linux-kernel - Re: [PATCH v2 1/2] binder: Refactor binder

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Message-ID: <aByhDepxNsCCr3rI@google.com>
Date: Thu, 8 May 2025 12:18:21 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Tiffany Yang <ynaffit@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Arve Hjønnevåg" <arve@...roid.com>, Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joel@...lfernandes.org>, Christian Brauner <brauner@...nel.org>, 
	Carlos Llamas <cmllamas@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>, linux-kernel@...r.kernel.org, 
	kernel-team@...roid.com
Subject: Re: [PATCH v2 1/2] binder: Refactor binder_node print synchronization

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