linux-kernel - Re: [PATCH v5 1/6] of: Decrement refcount of previous endpoint in of_graph_get_next

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Date:	Sun, 9 Nov 2014 16:36:44 +0100
From:	Philipp Zabel <pza@...gutronix.de>
To:	Guennadi Liakhovetski <g.liakhovetski@....de>
Cc:	Philipp Zabel <p.zabel@...gutronix.de>,
	Grant Likely <grant.likely@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	devel@...verdev.osuosl.org,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Mauro Carvalho Chehab <m.chehab@...sung.com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	kernel@...gutronix.de
Subject: Re: [PATCH v5 1/6] of: Decrement refcount of previous endpoint in
 of_graph_get_next_endpoint

Hi Guennadi,

On Fri, Nov 07, 2014 at 11:06:21PM +0100, Guennadi Liakhovetski wrote:
> Hi Philipp,
> 
> Thanks for the patch and sorry for a late reply. I did look at your 
> patches earlier too, but maybe not attentively enough, or maybe I'm 
> misunderstanding something now. In the scan_of_host() function in 
> soc_camera.c as of current -next I see:
> 
> 		epn = of_graph_get_next_endpoint(np, epn);
> 
> which already looks like a refcount leak to me. If epn != NULL, its 
> refcount is incremented, but then immediately the variable gets 
> overwritten, and there's no extra copy of that variable to fix this. If 
> I'm right, then that bug in itself should be fixed, ideally before your 
> patch is applied. But in fact, your patch fixes this, since it modifies 
> of_graph_get_next_endpoint() to return with prev's refcount not 
> incremented, right? Whereas the of_node_put(epn) later down in 
> scan_of_host() decrements refcount of the _next_ endpoint, not the 
> previous one, so, it should be left alone? I.e. AFAICT your modification 
> to of_graph_get_next_endpoint() fixes soc_camera.c with no further 
> modifications to it required?

You are right. With the old implementation, you'd have to do the
epn = of_graph_get_next_endpoint(np, prev); of_node_put(prev); prev = epn;
dance to avoid leaking a reference to the first endpoint. This series
accidentally fixes soc_camera by changing of_graph_get_next_endpoint
to decrement the reference count itself.

regards
Philipp
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