linux-kernel - Re: [PATCH net-next v2] net: dsa: Support bridge 802.1Q while untagging

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Date:   Thu, 1 Oct 2020 16:48:43 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v2] net: dsa: Support bridge 802.1Q while
 untagging



On 10/1/2020 4:24 PM, Vladimir Oltean wrote:
> On Wed, Sep 30, 2020 at 08:06:23PM -0700, Florian Fainelli wrote:
>> The intent of 412a1526d067 ("net: dsa: untag the bridge pvid from rx
>> skbs") is to transparently untag the bridge's default_pvid when the
>> Ethernet switch can only support egress tagged of that default_pvid
>> towards the CPU port.
>>
>> Prior to this commit, users would have to configure an 802.1Q upper on
>> the bridge master device when the bridge is configured with
>> vlan_filtering=0 in order to pop the VLAN tag:
>>
>> ip link add name br0 type bridge vlan_filtering 0
>> ip link add link br0 name br0.1 type vlan id 1
>>
>> After this commit we added support for managing a switch port 802.1Q
>> upper but those are not usually added as bridge members, and if they do,
>> they do not actually require any special management, the data path would
>> pop the desired VLAN tag accordingly.
>>
>> What we want to preserve is that use case and to manage when the user
>> creates that 802.1Q upper for the bridge port.
>>
>> While we are it, call __vlan_find_dev_deep_rcu() which makes use the
>> VLAN group array which is faster.
>>
>> As soon as we return the VLAN tagged SKB though it will be used by the
>> following call path:
>>
>> netif_receive_skb_list_internal
>>    -> __netif_receive_skb_list_core
>>      -> __netif_receive_skb_core
>>        -> vlan_do_receive()
>>
>> which uses skb->vlan_proto, if we do not set it to the appropriate VLAN
>> protocol, we will leave it set to what the DSA master has set
>> (ETH_P_XDSA).
>>
> 
> The explanation is super confusing, although I think the placement of
> the "skb->vlan_proto = vlan_dev_vlan_proto(upper_dev)" is correct.
> Here's what I think is going on. It has to do with what's upwards of the
> code you're changing:
> 
> 	/* Move VLAN tag from data to hwaccel */
> 	if (!skb_vlan_tag_present(skb) && hdr->h_vlan_proto == htons(proto)) {
> 		skb = skb_vlan_untag(skb);
> 		if (!skb)
> 			return NULL;
> 	}
> 
> So skb->vlan_proto should already be equal to the protocol of the 8021q
> upper, see the call path below.
> 
>                             this is the problem
>                                     |
> skb_vlan_untag()                   v
>    -> __vlan_hwaccel_put_tag(skb, skb->protocol, vlan_tci);
>      -> skb->vlan_proto = vlan_proto;

Ah, indeed!

> 
> But the problem is that skb_vlan_untag() calls __vlan_hwaccel_put_tag
> with the wrong vlan_proto, it calls it with the skb->protocol which is
> still ETH_P_XDSA because we haven't re-run eth_type_trans() yet.
> It looks like this function wants pretty badly to be called after
> eth_type_trans(), and it's getting pretty messy because of that, but we
> don't have any other driver-specific hook afterwards..
> 
> I don't have a lot of experience, the alternatives are either to:
> - move dsa_untag_bridge_pvid() after eth_type_trans(), similar to what
>    you did in your initial patch - maybe this is the cleanest

This would be my preference and it would not be hurting the fast-path 
that much.

> - make dsa_untag_bridge_pvid() call eth_type_trans() and this gets rid
>    of the extra step you need to do in tag_brcm.c

Sure, however this requires that we remove the call to eth_type_trans() 
in dsa_switch_rcv() or that we push/pull by an appropriate amount, not 
very effective.

> - document this very well

I doubt this would survive the test of time unfortunately.

> 
>> Fixes: 412a1526d067 ("net: dsa: untag the bridge pvid from rx skbs")
>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
>> ---
>> Changes in v2:
>>
>> - removed unused list_head iter argument
>>
>>   net/dsa/dsa_priv.h | 11 ++++-------
>>   1 file changed, 4 insertions(+), 7 deletions(-)
>>
>> diff --git a/net/dsa/dsa_priv.h b/net/dsa/dsa_priv.h
>> index 0348dbab4131..b4aafb2e90fa 100644
>> --- a/net/dsa/dsa_priv.h
>> +++ b/net/dsa/dsa_priv.h
>> @@ -205,7 +205,6 @@ static inline struct sk_buff *dsa_untag_bridge_pvid(struct sk_buff *skb)
>>   	struct net_device *br = dp->bridge_dev;
>>   	struct net_device *dev = skb->dev;
>>   	struct net_device *upper_dev;
>> -	struct list_head *iter;
>>   	u16 vid, pvid, proto;
>>   	int err;
>>   
>> @@ -247,12 +246,10 @@ static inline struct sk_buff *dsa_untag_bridge_pvid(struct sk_buff *skb)
>>   	 * supports because vlan_filtering is 0. In that case, we should
>>   	 * definitely keep the tag, to make sure it keeps working.
>>   	 */
>> -	netdev_for_each_upper_dev_rcu(dev, upper_dev, iter) {
>> -		if (!is_vlan_dev(upper_dev))
>> -			continue;
>> -
>> -		if (vid == vlan_dev_vlan_id(upper_dev))
>> -			return skb;
>> +	upper_dev = __vlan_find_dev_deep_rcu(br, htons(proto), vid);
>> +	if (upper_dev) {
>> +		skb->vlan_proto = vlan_dev_vlan_proto(upper_dev);
>> +		return skb;
>>   	}
>>   
>>   	__vlan_hwaccel_clear_tag(skb);
>> -- 
>> 2.25.1

-- 
Florian