[PATCH bpf-next] selftests/bpf: Mount debugfs in setns_by_fd

[Stanislav Fomichev <sdf@google.com>]

Jiri reports broken test_progs after recent commit 68f8e3d4b916
("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: a30338840fa5 ("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>
---
 tools/testing/selftests/bpf/network_helpers.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/testing/selftests/bpf/network_helpers.c b/tools/testing/selftests/bpf/network_helpers.c
index bec15558fd93..1f37adff7632 100644
--- a/tools/testing/selftests/bpf/network_helpers.c
+++ b/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;
 }
 
-- 
2.38.1.584.g0f3c55d4c2-goog