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

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Date:	Mon, 01 Dec 2014 13:35:43 +0100
From:	Philipp Zabel <p.zabel@...gutronix.de>
To:	Guennadi Liakhovetski <g.liakhovetski@....de>
Cc:	Philipp Zabel <pza@...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

Am Sonntag, den 09.11.2014, 16:51 +0100 schrieb Guennadi Liakhovetski:
> On Sun, 9 Nov 2014, Philipp Zabel wrote:
> 
> > 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.
> 
> Right, so, the patch has to be adjusted not to touch soc_camera.c at all.

No. As of the current media-tree we still need move the of_node_put(epn)
out of the loop:

diff --git a/drivers/media/platform/soc_camera/soc_camera.c b/drivers/media/platform/soc_camera/soc_camera.c
index f4308fe..619b2d4 100644
--- a/drivers/media/platform/soc_camera/soc_camera.c
+++ b/drivers/media/platform/soc_camera/soc_camera.c
@@ -1696,7 +1696,6 @@ static void scan_of_host(struct soc_camera_host *ici)
 		if (!i)
 			soc_of_bind(ici, epn, ren->parent);
 
-		of_node_put(epn);
 		of_node_put(ren);
 
 		if (i) {
@@ -1704,6 +1703,8 @@ static void scan_of_host(struct soc_camera_host *ici)
 			break;
 		}
 	}
+
+	of_node_put(epn);
 }
 
 #else

We can do this in two steps (step 1 fixing the current status quo, step
2 as part of this series). Step 1:

--- a/drivers/media/platform/soc_camera/soc_camera.c
+++ b/drivers/media/platform/soc_camera/soc_camera.c
@@ -1691,11 +1691,13 @@ static void scan_of_host(struct soc_camera_host *ici)
 {
 	struct device *dev = ici->v4l2_dev.dev;
 	struct device_node *np = dev->of_node;
-	struct device_node *epn = NULL, *ren;
+	struct device_node *epn, *prev = NULL, *ren;
 	unsigned int i;
 
 	for (i = 0; ; i++) {
-		epn = of_graph_get_next_endpoint(np, epn);
+		epn = of_graph_get_next_endpoint(np, prev);
+		of_node_put(prev);
+		prev = epn;
 		if (!epn)
 			break;
 
@@ -1710,7 +1712,6 @@ static void scan_of_host(struct soc_camera_host *ici)
 		if (!i)
 			soc_of_bind(ici, epn, ren->parent);
 
-		of_node_put(epn);
 		of_node_put(ren);
 
 		if (i) {

And step 2:

diff --git a/drivers/media/platform/soc_camera/soc_camera.c b/drivers/media/platform/soc_camera/soc_camera.c
index 3d44773..1de62bf 100644
--- a/drivers/media/platform/soc_camera/soc_camera.c
+++ b/drivers/media/platform/soc_camera/soc_camera.c
@@ -1691,13 +1691,11 @@ static void scan_of_host(struct soc_camera_host *ici)
 {
 	struct device *dev = ici->v4l2_dev.dev;
 	struct device_node *np = dev->of_node;
-	struct device_node *epn, *prev = NULL, *ren;
+	struct device_node *epn = NULL, *ren;
 	unsigned int i;
 
 	for (i = 0; ; i++) {
-		epn = of_graph_get_next_endpoint(np, prev);
-		of_node_put(prev);
-		prev = epn;
+		epn = of_graph_get_next_endpoint(np, epn);
 		if (!epn)
 			break;
 
@@ -1719,6 +1717,8 @@ static void scan_of_host(struct soc_camera_host *ici)
 			break;
 		}
 	}
+
+	of_node_put(epn);
 }
 
 #else

Would you prefer that option?

regards
Philipp


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