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

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Message-ID: <20210127165602.610b10c0@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 27 Jan 2021 16:56:02 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     xiyou.wangcong@...il.com
Cc:     Slava Bacherikov <mail@...va.cc>, willemb@...gle.com,
        open list <linux-kernel@...r.kernel.org>,
        netdev@...r.kernel.org
Subject: Re: BUG: Incorrect MTU on GRE device if remote is unspecified

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?