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

[Vadim Fedorenko]

On 28.01.2021 01:38, Cong Wang wrote:
> On Wed, Jan 27, 2021 at 4:56 PM Jakub Kicinski <kuba@xxxxxxxxxx> 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@NONE: <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@NONE: <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.

Hi!
The problem is in ip_tunnel_bind_dev() where mtu is set for tunnel device.

  	if (tdev) {
		hlen = tdev->hard_header_len + tdev->needed_headroom;
		mtu = min(tdev->mtu, IP_MAX_MTU);
	}

	dev->needed_headroom = t_hlen + hlen;
	mtu -= (dev->hard_header_len + t_hlen);

ipgre_tunnel_init sets hard_header_len to tunnel->hlen + sizeof(*iph) and
ip_tunnel_bind_dev adds header overhead once again.

I'll post a patch a bit later but I need someone with extended tests.

Thanks