From: Chen Ridong <chenridong@huawei.com>

Add a helper function named 'ns_next_child_pre' that performs
a pre-order traversal of a namespace's descendants.

Signed-off-by: Chen Ridong <chenridong@huawei.com>
---
 include/linux/user_namespace.h |  9 +++++++
 kernel/user_namespace.c        | 44 ++++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+)

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index d84b2703caab..823df9267a4a 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -169,6 +169,15 @@ static inline void set_userns_rlimit_max(struct user_namespace *ns,
 	ns->rlimit_max[type] = max <= LONG_MAX ? max : LONG_MAX;
 }
 
+struct user_namespace *ns_next_child(struct user_namespace *pos,
+					   struct user_namespace *parent);
+struct user_namespace *ns_next_child_pre(struct user_namespace *pos,
+						    struct user_namespace *root);
+
+#define ns_for_each_child_pre(pos, ns)				\
+	for ((pos) = ns_next_child_pre(NULL, (ns)); (pos);	\
+	     (pos) = ns_next_child_pre((pos), (ns)))
+
 #ifdef CONFIG_USER_NS
 
 static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index cbe8f96c3e60..9a2e77505b97 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -30,6 +30,50 @@ static bool new_idmap_permitted(const struct file *file,
 				struct uid_gid_map *map);
 static void free_user_ns(struct work_struct *work);
 
+struct user_namespace *ns_next_child(struct user_namespace *pos,
+					   struct user_namespace *parent)
+{
+	struct user_namespace *next;
+
+	if (!pos)
+		/* Get the first child of the parent. */
+		next = list_entry_rcu(parent->children.next, struct user_namespace, ns_node);
+	else
+		next = list_entry_rcu(pos->ns_node.next, struct user_namespace, ns_node);
+
+	if (&next->ns_node != &parent->children)
+		return next;
+
+	return NULL;
+}
+
+/* Should be called under rcu_read_lock() */
+struct user_namespace *ns_next_child_pre(struct user_namespace *pos,
+						    struct user_namespace *root)
+{
+	struct user_namespace *next;
+
+
+	/* if first iteration, visit @root */
+	if (!pos)
+		return root;
+
+	/* visit the first child if exists */
+	next = ns_next_child(NULL, pos);
+	if (next)
+		return next;
+
+	/* no child, visit my or the closest ancestor's next ns_node */
+	while (pos != root) {
+		next = ns_next_child(pos, pos->parent);
+		if (next)
+			return next;
+		pos = pos->parent;
+	}
+
+	return NULL;
+}
+
 static struct ucounts *inc_user_namespaces(struct user_namespace *ns, kuid_t uid)
 {
 	return inc_ucount(ns, uid, UCOUNT_USER_NAMESPACES);
-- 
2.34.1