bpf/selftests: Add check for updating XDP bpf_link with wrong program type

Add a check to the xdp_link selftest that the kernel rejects replacing an
XDP program with a different program type on link update.

v2:
- Split this out into its own patch.
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20220107221115.326171-3-toke@redhat.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/xdp_link.c

@@ -127,6 +127,11 @@ void serial_test_xdp_link(void)
 	ASSERT_EQ(link_info.prog_id, id1, "link_prog_id");
 	ASSERT_EQ(link_info.xdp.ifindex, IFINDEX_LO, "link_ifindex");
 
+	/* updating program under active BPF link with different type fails */
+	err = bpf_link__update_program(link, skel1->progs.tc_handler);
+	if (!ASSERT_ERR(err, "link_upd_invalid"))
+		goto cleanup;
+
 	err = bpf_link__detach(link);
 	if (!ASSERT_OK(err, "link_detach"))
 		goto cleanup;

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

@@ -10,3 +10,9 @@ int xdp_handler(struct xdp_md *xdp)
 {
 	return 0;
 }
+
+SEC("tc")
+int tc_handler(struct __sk_buff *skb)
+{
+	return 0;
+}