examples: Add example to traverse cgroup v2 hierarchy

Add simple example to traverse cgroup v2 hierarchy.

It can be run w/o arguments and in this case root cgroup will be used:

  # python36 -m drgn examples/linux/cgroup.py | wc -l
  1081
  # python36 -m drgn examples/linux/cgroup.py | shuf -n 3
  /system.slice/nfs-idmapd.service
  /user.slice/user-0.slice/user@0.service/dev-hugepages.mount
  /system.slice/crond.service

Or a path in cgroup v2 filesystem can be specified to limit the
traversal by this path:

  # python36 -m drgn examples/linux/cgroup.py /sys/fs/cgroup/www.slice
  /www.slice
  /www.slice/hack.service
  /www.slice/flow.service

Signed-off-by: Andrey Ignatov <rdna@fb.com>

examples/linux/cgroup.py

@@ -0,0 +1,40 @@
+"""List the paths of all descendants of a cgroup v2"""
+
+import os
+import sys
+
+from contextlib import contextmanager
+
+from drgn import cast
+from drgn.helpers.linux import (
+    cgroup_path,
+    css_for_each_descendant_pre,
+    fget,
+    find_task,
+)
+
+
+@contextmanager
+def open_dir(*args, **kwds):
+    # Built-in open() context manager can't deal with directories.
+    fd = os.open(*args, **kwds)
+    try:
+        yield fd
+    finally:
+        os.close(fd)
+
+
+def get_cgroup():
+    if len(sys.argv) == 1:
+        return prog["cgrp_dfl_root"].cgrp
+    task = find_task(prog, os.getpid())
+    with open_dir(sys.argv[1], os.O_RDONLY) as fd:
+        file_ = fget(task, fd)
+        kn = cast("struct kernfs_node *", file_.f_path.dentry.d_inode.i_private)
+        return cast("struct cgroup *", kn.priv)
+
+
+css = get_cgroup().self.address_of_()
+
+for pos in css_for_each_descendant_pre(css):
+    print(cgroup_path(pos.cgroup).decode())