linux-kernel - Re: Bug in sysfs_break_active

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Date: Wed, 13 Mar 2024 10:21:20 -1000
From: Tejun Heo <tj@...nel.org>
To: Alan Stern <stern@...land.harvard.edu>
Cc: Greg KH <gregkh@...uxfoundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Bug in sysfs_break_active_protection()

Sorry about late reply.

On Mon, Mar 04, 2024 at 02:17:27PM -0500, Alan Stern wrote:
..
> struct kernfs_node *sysfs_break_active_protection(struct kobject *kobj,
> 						  const struct attribute *attr)
> {
> 	struct kernfs_node *kn;
> 
> 	kobject_get(kobj);
> 	kn = kernfs_find_and_get(kobj->sd, attr->name);
> 	if (kn)
> 		kernfs_break_active_protection(kn);
> 	return kn;
> }
..
> If kn is NULL then the kobject_get(kobj) reference is never dropped.  
> It looks like this could happen if two processes want to unregister the 
> same kobject at the same time.
> 
> Shouldn't sysfs_break_active_protection() do this?
> 
> 	kobject_get(kobj);
> 	kn = kernfs_find_and_get(kobj->sd, attr->name);
> 	if (kn)
> 		kernfs_break_active_protection(kn);
> +	else
> +		kobject_put(kobj);
> 	return kn;

Yeah, I think you're right. It's an obvious ref leak. Would you mind writing
up a patch?

Thank you.

-- 
tejun