selftests: forwarding: lib: Avoid trapping soft devices

There are several cases where traffic that would normally be forwarded
in silicon needs to be observed in slow path. That's achieved by
trapping such traffic, and the functions trap_install() and
trap_uninstall() realize that. However, such treatment is obviously
wrong if the device in question is actually a soft device not backed by
an ASIC.

Therefore try to trap if possible, but fall back to inserting a continue
if not.

Signed-off-by: Petr Machata <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/forwarding/lib.sh

···
 	local dev=$1; shift
 	local direction=$1; shift
 
-	# For slow-path testing, we need to install a trap to get to
-	# slow path the packets that would otherwise be switched in HW.
-	tc filter add dev $dev $direction pref 1 flower skip_sw action trap
+	# Some devices may not support or need in-hardware trapping of traffic
+	# (e.g. the veth pairs that this library creates for non-existent
+	# loopbacks). Use continue instead, so that there is a filter in there
+	# (some tests check counters), and so that other filters are still
+	# processed.
+	tc filter add dev $dev $direction pref 1 \
+		flower skip_sw action trap 2>/dev/null \
+	    || tc filter add dev $dev $direction pref 1 \
+		       flower action continue
 }
 
 trap_uninstall()
···
 	local dev=$1; shift
 	local direction=$1; shift
 
-	tc filter del dev $dev $direction pref 1 flower skip_sw
+	tc filter del dev $dev $direction pref 1 flower
 }
 
 slow_path_trap_install()
 {
+	# For slow-path testing, we need to install a trap to get to
+	# slow path the packets that would otherwise be switched in HW.
 	if [ "${tcflags/skip_hw}" != "$tcflags" ]; then
 		trap_install "$@"
 	fi