selftests/xsk: add test when some packets are XDP_DROPed

Add a new test where some of the packets are not passed to the AF_XDP
socket and instead get a XDP_DROP verdict. This is important as it
tests the recycling mechanism of the buffer pool. If a packet is not
sent to the AF_XDP socket, the buffer the packet resides in is instead
recycled so it can be used again without the round-trip to user
space. The test introduces a new XDP program that drops every other
packet.
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/r/20230111093526.11682-13-magnus.karlsson@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/xsk_xdp_progs.c

@@ -11,9 +11,20 @@ struct {
 	__uint(value_size, sizeof(int));
 } xsk SEC(".maps");
 
+static unsigned int idx;
+
 SEC("xdp") int xsk_def_prog(struct xdp_md *xdp)
 {
 	return bpf_redirect_map(&xsk, 0, XDP_DROP);
 }
 
+SEC("xdp") int xsk_xdp_drop(struct xdp_md *xdp)
+{
+	/* Drop every other packet */
+	if (idx++ % 2)
+		return XDP_DROP;
+
+	return bpf_redirect_map(&xsk, 0, XDP_DROP);
+}
+
 char _license[] SEC("license") = "GPL";

tools/testing/selftests/bpf/xskxceiver.c

@@ -1646,6 +1646,34 @@ static void testapp_invalid_desc(struct test_spec *test)
 	pkt_stream_restore_default(test);
 }
 
+static void testapp_xdp_drop(struct test_spec *test)
+{
+	struct ifobject *ifobj = test->ifobj_rx;
+	int err;
+
+	test_spec_set_name(test, "XDP_DROP_HALF");
+	xsk_detach_xdp_program(ifobj->ifindex, ifobj->xdp_flags);
+	err = xsk_attach_xdp_program(ifobj->xdp_progs->progs.xsk_xdp_drop, ifobj->ifindex,
+				     ifobj->xdp_flags);
+	if (err) {
+		printf("Error attaching XDP_DROP program\n");
+		test->fail = true;
+		return;
+	}
+
+	pkt_stream_receive_half(test);
+	testapp_validate_traffic(test);
+
+	pkt_stream_restore_default(test);
+	xsk_detach_xdp_program(ifobj->ifindex, ifobj->xdp_flags);
+	err = xsk_attach_xdp_program(ifobj->xdp_progs->progs.xsk_def_prog, ifobj->ifindex,
+				     ifobj->xdp_flags);
+	if (err) {
+		printf("Error restoring default XDP program\n");
+		exit_with_error(-err);
+	}
+}
+
 static int xsk_load_xdp_programs(struct ifobject *ifobj)
 {
 	ifobj->xdp_progs = xsk_xdp_progs__open_and_load();
@@ -1796,6 +1824,9 @@ static void run_pkt_test(struct test_spec *test, enum test_mode mode, enum test_
 	case TEST_TYPE_HEADROOM:
 		testapp_headroom(test);
 		break;
+	case TEST_TYPE_XDP_DROP_HALF:
+		testapp_xdp_drop(test);
+		break;
 	default:
 		break;
 	}

tools/testing/selftests/bpf/xskxceiver.h

@@ -87,6 +87,7 @@ enum test_type {
 	TEST_TYPE_STATS_RX_FULL,
 	TEST_TYPE_STATS_FILL_EMPTY,
 	TEST_TYPE_BPF_RES,
+	TEST_TYPE_XDP_DROP_HALF,
 	TEST_TYPE_MAX
 };