[Qemu-devel] Re: [PATCH] tun: orphan an skb on tx

[Jan Kiszka]

Michael S. Tsirkin wrote:
> 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 <address@hidden>
>> Tested-by: Yan Vugenfirer <address@hidden>
>>
>> ---
>>
>> 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
> 
> This is commit 0110d6f22f392f976e84ab49da1b42f85b64a3c5 in net-2.6
> Please cherry-pick this fix in stable kernels (2.6.32 and 2.6.33).
> 
> Thanks!
> 

Before I forget again:

Tested-by: Jan Kiszka <address@hidden>
(namely the field test of our customer)

Jan

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