[PATCH net v2 2/2] selftests: net: add test for IPv4 route with loopback IPv6 nexthop

[Jiayuan Chen <jiayuan.chen@linux.dev>]

From: Jiayuan Chen <jiayuan.chen@shopee.com>

Add a regression test for a panic triggered when an IPv4 route
references a loopback IPv6 nexthop object. Without the fix, this
sequence causes a NULL pointer dereference in __mkroute_output()
because nhc_pcpu_rth_output is not allocated for the nexthop.

./fib_nexthops.sh
Tests passed: 249
Tests failed:   0
Tests skipped:  2

Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
---
 tools/testing/selftests/net/fib_nexthops.sh | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/tools/testing/selftests/net/fib_nexthops.sh b/tools/testing/selftests/net/fib_nexthops.sh
index 21026b667667..6eb7f95e70e1 100755
--- a/tools/testing/selftests/net/fib_nexthops.sh
+++ b/tools/testing/selftests/net/fib_nexthops.sh
@@ -1672,6 +1672,17 @@ ipv4_withv6_fcnal()
 
 	run_cmd "$IP ro replace 172.16.101.1/32 via inet6 2001:db8:50::1 dev veth1"
 	log_test $? 2 "IPv4 route with invalid IPv6 gateway"
+
+	# Test IPv4 route with loopback IPv6 nexthop
+	# Regression test: loopback IPv6 nexthop was misclassified as reject
+	# route, skipping nhc_pcpu_rth_output allocation, causing panic when
+	# an IPv4 route references it and triggers __mkroute_output().
+	run_cmd "$IP -6 nexthop add id 20 dev lo"
+	run_cmd "$IP ro add 172.20.20.0/24 nhid 20"
+	run_cmd "ip netns exec $me ping -c1 -W1 172.20.20.1"
+	log_test $? 1 "IPv4 route with loopback IPv6 nexthop (no crash)"
+	run_cmd "$IP ro del 172.20.20.0/24"
+	run_cmd "$IP nexthop del id 20"
 }
 
 ipv4_fcnal_runtime()
-- 
2.43.0