Re: [PATCH v2 1/2] binder: Refactor binder_node print synchronization

[Alice Ryhl]

On Mon, May 05, 2025 at 09:42:32PM +0000, Tiffany Yang wrote:
> +	if (node->proc)
> +		binder_inner_proc_unlock(node->proc);
> +	else
> +		spin_unlock(&binder_dead_nodes_lock);

I don't buy this logic. Imagine the following scenario:

1. print_binder_proc is called, and we loop over proc->nodes.
2. We call binder_inner_proc_unlock(node->proc).
3. On another thread, binder_deferred_release() is called.
4. The node is removed from proc->nodes and node->proc is set to NULL.
5. Back in print_next_binder_node_ilocked(), we now call
   spin_lock(&binder_dead_nodes_lock) and return.
6. In print_binder_proc(), we think that we hold the proc lock, but
   actually we hold the dead nodes lock instead. BOOM.

What happens with the current code is that print_binder_proc() takes the
proc lock again after the node was removed from proc->nodes, and then it
exits the loop because rb_next(n) returns NULL when called on a node not
in any rb-tree.

Alice