[PATCH net-next 11/11] selftests: net: cover the tunnel transmit drop reasons

[Anton Danilov <littlesmilingcloud@gmail.com>]

Extend the tunnel drop reason test with three failures of the transmit
path, all on GRE:

 - an underlay that cannot carry what the tunnel advertises, so the
   encapsulated packet does not fit the path MTU and is dropped after an
   ICMP fragmentation needed is sent back.  This is the case worth
   telling apart from the others: path MTU discovery is working as
   intended, and until now the drop was indistinguishable from a real
   failure,

 - a device in external mode receiving traffic that carries no tunnel
   metadata, reported as TUNNEL_TXINFO,

 - a tunnel whose remote endpoint has no route, reported as
   IP_OUTNOROUTES.

Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>
---
 .../selftests/net/tunnel_drop_reasons.sh      | 58 ++++++++++++++++++-
 1 file changed, 57 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/net/tunnel_drop_reasons.sh b/tools/testing/selftests/net/tunnel_drop_reasons.sh
index aad003f0efe5..5639e29365a4 100755
--- a/tools/testing/selftests/net/tunnel_drop_reasons.sh
+++ b/tools/testing/selftests/net/tunnel_drop_reasons.sh
@@ -26,6 +26,11 @@
 #    GRE_TUNNEL_NOT_FOUND.  It is triggered here by giving the two
 #    endpoints different keys.
 #
+# On the transmit side, the reasons reported while encapsulating are
+# checked too: a packet that does not fit the path MTU, a device in
+# external mode that receives no metadata, and a tunnel whose remote
+# endpoint has no route.
+#
 #  - a header with the routing bit set is reported as GRE_INVALID_HDR,
 #    and a header announcing a GRE version nobody handles is reported as
 #    UNHANDLED_PROTO.  Both are triggered by corrupting the GRE header
@@ -81,6 +86,7 @@ setup_tracing()
 
 setup_ns_pair()
 {
+	PING_ARGS=""
 	cleanup_all_ns
 	setup_ns NS_SND NS_RCV
 
@@ -122,9 +128,15 @@ addr_tunnels()
 	ip -n "$NS_RCV" addr add "$TUN_RCV/24" dev gre_test
 }
 
+# Traffic used by check_reason().  Tests that need something else set
+# PING_ARGS before calling it.
+PING_ARGS=""
+
 send_traffic()
 {
-	ip netns exec "$NS_SND" ping -c 2 -W 1 "$TUN_RCV" >/dev/null 2>&1
+	# shellcheck disable=SC2086
+	ip netns exec "$NS_SND" ping -c 2 -W 1 $PING_ARGS "$TUN_RCV" \
+		>/dev/null 2>&1
 	# Let the tracepoint records reach the trace buffer.
 	sleep 1
 }
@@ -254,6 +266,46 @@ test_corrupted_header()
 	check_reason "$name" "$want"
 }
 
+test_tx_pkt_too_big()
+{
+	setup_ns_pair
+	# The underlay cannot carry what the tunnel advertises, so the
+	# encapsulated packet does not fit the path MTU.  The kernel sends
+	# an ICMP fragmentation needed back and drops it, which is path MTU
+	# discovery working as intended.
+	ip -n "$NS_SND" link set veth_s mtu 1280
+	ip -n "$NS_RCV" link set veth_r mtu 1280
+	add_gre "$NS_SND" "$SND_V4" "$RCV_V4"
+	add_gre "$NS_RCV" "$RCV_V4" "$SND_V4"
+	ip -n "$NS_SND" link set gre_test mtu 1500
+	addr_tunnels
+
+	PING_ARGS="-s 1400 -M do"
+	check_reason "gre: packet does not fit the path MTU" PKT_TOO_BIG
+}
+
+test_tx_no_metadata()
+{
+	setup_ns_pair
+	# A device in external mode expects the destination to come from
+	# the tunnel metadata, which plain traffic does not carry.
+	ip -n "$NS_SND" link add gre_test type gre external
+	ip -n "$NS_SND" link set gre_test up
+	ip -n "$NS_SND" addr add "$TUN_SND/24" dev gre_test
+
+	check_reason "gre: external mode without metadata" TUNNEL_TXINFO
+}
+
+test_tx_no_route()
+{
+	setup_ns_pair
+	# Nothing knows how to reach the remote endpoint of the tunnel.
+	add_gre "$NS_SND" "$SND_V4" 172.31.255.254
+	ip -n "$NS_SND" addr add "$TUN_SND/24" dev gre_test
+
+	check_reason "gre: no route to the remote endpoint" IP_OUTNOROUTES
+}
+
 if [ "$(id -u)" -ne 0 ]; then
 	echo "SKIP: need root"
 	exit "$ksft_skip"
@@ -279,6 +331,10 @@ test_corrupted_header "gre: routing bit set" GRE_INVALID_HDR \
 test_corrupted_header "gre: unhandled GRE version" UNHANDLED_PROTO \
 	offset 21 u8 set 0x01
 
+test_tx_pkt_too_big
+test_tx_no_metadata
+test_tx_no_route
+
 if [ -e /proc/sys/net/ipv6 ]; then
 	test_opts_mismatch ip6gre iseq
 	test_old_seq ip6gre
-- 
2.47.3