linux-kernel - Re: [PATCH 1/2] device property: do not leak child nodes when using NULL/error pointers

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Message-ID: <Z06b0oTvxUi4DTlx@google.com>
Date: Mon, 2 Dec 2024 21:49:06 -0800
From: Dmitry Torokhov <dmitry.torokhov@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	Daniel Scally <djrscally@...il.com>, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] device property: do not leak child nodes when using
 NULL/error pointers

On Sat, Nov 30, 2024 at 11:44:04PM +0200, Andy Shevchenko wrote:
> On Fri, Nov 29, 2024 at 11:16:54PM -0800, Dmitry Torokhov wrote:
> > On Fri, Nov 29, 2024 at 04:50:15PM +0200, Andy Shevchenko wrote:
> > > On Thu, Nov 28, 2024 at 03:04:50PM -0800, Dmitry Torokhov wrote:
> > > > On Thu, Nov 28, 2024 at 03:13:16PM +0200, Andy Shevchenko wrote:
> > > > > On Wed, Nov 27, 2024 at 09:39:34PM -0800, Dmitry Torokhov wrote:
> 
> ...
> 
> > > > > > @@ struct fwnode_handle *device_get_next_child_node(const struct device *dev,
> > > > > >  	const struct fwnode_handle *fwnode = dev_fwnode(dev);
> > > > > >  	struct fwnode_handle *next;
> > > > > 
> > > > > > -	if (IS_ERR_OR_NULL(fwnode))
> > > > > > +	if (IS_ERR_OR_NULL(fwnode)) {
> > > > > > +		fwnode_handle_put(child);
> > > > > >  		return NULL;
> > > > > > +	}
> > > > > 
> > > > > >  	/* Try to find a child in primary fwnode */
> > > > > >  	next = fwnode_get_next_child_node(fwnode, child);
> > > > > 
> > > > > So, why not just moving the original check (w/o dropping the reference) here?
> > > > > Wouldn't it have the same effect w/o explicit call to the fwnode_handle_put()?
> > > > 
> > > > Because if you rely on check in fwnode_get_next_child_node() you would
> > > > not know if it returned NULL because there are no more children or
> > > > because the node is invalid. In the latter case you can't dereference
> > > > fwnode->secondary.
> > > 
> > > Yes, so, how does it contradict my proposal?
> > 
> > I guess I misunderstood your proposal then. Could you please explain it
> > in more detail?
> 
> 
> Current code (in steps):
> 	if (IS_ERR_OR_NULL()) check
> 	trying primary
> 	trying secondary if previous is NULL
> 
> 
> My proposal
> 
> 	trying primary
> 	return if not NULL
> 	if (IS_ERR_OR_NULL()) check in its current form (no put op)
> 	trying secondary
> 
> After your first patch IIUC this is possible as trying primary will put child uncoditionally.

Ah, I see. No, I do not think this is a good idea: it will make the code
harder to understand for a casual reader: "Why do we check node validity
only after we used it for the first time?"

For the code not in a hot path there is a lot of value in simplicity.

Thanks.

-- 
Dmitry