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Date: Wed, 20 Nov 2013 20:19:49 +0200
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Vlad Yasevich <vyasevic@...hat.com>
Cc: netdev@...r.kernel.org
Subject: Re: [RFC net-next PATCH] macvtap: Add packet capture support
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 <vyasevic@...hat.com>
> ---
> 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.
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.
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
--
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