netdev - Re: [Bridge] VLAN tags in mac

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Date:   Fri, 28 Jun 2019 13:02:22 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Toshiaki Makita <toshiaki.makita1@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        Jiri Pirko <jiri@...lanox.com>,
        Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
        netdev@...r.kernel.org, roopa@...ulusnetworks.com,
        bridge@...ts.linux-foundation.org,
        Zahari Doychev <zahari.doychev@...ux.com>, jhs@...atatu.com,
        Simon Horman <simon.horman@...ronome.com>,
        David Ahern <dsahern@...il.com>,
        Cong Wang <xiyou.wangcong@...il.com>
Subject: Re: [Bridge] VLAN tags in mac_len

On Mon, 2019-06-17 at 20:15 +0900, Toshiaki Makita wrote:

> I'll try to explain the problem I see, which cannot be fixed by option 1...
> The bug is in tcf_vlan_act(), and mainly in skb->data, not in mac_len.
> 
> Consider about vlan packets from NIC, but non-hw-accelerated, where
> vlan devices are configured to receive them.
> 
> When __netif_receive_skb_core() is called, skb is like this.
> 
> +-----+------+--------
> > eth | vlan | TCP/IP
> 
> +-----+------+--------
>        ^
>       data
> 
> skb->data is at the beginning of the vlan header.

Right.

> This is reasonable because we did not process the vlan tag at this point.

I think with this simple sentence you just threw a whole new semantic
issue into the mix, one that I at least hadn't considered.

However, it's not clear to me whether we should consider a tag as
processed or not when we push it.

In a sense, this means we should have two different VLAN tag push
options - considering it processed or unprocessed. Or maybe it should
always be considered unprocessed, but that's not what we do today.

> Then after vlan_do_receive() (receive the skb on a vlan device), the skb is like this.
> 
> +-----+--------
> > eth | TCP/IP
> 
> +-----+--------
>        ^
>       data
> 
> Or if reorder_hdr is off (which does not remove vlan tags when receiving on vlan devices),
> 
> +-----+------+--------
> > eth | vlan | TCP/IP
> 
> +-----+------+--------
>               ^
>              data
> 
> Relying on this mechanism, we are currently able to handle multiple vlan tags.
> 
> For example if we have 2 tags,
> 
> - On __netif_receive_skb_core() invocation
> 
> +-----+------+------+--------
> > eth | vlan | vlan | TCP/IP
> 
> +-----+------+------+--------
>        ^
>       data
> 
> - After first vlan_do_receive()
> 
> +-----+------+--------
> > eth | vlan | TCP/IP
> 
> +-----+------+--------
>        ^
>       data
> 
> Or if reorder_hdr is off,
> 
> +-----+------+------+--------
> > eth | vlan | vlan | TCP/IP
> 
> +-----+------+------+--------
>               ^
>              data
> 
> When we process one tag, the data goes forward by one tag.

Right, that's a very good point.

> Now looking at TC vlan case...
> 
> After it inserts two tags, the skb looks like:
> 
> (The first tag is in vlan_tci)
> +-----+------+--------
> > eth | vlan | TCP/IP
> 
> +-----+------+--------
>               ^
>              data
> 
> The data pointer went forward before we process it.
> This is apparently wrong. I think we don't want to (or cannot?) handle cases like this
> after tcf_vlan_act(). This is why I said we should remember mac_len there.

Right, makes a lot of sense.

If you consider a tc VLAN pop, you'd argue that it should pop the next
unprocessed tag I guess, since if it was processed then it doesn't
really exist any more (semantically, you still see it if reorder_hdr is
off), right?

> So, my opinion is:
> On ingress, data pointer can be at the end of vlan header and mac_len probably should
> include vlan tag length, but only after the vlan tag is processed.

You're basically arguing for option (3), I think, making VLAN push/pop
not manipulate mac_len since they can just push/pop *unprocessed* tags,
right?

I fear this will cause all kinds of trouble in other code. Perhaps we
need to make this processed/unprocessed state more explicit.

> Bridge may need to handle mac_len that is not equal to ETH_HLEN but to me it's a
> different problem.

Yes. Like I just said to Daniel, I think we should make bridge handle
mac_len so that we can just exclude it from this whole discussion.
Regardless of the mac_len and processed/unprocessed tags, it would just
work as expected.

johannes