selftest: net: Add GC test for temporary routes with exceptions.

Without the prior commit, IPv6 GC cannot track exceptions tied
to permanent routes if they were originally added as temporary
routes.

Let's add a test case for the issue.

  1. Add temporary routes
  2. Create exceptions for the temporary routes
  3. Promote the routes to permanent routes
  4. Check if GC can find and purge the exceptions

A few notes:

  + At step 4, unlike other test cases, we cannot wait for
    $GC_WAIT_TIME.  While the exceptions are always iterable via
    netlink (since it traverses the entire fib tree instead of
    tb6_gc_hlist), rt6_nh_dump_exceptions() skips expired entries.

    If we waited for the expiration time, we would be unable to
    distinguish whether the exceptions were truly purged by GC or
    just hidden due to being expired.

  + For the same reason, at step 2, we use ICMPv6 redirect message
    instead of Packet Too Big message.  This is because MTU exceptions
    always have RTF_EXPIRES, and rt6_age_examine_exception() does not
    respect the period specified by net.ipv6.route.flush=1.

  + We add a neighbour entry for the redirect target with NTF_ROUTER.
    Without this, the exceptions would be removed at step 3 when the
    fib6_may_remove_gc_list() is called.

Without the fix, the exceptions remain even after GC is triggered
by sysctl -wq net.ipv6.route.flush=1.

  FAIL: Expected 0 routes, got 5
      TEST: ipv6 route garbage collection (promote to permanent routes)   [FAIL]

With the fix, GC purges the exceptions properly.

      TEST: ipv6 route garbage collection (promote to permanent routes)   [ OK ]

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260320072317.2561779-4-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/fib_tests.sh

@@ -868,6 +868,64 @@ fib6_gc_test()
 	check_rt_num 5 $($IP -6 route list |grep -v expires|grep 2001:20::|wc -l)
 	log_test $ret 0 "ipv6 route garbage collection (replace with permanent)"
 
+	# Delete dummy_10 and remove all routes
+	$IP link del dev dummy_10
+
+	# rd6 is required for the next test. (ipv6toolkit)
+	if [ ! -x "$(command -v rd6)" ]; then
+	    echo "SKIP: rd6 not found."
+	    set +e
+	    cleanup &> /dev/null
+	    return
+	fi
+
+	setup_ns ns2
+	$IP link add veth1 type veth peer veth2 netns $ns2
+	$IP link set veth1 up
+	ip -netns $ns2 link set veth2 up
+	$IP addr add fe80:dead::1/64 dev veth1
+	ip -netns $ns2 addr add fe80:dead::2/64 dev veth2
+
+	# Add NTF_ROUTER neighbour to prevent rt6_age_examine_exception()
+	# from removing not-yet-expired exceptions.
+	ip -netns $ns2 link set veth2 address 00:11:22:33:44:55
+	$IP neigh add fe80:dead::3 lladdr 00:11:22:33:44:55 dev veth1 router
+
+	$NS_EXEC sysctl -wq net.ipv6.conf.veth1.accept_redirects=1
+	$NS_EXEC sysctl -wq net.ipv6.conf.veth1.forwarding=0
+
+	# Temporary routes
+	for i in $(seq 1 5); do
+	    # Expire route after $EXPIRE seconds
+	    $IP -6 route add 2001:10::$i \
+		via fe80:dead::2 dev veth1 expires $EXPIRE
+
+	    ip netns exec $ns2 rd6 -i veth2 \
+		-s fe80:dead::2 -d fe80:dead::1 \
+		-r 2001:10::$i -t fe80:dead::3 -p ICMP6
+	done
+
+	check_rt_num 5 $($IP -6 route list | grep expires | grep 2001:10:: | wc -l)
+
+	# Promote to permanent routes by "prepend" (w/o NLM_F_EXCL and NLM_F_REPLACE)
+	for i in $(seq 1 5); do
+	    # -EEXIST, but the temporary route becomes the permanent route.
+	    $IP -6 route append 2001:10::$i \
+		via fe80:dead::2 dev veth1 2>/dev/null || true
+	done
+
+	check_rt_num 5 $($IP -6 route list | grep -v expires | grep 2001:10:: | wc -l)
+	check_rt_num 5 $($IP -6 route list cache | grep 2001:10:: | wc -l)
+
+	# Trigger GC instead of waiting $GC_WAIT_TIME.
+	# rt6_nh_dump_exceptions() just skips expired exceptions.
+	$NS_EXEC sysctl -wq net.ipv6.route.flush=1
+	check_rt_num 0 $($IP -6 route list cache | grep 2001:10:: | wc -l)
+	log_test $ret 0 "ipv6 route garbage collection (promote to permanent routes)"
+
+	$IP neigh del fe80:dead::3 lladdr 00:11:22:33:44:55 dev veth1 router
+	$IP link del veth1
+
 	# ra6 is required for the next test. (ipv6toolkit)
 	if [ ! -x "$(command -v ra6)" ]; then
 	    echo "SKIP: ra6 not found."
@@ -876,9 +934,6 @@ fib6_gc_test()
 	    return
 	fi
 
-	# Delete dummy_10 and remove all routes
-	$IP link del dev dummy_10
-
 	# Create a pair of veth devices to send a RA message from one
 	# device to another.
 	$IP link add veth1 type veth peer name veth2