netdev - Re: [PATCH v3 4/5] net: tun: fix tun_xdp_one() for IFF

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Date:   Fri, 25 Jun 2021 20:00:54 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     netdev@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        Eugenio Pérez <eperezma@...hat.com>
Subject: Re: [PATCH v3 4/5] net: tun: fix tun_xdp_one() for IFF_TUN mode

On Fri, 2021-06-25 at 14:43 -0400, Willem de Bruijn wrote:
> On Thu, Jun 24, 2021 at 8:30 AM David Woodhouse <dwmw2@...radead.org> wrote:
> > 
> > From: David Woodhouse <dwmw@...zon.co.uk>
> > 
> > In tun_get_user(), skb->protocol is either taken from the tun_pi header
> > or inferred from the first byte of the packet in IFF_TUN mode, while
> > eth_type_trans() is called only in the IFF_TAP mode where the payload
> > is expected to be an Ethernet frame.
> > 
> > The equivalent code path in tun_xdp_one() was unconditionally using
> > eth_type_trans(), which is the wrong thing to do in IFF_TUN mode and
> > corrupts packets.
> > 
> > Pull the logic out to a separate tun_skb_set_protocol() function, and
> > call it from both tun_get_user() and tun_xdp_one().
> 
> I think this should be two patches. The support for parsing pi is an
> independent fix.

As things stand, this patch *doesn't* change the pskb_may_pull()
behaviour. I think we should make that unconditional, and can indeed do
so in a separate subsequent patch.

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