &&
@@ -2674,12 +2687,21 @@ backup_tests()
 
 	if reset "mpc backup both sides" &&
 	   continue_if mptcp_lib_kallsyms_doesnt_have "T mptcp_subflow_send_ack$"; then
-		pm_nl_add_endpoint $ns1 10.0.1.1 flags subflow,backup
+		pm_nl_set_limits $ns1 0 2
+		pm_nl_set_limits $ns2 1 2
+		pm_nl_add_endpoint $ns1 10.0.1.1 flags signal,backup
 		pm_nl_add_endpoint $ns2 10.0.1.2 flags subflow,backup
+
+		# 10.0.2.2 (non-backup) -> 10.0.1.1 (backup)
+		pm_nl_add_endpoint $ns2 10.0.2.2 flags subflow
+		# 10.0.1.2 (backup) -> 10.0.2.1 (non-backup)
+		pm_nl_add_endpoint $ns1 10.0.2.1 flags signal
+		ip -net "$ns2" route add 10.0.2.1 via 10.0.1.1 dev ns2eth1 # force this path
+
 		speed=slow \
 			run_tests $ns1 $ns2 10.0.1.1
-		chk_join_nr 0 0 0
-		chk_prio_nr 1 1 0 0
+		chk_join_nr 2 2 2
+		chk_prio_nr 1 1 1 1
 	fi
 
 	if reset "mpc switch to backup" &&