netdev - Re: Problem with queuing vlan tagged packets after migration from 3.16.0 to 4.9.0

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Date:   Wed, 2 Jan 2019 19:30:53 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Bartek Kois <bartek.kois@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Problem with queuing vlan tagged packets after migration from
 3.16.0 to 4.9.0

On Tue, Jan 1, 2019 at 11:46 AM Bartek Kois <bartek.kois@...il.com> wrote:
>
> Hi
> Yes it did work since I remember (like around 2.4.x) and it changed
> since I moved from Debian 8 to 9. I would appreciate fixing that in the
> future beacuse it is essential for queueing traffic on the routers, but
> the question is why these filters don`t work in that case:
>
>      tc filter add dev $LAN_ETH parent 1:0 protocol ip prio 4 u32 match
> u32 0x0a000c08 0xffffffff at 20 classid 1:2001      # for 10.0.12.8 ip
> address
>      tc filter add dev $LAN_ETH parent 1:0 protocol ip prio 4 u32 match
> u32 0x0a000c09 0xffffffff at 20 classid 1:2002      # for 10.0.12.9 ip
> address
>      tc filter add dev $LAN_ETH parent 1:0 protocol ip prio 4 u32 match
> u32 0x0a000c10 0xffffffff at 20 classid 1:2003      # for 10.0.12.10 ip
> address
>
> I`ve changed "at 16" which works without vlan tags to "at 20" to take
> vlan tag into account.

Yeah, this confirms my speculation.

The problem is essentially a design flaw of u32 filter, the IP header
and TCP header offsets are never fixed, for example VLAN tagging and
IP options. What's more, it is not easy for user-space to learn the offset
for different packets as it requires to parse into each packets.

I don't know whether we can fix this either, VLAN call path probably
already makes assumptions on the current skb->data position, if
we "fix" it for u32, it would probably break other things.