[PATCH 2/2] device property: fix infinite loop in fwnode_for_each_child_node()

[Xu Yang]

When iterate over children of a fwnode that has a secondary fwnode,
fwnode_get_next_child_node() can enter an infinite loop if the secondary
fwnode has more than one child.

                       Parent        Child
      (Primary fwnode)   FWa:   {FWa1, FWa2, FWa3}
    (Secondary fwnode)   FWb:   {FWb1, FWb2}

In this case:

 ┌─> fwnode_get_next_child_node(FWa, FWa1)
 │    - fwnode_call_ptr_op(FWa, get_next_child_node, FWa1) returns FWa2
 │
 │   ...
 │
 │   fwnode_get_next_child_node(FWa, FWa3)
 │    - fwnode_call_ptr_op(FWa, get_next_child_node, FWa3) returns NULL
 │    - fwnode_call_ptr_op(FWb, get_next_child_node, FWa3) returns FWb1
 │
 │   fwnode_get_next_child_node(FWa, FWb1)
 │    - fwnode_call_ptr_op(FWa, get_next_child_node, FWb1) returns FWa1
 └────┘

This cause fwnode_for_each_child_node() to loop indefinitely, reapeatedly
output {FWa1, FWa2, FWa3, FWb1, FWa1, ...}.

The root cause is that when the current child (FWb1) belongs to the
secondary fwnode, calling get_next_child_node() on the parimary fwnode
incorrectly returns the first child (FWa1) again instead of NULL.

Fix this by dynamically checking the parent fwnode of the current child
before calling get_next_child_node(). This approach follows the pattern
established in commit b5b41ab6b0c1 ("device property: Check
fwnode->secondary in fwnode_graph_get_next_endpoint()").

Fixes: 2692c614f8f0 ("device property: Allow secondary lookup in fwnode_get_next_child_node()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Xu Yang <xu.yang_2@xxxxxxx>
---
 drivers/base/property.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/drivers/base/property.c b/drivers/base/property.c
index 8e0148a37fff..9dce513f90cc 100644
--- a/drivers/base/property.c
+++ b/drivers/base/property.c
@@ -807,18 +807,32 @@ struct fwnode_handle *
 fwnode_get_next_child_node(const struct fwnode_handle *fwnode,
 			   struct fwnode_handle *child)
 {
-	struct fwnode_handle *next;
+	struct fwnode_handle *next, *child_parent = NULL;
+	const struct fwnode_handle *parent;
 
 	if (IS_ERR_OR_NULL(fwnode))
 		return NULL;
+	/*
+	 * If this function is in a loop and the previous iteration returned
+	 * an child from fwnode->secondary, then we need to use the secondary
+	 * as parent rather than @fwnode.
+	 */
+	if (child) {
+		child_parent = fwnode_get_parent(child);
+		parent = child_parent;
+	} else {
+		parent = fwnode;
+	}
 
-	/* Try to find a child in primary fwnode */
-	next = fwnode_call_ptr_op(fwnode, get_next_child_node, child);
+	next = fwnode_call_ptr_op(parent, get_next_child_node, child);
 	if (next)
-		return next;
+		goto put_child_parent;
+
+	next = fwnode_call_ptr_op(parent->secondary, get_next_child_node, NULL);
 
-	/* When no more children in primary, continue with secondary */
-	return fwnode_call_ptr_op(fwnode->secondary, get_next_child_node, child);
+put_child_parent:
+	fwnode_handle_put(child_parent);
+	return next;
 }
 EXPORT_SYMBOL_GPL(fwnode_get_next_child_node);
 

-- 
2.34.1