selftests/bpf: add test for BPF flow dissector in the root namespace

Make sure non-root namespaces get an error if root flow dissector is
attached.

Cc: Petar Penkov <ppenkov@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/test_flow_dissector.sh

@@ -18,19 +18,55 @@ fi
 # this is the case and run it with in_netns.sh if it is being run in the root
 # namespace.
 if [[ -z $(ip netns identify $$) ]]; then
+	err=0
+	if bpftool="$(which bpftool)"; then
+		echo "Testing global flow dissector..."
+
+		$bpftool prog loadall ./bpf_flow.o /sys/fs/bpf/flow \
+			type flow_dissector
+
+		if ! unshare --net $bpftool prog attach pinned \
+			/sys/fs/bpf/flow/flow_dissector flow_dissector; then
+			echo "Unexpected unsuccessful attach in namespace" >&2
+			err=1
+		fi
+
+		$bpftool prog attach pinned /sys/fs/bpf/flow/flow_dissector \
+			flow_dissector
+
+		if unshare --net $bpftool prog attach pinned \
+			/sys/fs/bpf/flow/flow_dissector flow_dissector; then
+			echo "Unexpected successful attach in namespace" >&2
+			err=1
+		fi
+
+		if ! $bpftool prog detach pinned \
+			/sys/fs/bpf/flow/flow_dissector flow_dissector; then
+			echo "Failed to detach flow dissector" >&2
+			err=1
+		fi
+
+		rm -rf /sys/fs/bpf/flow
+	else
+		echo "Skipping root flow dissector test, bpftool not found" >&2
+	fi
+
+	# Run the rest of the tests in a net namespace.
 	../net/in_netns.sh "$0" "$@"
-	exit $?
-fi
+	err=$(( $err + $? ))
 
-# Determine selftest success via shell exit code
-exit_handler()
-{
-	if (( $? == 0 )); then
+	if (( $err == 0 )); then
 		echo "selftests: $TESTNAME [PASS]";
 	else
 		echo "selftests: $TESTNAME [FAILED]";
 	fi
 
+	exit $err
+fi
+
+# Determine selftest success via shell exit code
+exit_handler()
+{
 	set +e
 
 	# Cleanup