can-raw: Fix skb_orphan_try handling

Commit fc6055a5 (net: Introduce
skb_orphan_try()) allows an early orphan of the skb and takes care on
tx timestamping, which needs the sk-reference in the skb on driver level.
So does the can-raw socket, which has not been taken into account here.

The patch below adds a 'prevent_sk_orphan' bit in the skb tx shared info,
which fixes the problem discovered by Matthias Fuchs here:

      http://marc.info/?t=128030411900003&r=1&w=2

Even if it's not a primary tx timestamp topic it fits well into some skb
shared tx context. Or should be find a different place for the information to
protect the sk reference until it reaches the driver level?
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -169,6 +169,7 @@ struct skb_shared_hwtstamps {
  * @software:		generate software time stamp
  * @in_progress:	device driver is going to provide
  *			hardware time stamp
+ * @prevent_sk_orphan:	make sk reference available on driver level
  * @flags:		all shared_tx flags
  *
  * These flags are attached to packets as part of the
@@ -178,7 +179,8 @@ union skb_shared_tx {
 	struct {
 		__u8	hardware:1,
 			software:1,
-			in_progress:1;
+			in_progress:1,
+			prevent_sk_orphan:1;
 	};
 	__u8 flags;
 };

net/can/raw.c

@@ -650,6 +650,10 @@ static int raw_sendmsg(struct kiocb *iocb, struct socket *sock,
 	err = sock_tx_timestamp(msg, sk, skb_tx(skb));
 	if (err < 0)
 		goto free_skb;
+
+	/* to be able to check the received tx sock reference in raw_rcv() */
+	skb_tx(skb)->prevent_sk_orphan = 1;
+
 	skb->dev = dev;
 	skb->sk  = sk;