Subject: Re: [PATCH net] net: Account for all vlan headers in skb_mac_gso_segment

From: Vlad Yasevich <[email-masked]>
Date: Mon, 31 Mar 2014 17:37:15 -0400

On 03/31/2014 05:22 PM, Neil Jerram wrote:
> Hi Vlad,
> 
>> skb_network_protocol() already accounts for multiple vlan
>> headers that may be present in the skb.  However, skb_mac_gso_segment()
>> doesn't know anything about it and assumes that skb->mac_len
>> is set correctly to skip all mac headers.  That may not
>> always be the case.  If we are simply forwarding the packet (via
>> bridge or macvtap), all vlan headers may not be accounted for.
> 
> When is it the case that skb->mac_len does not include all VLAN tags?  If you can clearly describe when this is true, and when not, it would be nice to add a comment above the mac_len field in skbuff.h, to explain.
> 

In the case of skb_mac_gso_segment(), the problem can be observed
if the we receive a packet with multiple vlan tags that has
to go through the gso code (ex: GSO_DODGY is set).

In this case, mac_len will be adjust for the outer header, but
not the inner header.  skb_network_header will be pointing to
the wrong offset and GSO will fail causing retransmissions.

V2 of this patch has already been applied.  I can submit a clean-up
follow-on to rename the variable.

-vlad

>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>> index d855794..18b8c1b 100644
>> --- a/include/linux/netdevice.h
>> +++ b/include/linux/netdevice.h
>> @@ -3015,7 +3015,7 @@ struct sk_buff *skb_gso_segment(struct sk_buff *skb,
>> netdev_features_t features)
>>  {
>>  	return __skb_gso_segment(skb, features, true);
>>  }
>> -__be16 skb_network_protocol(struct sk_buff *skb);
>> +__be16 skb_network_protocol(struct sk_buff *skb, int *depth);
> 
> For me, eth_hdr_len or l2_hdr_len would be much clearer names than "depth".
> 
>>  static inline bool can_checksum_protocol(netdev_features_t features,
>>  					 __be16 protocol)
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index a98f7fa..49c41e6 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -2287,7 +2287,7 @@ out:
>>  }
>>  EXPORT_SYMBOL(skb_checksum_help);
>>
>> -__be16 skb_network_protocol(struct sk_buff *skb)
>> +__be16 skb_network_protocol(struct sk_buff *skb, int *depth)
>>  {
>>  	__be16 type = skb->protocol;
>>  	int vlan_depth = ETH_HLEN;
>> @@ -2314,6 +2314,9 @@ __be16 skb_network_protocol(struct sk_buff *skb)
>>  		vlan_depth += VLAN_HLEN;
>>  	}
>>
>> +	if (depth)
>> +		*depth = vlan_depth;
>> +
>>  	return type;
>>  }
> 
> Similarly here ...
> 
>> @@ -2327,12 +2330,13 @@ struct sk_buff *skb_mac_gso_segment(struct sk_buff
>> *skb,
>>  {
>>  	struct sk_buff *segs = ERR_PTR(-EPROTONOSUPPORT);
>>  	struct packet_offload *ptype;
>> -	__be16 type = skb_network_protocol(skb);
>> +	int vlan_depth = 0;
>> +	__be16 type = skb_network_protocol(skb, &vlan_depth);
>>
>>  	if (unlikely(!type))
>>  		return ERR_PTR(-EINVAL);
>>
>> -	__skb_pull(skb, skb->mac_len);
>> +	__skb_pull(skb, vlan_depth > skb->mac_len ? vlan_depth : skb->mac_len);
> 
> ... and here (obviously).
> 
> Regards,
> 	Neil
>