Re: [PATCH] ftrace: tracepoint of net_dev_xmit sees freed skb and causes panic

On Tue, May 31, 2011 at 11:13:11AM -0400, Steven Rostedt wrote:

On Tue, 2011-05-31 at 16:48 +0900, Koki Sanagi wrote:

quoted

Because there is a possibility that skb is kfree_skb()ed and zero cleared
after ndo_start_xmit, we should not see the contents of skb like skb->len and
skb->dev->name after ndo_start_xmit. But trace_net_dev_xmit does that
and causes panic by NULL pointer dereference.
This patch fixes trace_net_dev_xmit not to see the contents of skb directly.

quoted

 
 	if (likely(!skb->next)) {
 		u32 features;

@@ -2139,8 +2140,9 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 			}
 		}
 
+		skb_len = skb->len;
 		rc = ops->ndo_start_xmit(skb, dev);
-		trace_net_dev_xmit(skb, rc);
+		trace_net_dev_xmit(skb, rc, dev, skb_len);
 		if (rc == NETDEV_TX_OK)
 			txq_trans_update(txq);
 		return rc;

@@ -2160,8 +2162,9 @@ gso:
 		if (dev->priv_flags & IFF_XMIT_DST_RELEASE)
 			skb_dst_drop(nskb);
 
+		skb_len = nskb->len;
 		rc = ops->ndo_start_xmit(nskb, dev);
-		trace_net_dev_xmit(nskb, rc);
+		trace_net_dev_xmit(nskb, rc, dev, skb_len);

What if you just put the tracepoint before the call to
ops->ndo_start_xmit?

Then you won't know the return code of ndo_start_xmit, which this tracepoint
records.
Neil

-- Steve

quoted

 		if (unlikely(rc != NETDEV_TX_OK)) {
 			if (rc & ~NETDEV_TX_MASK)
 				goto out_kfree_gso_skb;