selftests/bpf: Fix flaky cgroup_iter_sleepable subtest

Occasionally, with './test_progs -j' on my vm, I will hit the
following failure:

test_cgrp_local_storage:PASS:join_cgroup /cgrp_local_storage 0 nsec
test_cgroup_iter_sleepable:PASS:skel_open 0 nsec
test_cgroup_iter_sleepable:PASS:skel_load 0 nsec
test_cgroup_iter_sleepable:PASS:attach_iter 0 nsec
test_cgroup_iter_sleepable:PASS:iter_create 0 nsec
test_cgroup_iter_sleepable:FAIL:cgroup_id unexpected cgroup_id: actual 1 != expected 2812
#48/5 cgrp_local_storage/cgroup_iter_sleepable:FAIL
#48 cgrp_local_storage:FAIL

Finally, I decided to do some investigation since the test is introduced
by myself. It turns out the reason is due to cgroup_fd with value 0.
In cgroup_iter, a cgroup_fd of value 0 means the root cgroup.

/* from cgroup_iter.c */
if (fd)
cgrp = cgroup_v1v2_get_from_fd(fd);
else if (id)
cgrp = cgroup_get_from_id(id);
else /* walk the entire hierarchy by default. */
cgrp = cgroup_get_from_path("/");

That is why we got cgroup_id 1 instead of expected 2812.

Why we got a cgroup_fd 0? Nobody should really touch 'stdin' (fd 0) in
test_progs. I traced 'close' syscall with stack trace and found the root
cause, which is a bug in bpf_obj_pinning.c. Basically, the code closed
fd 0 although it should not. Fixing the bug in bpf_obj_pinning.c also
resolved the above cgroup_iter_sleepable subtest failure.

Fixes: 3b22f98e5a05 ("selftests/bpf: Add path_fd-based BPF_OBJ_PIN and BPF_OBJ_GET tests")
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230827150551.1743497-1-yonghong.song@linux.dev

selftests/bpf: Add path_fd-based BPF_OBJ_PIN and BPF_OBJ_GET tests

Add a selftest demonstrating using detach-mounted BPF FS using new mount
APIs, and pinning and getting BPF map using such mount. This
demonstrates how something like container manager could setup BPF FS,
pin and adjust all the necessary objects in it, all before exposing BPF
FS to a particular mount namespace.

Also add a few subtests validating all meaningful combinations of
path_fd and pathname. We use mounted /sys/fs/bpf location for these.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230523170013.728457-5-andrii@kernel.org