netdev - Re: [PATCH] net: Fix missing meta data in skb with vlan packet

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Date:   Mon, 15 Apr 2019 11:57:26 +0200
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
Cc:     Yuya Kusakabe <yuya.kusakabe@...il.com>, davem@...emloft.net,
        taketarou2@...il.com, netdev@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH] net: Fix missing meta data in skb with vlan packet

On Mon, 15 Apr 2019 15:05:36 +0900
Toshiaki Makita <makita.toshiaki@....ntt.co.jp> wrote:

> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index ef2cd5712098..6bc663249c4c 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
[...]
> > @@ -5095,6 +5096,13 @@ static struct sk_buff
> > *skb_reorder_vlan_header(struct sk_buff *skb)
> >                 memmove(skb_mac_header(skb) + VLAN_HLEN, skb_mac_header(skb),
> >                         mac_len - VLAN_HLEN - ETH_TLEN);
> >         }
> > +
> > +       meta_len = skb_metadata_len(skb);
> > +       if (meta_len) {  
> 
> Since this is not used by non-XDP skb and skb path is slow-path for XDP
> anyway, should add unlikely here in favor of non-XDP case?

I want to stress, that XDP is meant to cooperate with network stack. 
The XDP metadata is meant as a communication channel between XDP and
network-stack SKBs.  

Thus, there are use-cases where this is not considered a slow-path.
That said I don't care if this is marked unlikely(), as not many people
are using this metadata communication channel yet.

In one customer use-case I have seen, they wanted to pop VLAN tags
(Q-in-Q) at XDP layer, but keep some of the info in metadata for TC-bpf
hook. I don't think they kept/left any VLAN headers in the SKB.


> > +               meta = skb_metadata_end(skb) - meta_len;
> > +               memmove(meta + VLAN_HLEN, meta, meta_len);
> > +       };
> > +



-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer