From: Hao Luo <redacted>

Cgroup_iter is a type of bpf_iter. It walks over cgroups in three modes:

 - walking a cgroup's descendants in pre-order.
 - walking a cgroup's descendants in post-order.
 - walking a cgroup's ancestors.

When attaching cgroup_iter, one can set a cgroup to the iter_link
created from attaching. This cgroup is passed as a file descriptor and
serves as the starting point of the walk. If no cgroup is specified,
the starting point will be the root cgroup.

For walking descendants, one can specify the order: either pre-order or
post-order. For walking ancestors, the walk starts at the specified
cgroup and ends at the root.

One can also terminate the walk early by returning 1 from the iter
program.

Note that because walking cgroup hierarchy holds cgroup_mutex, the iter
program is called with cgroup_mutex held.

Currently only one session is supported, which means, depending on the
volume of data bpf program intends to send to user space, the number
of cgroups that can be walked is limited. For example, given the current
buffer size is 8 * PAGE_SIZE, if the program sends 64B data for each
cgroup, the total number of cgroups that can be walked is 512. This is
a limitation of cgroup_iter. If the output data is larger than the
buffer size, the second read() will signal EOPNOTSUPP. In order to work
around, the user may have to update their program to reduce the volume
of data sent to output. For example, skip some uninteresting cgroups.
In future, we may extend bpf_iter flags to allow customizing buffer
size.

Signed-off-by: Hao Luo <redacted>
Signed-off-by: Yosry Ahmed <redacted>
Acked-by: Yonghong Song <redacted>
---
 include/linux/bpf.h                           |   8 +
 include/uapi/linux/bpf.h                      |  30 +++
 kernel/bpf/Makefile                           |   3 +
 kernel/bpf/cgroup_iter.c                      | 252 ++++++++++++++++++
 tools/include/uapi/linux/bpf.h                |  30 +++
 .../selftests/bpf/prog_tests/btf_dump.c       |   4 +-
 6 files changed, 325 insertions(+), 2 deletions(-)
 create mode 100644 kernel/bpf/cgroup_iter.c

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index a97751d845c9..9061618fe929 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -47,6 +47,7 @@ struct kobject;
 struct mem_cgroup;
 struct module;
 struct bpf_func_state;
+struct cgroup;

 extern struct idr btf_idr;
 extern spinlock_t btf_idr_lock;

@@ -1717,7 +1718,14 @@ int bpf_obj_get_user(const char __user *pathname, int flags);
        int __init bpf_iter_ ## target(args) { return 0; }

 struct bpf_iter_aux_info {
+       /* for map_elem iter */
        struct bpf_map *map;
+
+       /* for cgroup iter */
+       struct {
+               struct cgroup *start; /* starting cgroup */
+               int order;
+       } cgroup;
 };

 typedef int (*bpf_iter_attach_target_t)(struct bpf_prog *prog,

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index ffcbf79a556b..fe50c2489350 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -87,10 +87,30 @@ struct bpf_cgroup_storage_key {
        __u32   attach_type;            /* program attach type (enum bpf_attach_type) */
 };

+enum bpf_iter_cgroup_traversal_order {
+       BPF_ITER_CGROUP_PRE = 0,        /* pre-order traversal */
+       BPF_ITER_CGROUP_POST,           /* post-order traversal */
+       BPF_ITER_CGROUP_PARENT_UP,      /* traversal of ancestors up to the root */