netdev - Re: [PATCH 0/3] net: time stamping fixes

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Date:	Wed, 19 Oct 2011 16:08:25 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Richard Cochran <richardcochran@...il.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 0/3] net: time stamping fixes

Le mercredi 19 octobre 2011 à 15:57 +0200, Johannes Berg a écrit :
> On Wed, 2011-10-19 at 15:44 +0200, Eric Dumazet wrote:
> > Le mercredi 19 octobre 2011 à 15:35 +0200, Johannes Berg a écrit :
> > 
> > > Even with that fixed I'm not really convinced of it all -- need to
> > > really really really make sure that no skb->sk that was owned by a TX
> > > skb is ever passed to sock_hold(). Can we really guarantee that?
> > 
> > Either we can guarantee that, or kernel is a piece of crap, all bets are
> > off.
> 
> Fair enough :-)
> 
> > Yes, we need to make an audit, since we assumed sock_hold() was the
> > right thing to do in all contexts.
> 
> Ok.
> 
> Anyway, I guess you agree that the patches as-is aren't actually the
> right solution since we can't sock_hold() a TX skb socket reference?

Yes, the sock_hold() could be changed by an atomic_inc_not_zero()

What about doing this ?

diff --git a/include/linux/phy.h b/include/linux/phy.h
index 54fc413..79f337c 100644
--- a/include/linux/phy.h
+++ b/include/linux/phy.h
@@ -420,7 +420,7 @@ struct phy_driver {
 
 	/*
 	 * Requests a Tx timestamp for 'skb'. The phy driver promises
-	 * to deliver it to the socket's error queue as soon as a
+	 * to deliver it using skb_complete_tx_timestamp() as soon as a
 	 * timestamp becomes available. One of the PTP_CLASS_ values
 	 * is passed in 'type'.
 	 */
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 8bd383c..0f96646 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2020,8 +2020,13 @@ static inline bool skb_defer_rx_timestamp(struct sk_buff *skb)
 /**
  * skb_complete_tx_timestamp() - deliver cloned skb with tx timestamps
  *
+ * PHY drivers may accept clones of transmitted packets for
+ * timestamping via their phy_driver.txtstamp method. These drivers
+ * must call this function to return the skb back to the stack, with
+ * or without a timestamp.
+ *
  * @skb: clone of the the original outgoing packet
- * @hwtstamps: hardware time stamps
+ * @hwtstamps: hardware time stamps, may be NULL if not available
  *
  */
 void skb_complete_tx_timestamp(struct sk_buff *skb,
diff --git a/net/core/timestamping.c b/net/core/timestamping.c
index 98a5264..82fb288 100644
--- a/net/core/timestamping.c
+++ b/net/core/timestamping.c
@@ -57,9 +57,13 @@ void skb_clone_tx_timestamp(struct sk_buff *skb)
 	case PTP_CLASS_V2_VLAN:
 		phydev = skb->dev->phydev;
 		if (likely(phydev->drv->txtstamp)) {
+			if (!atomic_inc_not_zero(&sk->sk_refcnt))
+				return;
 			clone = skb_clone(skb, GFP_ATOMIC);
-			if (!clone)
+			if (!clone) {
+				sock_put(sk);
 				return;
+			}
 			clone->sk = sk;
 			phydev->drv->txtstamp(phydev, clone, type);
 		}
@@ -77,8 +81,11 @@ void skb_complete_tx_timestamp(struct sk_buff *skb,
 	struct sock_exterr_skb *serr;
 	int err;
 
-	if (!hwtstamps)
+	if (!hwtstamps) {
+		sock_put(sk);
+		kfree_skb(skb);
 		return;
+	}
 
 	*skb_hwtstamps(skb) = *hwtstamps;
 	serr = SKB_EXT_ERR(skb);
@@ -87,6 +94,7 @@ void skb_complete_tx_timestamp(struct sk_buff *skb,
 	serr->ee.ee_origin = SO_EE_ORIGIN_TIMESTAMPING;
 	skb->sk = NULL;
 	err = sock_queue_err_skb(sk, skb);
+	sock_put(sk);
 	if (err)
 		kfree_skb(skb);
 }


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