linux-kernel - Re: BUG: Incorrect MTU on GRE device if remote is unspecified

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Date:   Wed, 27 Jan 2021 17:38:44 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Slava Bacherikov <mail@...va.cc>,
        Willem de Bruijn <willemb@...gle.com>,
        open list <linux-kernel@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Xie He <xie.he.0141@...il.com>
Subject: Re: BUG: Incorrect MTU on GRE device if remote is unspecified

On Wed, Jan 27, 2021 at 4:56 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 25 Jan 2021 22:10:10 +0200 Slava Bacherikov wrote:
> > Hi, I'd like to report a regression. Currently, if you create GRE
> > interface on the latest stable or LTS kernel (5.4 branch) with
> > unspecified remote destination it's MTU will be adjusted for header size
> > twice. For example:
> >
> > $ ip link add name test type gre local 127.0.0.32
> > $ ip link show test | grep mtu
> > 27: test@...E: <NOARP> mtu 1452 qdisc noop state DOWN mode DEFAULT group
> > default qlen 1000
> >
> > or with FOU
> >
> > $ ip link add name test2   type gre local 127.0.0.32 encap fou
> > encap-sport auto encap-dport 6666
> > $ ip link show test2 | grep mtu
> > 28: test2@...E: <NOARP> mtu 1436 qdisc noop state DOWN mode DEFAULT
> > group default qlen 1000
> >
> > The same happens with GUE too (MTU is 1428 instead of 1464).
> > As you can see that MTU in first case is 1452 (1500 - 24 - 24) and with
> > FOU it's 1436 (1500 - 32 - 32), GUE 1428 (1500 - 36 - 36). If remote
> > address is specified MTU is correct.
> >
> > This regression caused by fdafed459998e2be0e877e6189b24cb7a0183224 commit.
>
> Cong is this one on your radar?

Yeah, I guess ipgre_link_update() somehow gets called twice,
but I will need to look into it.

Thanks.