netdev - Re: [PATCH net-2.6 resent] af_packet: Don't use skb after dev_queue

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Date:	Sun, 10 Jan 2010 13:51:35 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	jarkao2@...il.com
Cc:	mbreuer@...jas.com, shemminger@...tta.com,
	akpm@...ux-foundation.org, flyboy@...il.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-2.6 resent] af_packet: Don't use skb after
 dev_queue_xmit()

From: Jarek Poplawski <jarkao2@...il.com>
Date: Sat, 9 Jan 2010 13:38:27 +0100

> tpacket_snd() can change and kfree an skb after dev_queue_xmit(),
> which is illegal.
> 
> With debugging by: Stephen Hemminger <shemminger@...tta.com>
> 
> Reported-by: Michael Breuer <mbreuer@...jas.com>
> Tested-by: Michael Breuer <mbreuer@...jas.com>
> Signed-off-by: Jarek Poplawski <jarkao2@...il.com>
> Acked-by: Stephen Hemminger <shemminger@...tta.com>

Jarek, if this code path triggers, it will deadlock the
send ring with your changes.

We will now leave the ring packet status in the "SENDING" state.

That's not right.

Then, if the application calls send again, we will just return
immediately since we only make progress if the head ring entry is in
SEND_REQUEST state.

This is really bogus behavior.  When the qdisc or mid-layer
drops the packet, we should at least mark the packet state
properly (which is what the current code would does, sans
the "reference SKB after dev_queue_xmit()" issue).  And
advance the packet ring pointer.

This way the user:

1) can see that the packet got dropped and couldn't be sent

2) can call send again to try sending the rest of the ring

Fix the use after dev_queue_xmit() issue, but don't change other side
effects which are important for correct AF_PACKET TX ring semantics.
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