selftests: openvswitch: don't hardcode the drop reason subsys

JIRA: https://issues.redhat.com/browse/RHEL-84558
Upstream Status: net.git commit 1cc3462

commit 1cc3462
Author: Jakub Kicinski <kuba@kernel.org>
Date:   Tue Mar 4 10:06:15 2025 -0800

    selftests: openvswitch: don't hardcode the drop reason subsys

    WiFi removed one of their subsys entries from drop reasons, in
    commit 286e696 ("wifi: mac80211: Drop cooked monitor support")
    SKB_DROP_REASON_SUBSYS_OPENVSWITCH is now 2 not 3.
    The drop reasons are not uAPI, read the correct value
    from debug info.

    We need to enable vmlinux BTF, otherwise pahole needs
    a few GB of memory to decode the enum name.

    Acked-by: Stanislav Fomichev <sdf@fomichev.me>
    Acked-by: Aaron Conole <aconole@redhat.com>
    Link: https://patch.msgid.link/20250304180615.945945-1-kuba@kernel.org
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Hangbin Liu <haliu@redhat.com>

Author: Hangbin Liu

tools/testing/selftests/net/config

@@ -18,6 +18,8 @@ CONFIG_DUMMY=y
 CONFIG_BRIDGE_VLAN_FILTERING=y
 CONFIG_BRIDGE=y
 CONFIG_CRYPTO_CHACHA20POLY1305=m
+CONFIG_DEBUG_INFO_BTF=y
+CONFIG_DEBUG_INFO_BTF_MODULES=n
 CONFIG_VLAN_8021Q=y
 CONFIG_GENEVE=m
 CONFIG_IFB=y

tools/testing/selftests/net/openvswitch/openvswitch.sh

@@ -330,6 +330,11 @@ test_psample() {
 # - drop packets and verify the right drop reason is reported
 test_drop_reason() {
 	which perf >/dev/null 2>&1 || return $ksft_skip
+	which pahole >/dev/null 2>&1 || return $ksft_skip
+
+	ovs_drop_subsys=$(pahole -C skb_drop_reason_subsys |
+			      awk '/OPENVSWITCH/ { print $3; }' |
+			      tr -d ,)
 
 	sbx_add "test_drop_reason" || return $?
 
@@ -373,7 +378,7 @@ test_drop_reason() {
 		"in_port(2),eth(),eth_type(0x0800),ipv4(src=172.31.110.20,proto=1),icmp()" 'drop'
 
 	ovs_drop_record_and_run "test_drop_reason" ip netns exec client ping -c 2 172.31.110.20
-	ovs_drop_reason_count 0x30001 # OVS_DROP_FLOW_ACTION
+	ovs_drop_reason_count 0x${ovs_drop_subsys}0001 # OVS_DROP_FLOW_ACTION
 	if [[ "$?" -ne "2" ]]; then
 		info "Did not detect expected drops: $?"
 		return 1
@@ -390,15 +395,15 @@ test_drop_reason() {
 
 	ovs_drop_record_and_run \
             "test_drop_reason" ip netns exec client nc -i 1 -zuv 172.31.110.20 6000
-	ovs_drop_reason_count 0x30004 # OVS_DROP_EXPLICIT_ACTION_ERROR
+	ovs_drop_reason_count 0x${ovs_drop_subsys}0004 # OVS_DROP_EXPLICIT_ACTION_ERROR
 	if [[ "$?" -ne "1" ]]; then
 		info "Did not detect expected explicit error drops: $?"
 		return 1
 	fi
 
 	ovs_drop_record_and_run \
             "test_drop_reason" ip netns exec client nc -i 1 -zuv 172.31.110.20 7000
-	ovs_drop_reason_count 0x30003 # OVS_DROP_EXPLICIT_ACTION
+	ovs_drop_reason_count 0x${ovs_drop_subsys}0003 # OVS_DROP_EXPLICIT_ACTION
 	if [[ "$?" -ne "1" ]]; then
 		info "Did not detect expected explicit drops: $?"
 		return 1