selftests/bpf: Mount debugfs in setns_by_fd

Jiri reports broken test_progs after recent commit 68f8e3d4b9
("selftests/bpf: Make sure zero-len skbs aren't redirectable").
Apparently we don't remount debugfs when we switch back networking namespace.
Let's explicitly mount /sys/kernel/debug.

0: https://lore.kernel.org/bpf/63b85917-a2ea-8e35-620c-808560910819@meta.com/T/#ma66ca9c92e99eee0a25e40f422489b26ee0171c1

Fixes: a30338840f ("selftests/bpf: Move open_netns() and close_netns() into network_helpers.c")
Reported-by: Jiri Olsa <olsajiri@gmail.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20221123200829.2226254-1-sdf@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/network_helpers.c

@@ -426,6 +426,10 @@ static int setns_by_fd(int nsfd)
 	if (!ASSERT_OK(err, "mount /sys/fs/bpf"))
 		return err;
 
+	err = mount("debugfs", "/sys/kernel/debug", "debugfs", 0, NULL);
+	if (!ASSERT_OK(err, "mount /sys/kernel/debug"))
+		return err;
+
 	return 0;
 }
 

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

@@ -9,7 +9,7 @@
 		goto out; \
 })
 
-void test_empty_skb(void)
+void serial_test_empty_skb(void)
 {
 	LIBBPF_OPTS(bpf_test_run_opts, tattr);
 	struct empty_skb *bpf_obj = NULL;

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

@@ -85,7 +85,7 @@ static void test_max_pkt_size(int fd)
 }
 
 #define NUM_PKTS 10000
-void test_xdp_do_redirect(void)
+void serial_test_xdp_do_redirect(void)
 {
 	int err, xdp_prog_fd, tc_prog_fd, ifindex_src, ifindex_dst;
 	char data[sizeof(pkt_udp) + sizeof(__u32)];

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

@@ -174,7 +174,7 @@ out:
 	system("ip netns del synproxy");
 }
 
-void test_xdp_synproxy(void)
+void serial_test_xdp_synproxy(void)
 {
 	if (test__start_subtest("xdp"))
 		test_synproxy(true);