Re: [PATCH v2 2/2] device property: fix infinite loop in fwnode_for_each_child_node()

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Wed, Jun 03, 2026 at 04:44:32PM +0800, Xu Yang wrote:

> 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()").

...

TBH, this code becomes twisted and complicated. Can we add some test cases to
show the problem? Also we need to add other possible combinations (somewhat
about ~5-6) of the different types of fwnode in a relationship.

-- 
With Best Regards,
Andy Shevchenko