Subject: Re: [RFC net-next PATCH] macvtap: Add packet capture support

From: Vlad Yasevich <[email-masked]>
Date: Wed, 20 Nov 2013 14:36:40 -0500

On 11/20/2013 01:19 PM, Michael S. Tsirkin wrote:
> On Wed, Nov 20, 2013 at 01:04:09PM -0500, Vlad Yasevich wrote:
>> Currently it is impossible to capture traffic when using a macvtap
>> device.  The reason is that all capture handling is done either in
>> dev_hard_start_xmit() or in __netif_receive_skb_core().  Macvtap
>> currenlty doesn't use dev_hard_start_xmit(), and at the time the
>> packet traverses __netif_receive_skb_core, the skb->dev is set to
>> the lower-level device that doesn't end up matching macvtap.
>>
>> To solve the issue, use dev_hard_start_xmit() on the output path.
>> On the input path, it is toughter to solve since macvtap ends up
>> consuming the skb so there is nothing more left for
>> __netif_receive_skb_core() to do.  A simple solution is to
>> pull the code that delivers things to the taps/captures into
>> its own function and allow macvtap to call it from its receive
>> routine.
>>
>> Signed-off-by: Vlad Yasevich <[email-masked]>
>> ---
>> This is only an RFC.  I'd like to solicit comments on this simple
>> approach.
> 
> I'm kind of worried about this. What worries me is that normally
> if you have a packet socket bound to all interfaces, what it shows is
> traffic to/from the box.
> 
> This might be a bug for someone, but I suspect at this point this
> is part of the ABI.
> 
> But macvtap bypasses most of the host networking stack,
> So I worry that suddenly showing these packets would be confusing.

Is it really different from using bridge and tap?  If someone
does 'tcpdump -i any', they will see packets sent by the guest with
bridge+tap.  It makes sense to do that same thing for macvtap, no?

> 
> Assuming we want to show this traffic, I think it's preferable to
> -		if (!ptype->dev || ptype->dev == skb->dev) {
> +		if (ptype->dev == skb->dev) {
> so you have to bind to macvtap explicitly to see the traffic.
> 
> 
> Of course when you start binding to specific macvtaps
> it doesn't scale well because we'll have a long list
> on ptype_all suddenly.

How likely is that really?  ptype_all doesn't scale as it.
Any time you don't set the protocol field, the entry goes
into ptype_all.  Macvtap doesn't change that.  You can
create a long list in ptype all if you have lots of guests and
you want to snoop on each guest separately thus binding to tap
device.

-vlad

> This might mean we need to keep this list per-device ...
> 
>> A more complicated solution would have been to give
>> macvtap its own rx_handler and return RX_HANDLER_ANOTHER during
>> receive operation to make the packet go through another round of
>> capturing and hit the macvtap rx_handler.  I thought this would
>> hurt performance too much for no real gain.
>>
>> Thanks
>> -vlad
>>
>>  drivers/net/macvtap.c     |  4 +++-
>>  include/linux/netdevice.h |  2 ++
>>  net/core/dev.c            | 33 ++++++++++++++++++++++++++-------
>>  3 files changed, 31 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
>> index dc76670..0ed8fae 100644
>> --- a/drivers/net/macvtap.c
>> +++ b/drivers/net/macvtap.c
>> @@ -334,6 +334,7 @@ drop:
>>   */
>>  static int macvtap_receive(struct sk_buff *skb)
>>  {
>> +	netif_receive_skb_taps(skb, true);
>>  	skb_push(skb, ETH_HLEN);
>>  	return macvtap_forward(skb->dev, skb);
>>  }
>> @@ -727,8 +728,9 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
>>  		skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
>>  	}
>>  	if (vlan) {
>> +		skb->dev = vlan->dev;
>>  		local_bh_disable();
>> -		macvlan_start_xmit(skb, vlan->dev);
>> +		dev_hard_start_xmit(skb, vlan->dev, NULL, NULL);
>>  		local_bh_enable();
>>  	} else {
>>  		kfree_skb(skb);
>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>> index 8b3de7c..84880eb 100644
>> --- a/include/linux/netdevice.h
>> +++ b/include/linux/netdevice.h
>> @@ -2383,6 +2383,8 @@ void dev_kfree_skb_any(struct sk_buff *skb);
>>  int netif_rx(struct sk_buff *skb);
>>  int netif_rx_ni(struct sk_buff *skb);
>>  int netif_receive_skb(struct sk_buff *skb);
>> +struct packet_type *netif_receive_skb_taps(struct sk_buff *skb,
>> +					   bool last_deliver);
>>  gro_result_t napi_gro_receive(struct napi_struct *napi, struct sk_buff *skb);
>>  void napi_gro_flush(struct napi_struct *napi, bool flush_old);
>>  struct sk_buff *napi_get_frags(struct napi_struct *napi);
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index da9c5e1..50f0ac4 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -3484,6 +3484,31 @@ static bool skb_pfmemalloc_protocol(struct sk_buff *skb)
>>  	}
>>  }
>>  
>> +struct packet_type *netif_receive_skb_taps(struct sk_buff *skb,
>> +					   bool last_deliver)
>> +{
>> +	struct packet_type *ptype, *pt_prev = NULL;
>> +	struct net_device *orig_dev;
>> +
>> +	if (list_empty(&ptype_all))
>> +		return NULL;
>> +
>> +	orig_dev = skb->dev;
>> +	list_for_each_entry_rcu(ptype, &ptype_all, list) {
>> +		if (!ptype->dev || ptype->dev == skb->dev) {
>> +			if (pt_prev)
>> +				deliver_skb(skb, pt_prev, orig_dev);
>> +			pt_prev = ptype;
>> +		}
>> +	}
>> +
>> +	if (last_deliver && pt_prev)
>> +		deliver_skb(skb, pt_prev, orig_dev);
>> +
>> +	return pt_prev;
>> +}
>> +EXPORT_SYMBOL_GPL(netif_receive_skb_taps);
>> +
>>  static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc)
>>  {
>>  	struct packet_type *ptype, *pt_prev;
>> @@ -3535,13 +3560,7 @@ another_round:
>>  	if (pfmemalloc)
>>  		goto skip_taps;
>>  
>> -	list_for_each_entry_rcu(ptype, &ptype_all, list) {
>> -		if (!ptype->dev || ptype->dev == skb->dev) {
>> -			if (pt_prev)
>> -				ret = deliver_skb(skb, pt_prev, orig_dev);
>> -			pt_prev = ptype;
>> -		}
>> -	}
>> +	pt_prev = netif_receive_skb_taps(skb, false);
>>  
>>  skip_taps:
>>  #ifdef CONFIG_NET_CLS_ACT
>> -- 
>> 1.8.4.2