[PATCH net-next v2] selftests: net: fou_mcast_encap: load the fou module

setup_ipv4() creates a GRETAP tunnel with FOU encap and configures a FOU
port via "ip fou add". Both require the fou module, which cannot be
auto-loaded.

The test assumes CONFIG_NET_FOU=y, which matches the default selftests
config, but fails on kernels built with CONFIG_NET_FOU=m. Without the fou
module, tunnel creation fails and the test reports a false failure with
0/100 received packets:

  RTNETLINK answers: Invalid argument
  TEST: FOU/GRETAP IPv4 multicast encap resubmit (received 0/100)  [FAIL]

Load the fou module and skip the subtest cleanly when unavailable. This
matches the existing fou6 handling in this script and the pattern used in
pmtu.sh.

modprobe is a no-op when CONFIG_NET_FOU=y, so default selftest builds
remain unchanged.

Signed-off-by: Qingshuang Fu <redacted>
---
v2:
- Dropped Fixes tag, target net-next instead
- Dropped comment inside setup_ipv4() per review feedback
- v1: https://lore.kernel.org/all/20260911100158.854183-1-fuqingshuang@kylinos.cn/ (local)

 tools/testing/selftests/net/fou_mcast_encap.sh | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/net/fou_mcast_encap.sh b/tools/testing/selftests/net/fou_mcast_encap.sh
index 70210d39fba3..5f84d58d3075 100755
--- a/tools/testing/selftests/net/fou_mcast_encap.sh
+++ b/tools/testing/selftests/net/fou_mcast_encap.sh

@@ -51,8 +51,6 @@ setup_common() {
 }
 
 setup_ipv4() {
-	# IPv4 FOU (CONFIG_NET_FOU) is built in on kernels configured for
-	# these tests, so no module load is needed here.
 	ip -n "$NSENDER" addr add 10.0.0.1/24 dev veth_s
 	ip -n "$NRECV" addr add 10.0.0.2/24 dev veth_r
 

@@ -160,8 +158,13 @@ run_ping_test() {
 }
 
 setup_common
-setup_ipv4
-run_ping_test -4 eoudp4 "$TUN4_R" "FOU/GRETAP IPv4 multicast encap resubmit"
+
+if modprobe -q fou; then
+	setup_ipv4
+	run_ping_test -4 eoudp4 "$TUN4_R" "FOU/GRETAP IPv4 multicast encap resubmit"
+else
+	log_test_skip "FOU/GRETAP IPv4 multicast encap resubmit"
+fi
 
 if setup_ipv6; then
 	run_ping_test -6 eoudp6 "$TUN6_R" "FOU/ip6gretap IPv6 multicast encap resubmit"

-- 
2.25.1