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

[Neil Horman <nhorman@tuxdriver.com>]

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