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: Tejun Heo <tj@kernel.org>
Acked-by: Greg Kroah-Hartman <greg@kroah.com>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Daniel Mack <daniel@zonque.org>

2 files changed, 34 insertions, 0 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 5047355b9a0f..383c630f36f9 100644
--- a/include/linux/cgroup.h
+++ b/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);
 
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index dcb417c6c242..6b2b1d945df2 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1827,6 +1827,38 @@ out:
 }
 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
  */