netdev - Re: [Bridge] [PATCH 1/2] net: bridge: fix tc added QinQ forwarding

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Date:   Fri, 18 Jan 2019 11:29:52 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Zahari Doychev <zahari.doychev@...ux.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bridge@...ts.linux-foundation.org,
        Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
        Roopa Prabhu <roopa@...ulusnetworks.com>,
        Johannes Berg <johannes@...solutions.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>
Subject: Re: [Bridge] [PATCH 1/2] net: bridge: fix tc added QinQ forwarding

On 2019/01/18 4:19, Cong Wang wrote:
> On Thu, Jan 17, 2019 at 12:59 AM Toshiaki Makita
> <makita.toshiaki@....ntt.co.jp> wrote:
>> On 2019/01/17 17:17, Zahari Doychev wrote:
>>> On Tue, Jan 15, 2019 at 03:11:28PM +0900, Toshiaki Makita wrote:
>>>> On 2019/01/13 22:59, Zahari Doychev wrote:
>>>>> Use the skb->mac_len instead of using the ETH_HLEN when pushing the skb
>>>>> data pointer. This fixes sending incorrect packets when more than one
>>>>> vlan tags are pushed by tc-vlan and the mac header length is bigger than
>>>>> ETH_HLEN. In this way the vlan tagged contained in the skb is inserted at
>>>>> right offset in the packet payload before sending the packet.
>>>>>
>>>>> Signed-off-by: Zahari Doychev <zahari.doychev@...ux.com>
>>>>> ---
>>>>>  net/bridge/br_forward.c | 2 +-
>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/net/bridge/br_forward.c b/net/bridge/br_forward.c
>>>>> index 5372e2042adf..55f928043f77 100644
>>>>> --- a/net/bridge/br_forward.c
>>>>> +++ b/net/bridge/br_forward.c
>>>>> @@ -39,7 +39,7 @@ int br_dev_queue_push_xmit(struct net *net, struct sock *sk, struct sk_buff *skb
>>>>>     if (!is_skb_forwardable(skb->dev, skb))
>>>>>             goto drop;
>>>>>
>>>>> -   skb_push(skb, ETH_HLEN);
>>>>> +   skb_push(skb, skb->mac_len);
>>>>>     br_drop_fake_rtable(skb);
>>>>>
>>>>>     if (skb->ip_summed == CHECKSUM_PARTIAL &&
>>>>>
>>>>
>>>> I guess you mean skb->data points to mac_header + ETH_HLEN + VLAN_HLEN
>>>> when bridge receives skbs in br_handle_frame()?
>>>
>>> yes, this is what I see.
>>>
>>>> If so, the behavior of act_vlan is odd. Normal double tagged skbs from
>>>> hardware devices should have skb->data pointing to mac_header + ETH_HLEN
>>>> because they just call eth_type_trans() before entering
>>>> netif_receive_skb()...
>>>> I think act_vlan needs some fix.
>>>
>>> The act_valn is using the skb_vlan_push(...) to add the vlan tags and in this
>>> way increasing the skb->data and mac_len. So I think I can add a fix there to
>>> set the skb->data to point to mac_header + ETH_HLEN when more tags are added.
>>
>> As skb->data always points to mac_header after calling skb_vlan_push(),
>> we probably need to remember mac_len before invocation of it?
>>
>> The problem should be this part in tcf_vlan_act():
>>
>>> out:
>>>       if (skb_at_tc_ingress(skb))
>>>               skb_pull_rcsum(skb, skb->mac_len);
>>
>> skb->mac_len should not be used here.
> 
> I am confused. This code is to push skb->data back to network header.
> If skb_vlan_push() pushes skb->data to mac header, then this code
> is correct for pulling it back to network header, as skb->mac_len is
> updated accordingly inside skb_vlan_push() too.
> 
> What goes wrong here? skb->mac_len isn't correct for double tagging?

Bridge and VLAN code (skb_vlan_untag in __netif_receive_skb_core)
expects skb->data to point to the start of VLAN header, not the next
(network) header. After calling tcf_vlan_act() on ingress filter,
skb->data points to the next of VLAN header (network header), while
non-hwaccel VLAN packets (including double tagged ones) from NIC drivers
have skb->data pointing to the start of VLAN header as expected.

I'm not sure if mac_len should or should not be updated in
skb_vlan_push(). Anyway IIRC skb_vlan_untag() and bridge code do not
rely on mac_len so mac_len should not cause problems there.

-- 
Toshiaki Makita