selftests/bpf: Check for correct program attach/detach in xdp_attach test

David Ahern noticed that there was a bug in the EXPECTED_FD code so
programs did not get detached properly when that parameter was supplied.
This case was not included in the xdp_attach tests; so let's add it to be
sure that such a bug does not sneak back in down.

Fixes: 87854a0b57 ("selftests/bpf: Add tests for attaching XDP programs")
Reported-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200414145025.182163-2-toke@redhat.com

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

@@ -6,19 +6,34 @@
 
 void test_xdp_attach(void)
 {
+	__u32 duration = 0, id1, id2, id0 = 0, len;
 	struct bpf_object *obj1, *obj2, *obj3;
 	const char *file = "./test_xdp.o";
+	struct bpf_prog_info info = {};
 	int err, fd1, fd2, fd3;
-	__u32 duration = 0;
 	DECLARE_LIBBPF_OPTS(bpf_xdp_set_link_opts, opts,
 			    .old_fd = -1);
 
+	len = sizeof(info);
+
 	err = bpf_prog_load(file, BPF_PROG_TYPE_XDP, &obj1, &fd1);
 	if (CHECK_FAIL(err))
 		return;
+	err = bpf_obj_get_info_by_fd(fd1, &info, &len);
+	if (CHECK_FAIL(err))
+		goto out_1;
+	id1 = info.id;
+
 	err = bpf_prog_load(file, BPF_PROG_TYPE_XDP, &obj2, &fd2);
 	if (CHECK_FAIL(err))
 		goto out_1;
+
+	memset(&info, 0, sizeof(info));
+	err = bpf_obj_get_info_by_fd(fd2, &info, &len);
+	if (CHECK_FAIL(err))
+		goto out_2;
+	id2 = info.id;
+
 	err = bpf_prog_load(file, BPF_PROG_TYPE_XDP, &obj3, &fd3);
 	if (CHECK_FAIL(err))
 		goto out_2;
@@ -28,6 +43,11 @@ void test_xdp_attach(void)
 	if (CHECK(err, "load_ok", "initial load failed"))
 		goto out_close;
 
+	err = bpf_get_link_xdp_id(IFINDEX_LO, &id0, 0);
+	if (CHECK(err || id0 != id1, "id1_check",
+		  "loaded prog id %u != id1 %u, err %d", id0, id1, err))
+		goto out_close;
+
 	err = bpf_set_link_xdp_fd_opts(IFINDEX_LO, fd2, XDP_FLAGS_REPLACE,
 				       &opts);
 	if (CHECK(!err, "load_fail", "load with expected id didn't fail"))
@@ -37,6 +57,10 @@ void test_xdp_attach(void)
 	err = bpf_set_link_xdp_fd_opts(IFINDEX_LO, fd2, 0, &opts);
 	if (CHECK(err, "replace_ok", "replace valid old_fd failed"))
 		goto out;
+	err = bpf_get_link_xdp_id(IFINDEX_LO, &id0, 0);
+	if (CHECK(err || id0 != id2, "id2_check",
+		  "loaded prog id %u != id2 %u, err %d", id0, id2, err))
+		goto out_close;
 
 	err = bpf_set_link_xdp_fd_opts(IFINDEX_LO, fd3, 0, &opts);
 	if (CHECK(!err, "replace_fail", "replace invalid old_fd didn't fail"))
@@ -51,6 +75,10 @@ void test_xdp_attach(void)
 	if (CHECK(err, "remove_ok", "remove valid old_fd failed"))
 		goto out;
 
+	err = bpf_get_link_xdp_id(IFINDEX_LO, &id0, 0);
+	if (CHECK(err || id0 != 0, "unload_check",
+		  "loaded prog id %u != 0, err %d", id0, err))
+		goto out_close;
 out:
 	bpf_set_link_xdp_fd(IFINDEX_LO, -1, 0);
 out_close: