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: 3b22f98e ("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

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

@@ -8,6 +8,7 @@
 #include <linux/unistd.h>
 #include <linux/mount.h>
 #include <sys/syscall.h>
+#include "bpf/libbpf_internal.h"
 
 static inline int sys_fsopen(const char *fsname, unsigned flags)
 {
@@ -155,7 +156,7 @@ static void validate_pin(int map_fd, const char *map_name, int src_value,
 	ASSERT_OK(err, "obj_pin");
 
 	/* cleanup */
-	if (pin_opts.path_fd >= 0)
+	if (path_kind == PATH_FD_REL && pin_opts.path_fd >= 0)
 		close(pin_opts.path_fd);
 	if (old_cwd[0])
 		ASSERT_OK(chdir(old_cwd), "restore_cwd");
@@ -220,7 +221,7 @@ static void validate_get(int map_fd, const char *map_name, int src_value,
 		goto cleanup;
 
 	/* cleanup */
-	if (get_opts.path_fd >= 0)
+	if (path_kind == PATH_FD_REL && get_opts.path_fd >= 0)
 		close(get_opts.path_fd);
 	if (old_cwd[0])
 		ASSERT_OK(chdir(old_cwd), "restore_cwd");