[RFC PATCH bpf-next 1/5] cgroup: Enable task_under_cgroup_hierarchy() on cgroup1

[Yafang Shao <laoar.shao@gmail.com>]

Currently, the function task_under_cgroup_hierarchy() allows us to
determine if a task resides exclusively within a cgroup2 hierarchy.
Nevertheless, given the continued prevalence of cgroup1, it's useful that
we make a minor adjustment to extend its functionality to cgroup1 as well.
Once this modification is implemented, we will have the ability to
effortlessly verify a task's cgroup membership within BPF programs. For
instance, we can easily check if a task belongs to a cgroup1 directory,
such as /sys/fs/cgroup/cpu,cpuacct/kubepods/burstable/ or
/sys/fs/cgroup/cpu,cpuacct/kubepods/besteffort/.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 include/linux/cgroup.h | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index b307013..5414a2c 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -543,15 +543,33 @@ static inline struct cgroup *cgroup_ancestor(struct cgroup *cgrp,
  * @ancestor: possible ancestor of @task's cgroup
  *
  * Tests whether @task's default cgroup hierarchy is a descendant of @ancestor.
- * It follows all the same rules as cgroup_is_descendant, and only applies
- * to the default hierarchy.
+ * It follows all the same rules as cgroup_is_descendant.
  */
 static inline bool task_under_cgroup_hierarchy(struct task_struct *task,
 					       struct cgroup *ancestor)
 {
 	struct css_set *cset = task_css_set(task);
+	struct cgroup *cgrp;
+	bool ret = false;
+	int ssid;
+
+	if (ancestor->root == &cgrp_dfl_root)
+		return cgroup_is_descendant(cset->dfl_cgrp, ancestor);
+
+	for (ssid = 0; ssid < CGROUP_SUBSYS_COUNT; ssid++) {
+		if (!ancestor->subsys[ssid])
+			continue;
 
-	return cgroup_is_descendant(cset->dfl_cgrp, ancestor);
+		cgrp = task_css(task, ssid)->cgroup;
+		if (!cgrp)
+			continue;
+
+		if (!cgroup_is_descendant(cgrp, ancestor))
+			return false;
+		if (!ret)
+			ret = true;
+	}
+	return ret;
 }
 
 /* no synchronization, the result can only be used as a hint */
-- 
1.8.3.1