netdev - Re: [PATCH v3 net-next 2/3] openvswitch: Use is_skb

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Date:   Sun, 4 Dec 2016 16:22:40 -0800
From:   Pravin Shelar <pshelar@....org>
To:     Jiri Benc <jbenc@...hat.com>
Cc:     Jarno Rajahalme <jarno@....org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Eric Garver <e@...g.me>
Subject: Re: [PATCH v3 net-next 2/3] openvswitch: Use is_skb_forwardable() for
 length check.

On Fri, Dec 2, 2016 at 1:25 AM, Jiri Benc <jbenc@...hat.com> wrote:
> On Thu, 1 Dec 2016 11:50:00 -0800, Pravin Shelar wrote:
>> This is not changing any behavior compared to current OVS vlan checks.
>> Single vlan header is not considered for MTU check.
>
> It is changing it.
>
> Consider the case when there's an interface with MTU 1500 forwarding to
> an interface with MTU 1496. Obviously, full-sized vlan frames
> ingressing on the first interface are not forwardable to the second
> one. Yet, if the vlan tag is accelerated (and thus not counted in
> skb->len), is_skb_forwardable happily returns true because of the check
>
>         len = dev->mtu + dev->hard_header_len + VLAN_HLEN;
>         if (skb->len <= len)
>
ok, This case would be allowed due to this patch. But core linux stack
and bridge is using this check then why not just use same forwarding
check in OVS too, this make it consistent with core networking
forwarding expectations.