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

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Date:   Thu, 8 Dec 2016 15:50:41 -0500
From:   Eric Garver <e@...g.me>
To:     Pravin Shelar <pshelar@....org>
Cc:     Jiri Benc <jbenc@...hat.com>, Jarno Rajahalme <jarno@....org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH v3 net-next 2/3] openvswitch: Use is_skb_forwardable()
 for length check.

On Sun, Dec 04, 2016 at 04:22:40PM -0800, Pravin Shelar wrote:
> 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.

Should we not also follow the "skbs are untagged" approach that the rest
of the kernel uses? I'm referring to patches 1 and 2 form Jiri's series
"openvswitch: make vlan handling consistent".

With those changes is_skb_forwardable() would behave as expected here.