Re: [PATCH] flow_dissector: Fix vlan header offset in __skb_flow_dissect

[Yuehaibing]

On 2019/6/20 2:39, Stanislav Fomichev wrote:
> On 06/20, YueHaibing wrote:
>> We build vlan on top of bonding interface, which vlan offload
>> is off, bond mode is 802.3ad (LACP) and xmit_hash_policy is
>> BOND_XMIT_POLICY_ENCAP34.
>>
>> __skb_flow_dissect() fails to get information from protocol headers
>> encapsulated within vlan, because 'nhoff' is points to IP header,
>> so bond hashing is based on layer 2 info, which fails to distribute
>> packets across slaves.
>>
>> Fixes: d5709f7ab776 ("flow_dissector: For stripped vlan, get vlan info from skb->vlan_tci")
>> Signed-off-by: YueHaibing <yuehaibing@xxxxxxxxxx>
>> ---
>>  net/core/flow_dissector.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
>> index 415b95f..2a52abb 100644
>> --- a/net/core/flow_dissector.c
>> +++ b/net/core/flow_dissector.c
>> @@ -785,6 +785,9 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
>>  		    skb && skb_vlan_tag_present(skb)) {
>>  			proto = skb->protocol;
>>  		} else {
>> +			if (dissector_vlan == FLOW_DISSECTOR_KEY_MAX)
>> +				nhoff -=  sizeof(*vlan);
>> +
> Should we instead fix the place where the skb is allocated to properly
> pull vlan (skb_vlan_untag)? I'm not sure this particular place is
> supposed to work with an skb. Having an skb with nhoff pointing to
> IP header but missing skb_vlan_tag_present() when with
> proto==ETH_P_8021xx seems weird.

The skb is a forwarded vxlan packet, it send through vlan interface like this:

   vlan_dev_hard_start_xmit
    --> __vlan_hwaccel_put_tag //vlan_tci and VLAN_TAG_PRESENT is set
    --> dev_queue_xmit
        --> validate_xmit_skb
          --> validate_xmit_vlan // vlan_hw_offload_capable is false
             --> __vlan_hwaccel_push_inside //here skb_push vlan_hlen, then clear skb->tci

    --> bond_start_xmit
       --> bond_xmit_hash
         --> __skb_flow_dissect // nhoff point to IP header
            -->  case htons(ETH_P_8021Q)
            // skb_vlan_tag_present is false, so
              vlan = __skb_header_pointer(skb, nhoff, sizeof(_vlan), //vlan point to ip header wrongly

> 
>>  			vlan = __skb_header_pointer(skb, nhoff, sizeof(_vlan),
>>  						    data, hlen, &_vlan);
>>  			if (!vlan) {
>> -- 
>> 2.7.0
>>
>>
> 
> .
>