cgroup: implement task_cgroup_path_from_hierarchy()

kdbus folks want a sane way to determine the cgroup path that a given
task belongs to on a given hierarchy, which is a reasonble thing to
expect from cgroup core.

Implement task_cgroup_path_from_hierarchy().

v2: Dropped unnecessary NULL check on the return value of
    task_cgroup_from_root() as suggested by Li Zefan.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NGreg Kroah-Hartman <greg@kroah.com>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Daniel Mack <daniel@zonque.org>

include/linux/cgroup.h

@@ -542,6 +542,8 @@ int cgroup_is_removed(const struct cgroup *cgrp);
 bool cgroup_is_descendant(struct cgroup *cgrp, struct cgroup *ancestor);
 
 int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen);
+int task_cgroup_path_from_hierarchy(struct task_struct *task, int hierarchy_id,
+				    char *buf, size_t buflen);
 
 int cgroup_task_count(const struct cgroup *cgrp);
 

kernel/cgroup.c

@@ -1827,6 +1827,38 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
 }
 EXPORT_SYMBOL_GPL(cgroup_path);
 
+/**
+ * task_cgroup_path_from_hierarchy - cgroup path of a task on a hierarchy
+ * @task: target task
+ * @hierarchy_id: the hierarchy to look up @task's cgroup from
+ * @buf: the buffer to write the path into
+ * @buflen: the length of the buffer
+ *
+ * Determine @task's cgroup on the hierarchy specified by @hierarchy_id and
+ * copy its path into @buf.  This function grabs cgroup_mutex and shouldn't
+ * be used inside locks used by cgroup controller callbacks.
+ */
+int task_cgroup_path_from_hierarchy(struct task_struct *task, int hierarchy_id,
+				    char *buf, size_t buflen)
+{
+	struct cgroupfs_root *root;
+	struct cgroup *cgrp = NULL;
+	int ret = -ENOENT;
+
+	mutex_lock(&cgroup_mutex);
+
+	root = idr_find(&cgroup_hierarchy_idr, hierarchy_id);
+	if (root) {
+		cgrp = task_cgroup_from_root(task, root);
+		ret = cgroup_path(cgrp, buf, buflen);
+	}
+
+	mutex_unlock(&cgroup_mutex);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(task_cgroup_path_from_hierarchy);
+
 /*
  * Control Group taskset
  */