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

From: Stanislav Fomichev <[email-masked]>
Date: Wed, 19 Jun 2019 11:39:38 -0700

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 <[email-masked]>
> ---
>  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.

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