linux-kernel - Re: [PATCH] tun: orphan an skb on tx

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Date:	Tue, 14 Sep 2010 17:20:25 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Herbert Xu <herbert@...dor.hengli.com.au>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] tun: orphan an skb on tx

I looked at the macvtap driver and it seems that it should
have the below issue, same as tap.
Arnd?

On Tue, Apr 13, 2010 at 05:59:44PM +0300, Michael S. Tsirkin wrote:
> The following situation was observed in the field:
> tap1 sends packets, tap2 does not consume them, as a result
> tap1 can not be closed. This happens because
> tun/tap devices can hang on to skbs undefinitely.
> 
> As noted by Herbert, possible solutions include a timeout followed by a
> copy/change of ownership of the skb, or always copying/changing
> ownership if we're going into a hostile device.
> 
> This patch implements the second approach.
> 
> Note: one issue still remaining is that since skbs
> keep reference to tun socket and tun socket has a
> reference to tun device, we won't flush backlog,
> instead simply waiting for all skbs to get transmitted.
> At least this is not user-triggerable, and
> this was not reported in practice, my assumption is
> other devices besides tap complete an skb
> within finite time after it has been queued.
> 
> A possible solution for the second issue
> would not to have socket reference the device,
> instead, implement dev->destructor for tun, and
> wait for all skbs to complete there, but this
> needs some thought, probably too risky for 2.6.34.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> Tested-by: Yan Vugenfirer <yvugenfi@...hat.com>
> 
> ---
> 
> Please review the below, and consider for 2.6.34,
> and stable trees.
> 
>  drivers/net/tun.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 96c39bd..4326520 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -387,6 +387,10 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  		}
>  	}
>  
> +	/* Orphan the skb - required as we might hang on to it
> +	 * for indefinite time. */
> +	skb_orphan(skb);
> +
>  	/* Enqueue packet */
>  	skb_queue_tail(&tun->socket.sk->sk_receive_queue, skb);
>  	dev->trans_start = jiffies;
> -- 
> 1.7.0.2.280.gc6f05
> --
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