[PATCH v3 1/2] list: add missing empty list check to list_cut_before()

[Ziran Zhang]

list_cut_before() lacks the list_empty() guard present in
list_cut_position().  With an empty head, the function falls through
to the pointer updates instead of returning early.

Both helpers share the same kernel-doc wording:

  'You should pass on @entry an element you know is on @head.'

Nevertheless, list_cut_position() returns early for an empty head,
and list_splice() also checks its source list before doing any
pointer updates.  Make list_cut_before() consistent with them for
the empty-list case.

Signed-off-by: Ziran Zhang <zhangcoder@xxxxxxxx>
---
 include/linux/list.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/linux/list.h b/include/linux/list.h
index 77fb62f79..4d2061d35 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -555,6 +555,11 @@ static inline void list_cut_before(struct list_head *list,
 				   struct list_head *head,
 				   struct list_head *entry)
 {
+	if (list_empty(head)) {
+		INIT_LIST_HEAD(list);
+		return;
+	}
+
 	if (head->next == entry) {
 		INIT_LIST_HEAD(list);
 		return;
-- 
2.51.0