helpers: Add cgroup-bpf helpers

Add a new module `bpf` and a bunch of cgroup-bpf helpers. More bpf
helpers can be added in the future (e.g. iterate over all programs,
maps, etc).

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

drgn/helpers/linux/bpf.py

@@ -0,0 +1,55 @@
+# SPDX-License-Identifier: GPL-3.0+
+
+"""
+BPF
+---
+
+The ``drgn.helpers.linux.bpf`` module provides helpers for working with BPF
+interface in :linux:`include/linux/bpf.h`, :linux:`include/linux/bpf-cgroup.h`,
+etc.
+"""
+
+
+import itertools
+
+from drgn.helpers import enum_type_to_class
+from drgn.helpers.linux.list import list_for_each_entry
+
+
+__all__ = [
+    "cgroup_bpf_prog_for_each",
+    "cgroup_bpf_prog_for_each_effective",
+]
+
+
+def cgroup_bpf_prog_for_each(cgrp, bpf_attach_type):
+    """
+    .. c:function:: cgroup_bpf_prog_for_each(struct cgroup *cgrp, int bpf_attach_type)
+
+    Iterate over all cgroup bpf programs of the given attach type attached to
+    the given cgroup.
+
+    :return: Iterator of ``struct bpf_prog *`` objects.
+    """
+    progs_head = cgrp.bpf.progs[bpf_attach_type]
+    for pl in list_for_each_entry(
+        "struct bpf_prog_list", progs_head.address_of_(), "node"
+    ):
+        yield pl.prog
+
+
+def cgroup_bpf_prog_for_each_effective(cgrp, bpf_attach_type):
+    """
+    .. c:function:: cgroup_bpf_prog_for_each(struct cgroup *cgrp, int bpf_attach_type)
+
+    Iterate over all effective cgroup bpf programs of the given attach type for
+    the given cgroup.
+
+    :return: Iterator of ``struct bpf_prog *`` objects.
+    """
+    prog_array_items = cgrp.bpf.effective[bpf_attach_type].items
+    for i in itertools.count():
+        prog = prog_array_items[i].prog.read_()
+        if not prog:
+            break
+        yield prog