netdev - [PATCH v3] smsc95xx: Use skb_cow_head to deal with cloned skbs

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Date:   Wed, 19 Apr 2017 11:07:22 +0100
From:   James Hughes <james.hughes@...pberrypi.org>
To:     netdev@...r.kernel.org,
        Steve Glendinning <steve.glendinning@...well.net>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>
Cc:     James Hughes <james.hughes@...pberrypi.org>
Subject: [PATCH v3] smsc95xx: Use skb_cow_head to deal with cloned skbs

The driver was failing to check that the SKB wasn't cloned
before adding checksum data.
Replace existing handling to extend/copy the header buffer
with skb_cow_head.

Signed-off-by: James Hughes <james.hughes@...pberrypi.org>
---
Change for V3
- Added the dev_kfree_skb_any, otherwise the skb would not 
  be deallocated due to returning NULL.

 drivers/net/usb/smsc95xx.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
index df60c98..4cb8144 100644
--- a/drivers/net/usb/smsc95xx.c
+++ b/drivers/net/usb/smsc95xx.c
@@ -2067,13 +2067,13 @@ static struct sk_buff *smsc95xx_tx_fixup(struct usbnet *dev,
 	/* We do not advertise SG, so skbs should be already linearized */
 	BUG_ON(skb_shinfo(skb)->nr_frags);
 
-	if (skb_headroom(skb) < overhead) {
-		struct sk_buff *skb2 = skb_copy_expand(skb,
-			overhead, 0, flags);
+	/* Make writable and expand header space by overhead if required */
+	if (skb_cow_head(skb, overhead)) {
+		/* Must reallocate here as returning NULL to indicate error
+		 * means the skb won't be deallocated in the caller.
+		 */
 		dev_kfree_skb_any(skb);
-		skb = skb2;
-		if (!skb)
-			return NULL;
+		return NULL;
 	}
 
 	if (csum) {
-- 
2.9.3