On Fri, 2017-09-22 at 10:56 +0200, Sabrina Dubroca wrote:
> 2017-09-21, 08:02:18 -0700, Eric Dumazet wrote:
> > On Thu, 2017-09-21 at 21:32 +0800, Zhang Shengju wrote:
> > > Currently, any mtu value can be assigned when adding a new dummy device:
> > > [~]# ip link add name dummy1 mtu 100000 type dummy
> > > [~]# ip link show dummy1
> > > 15: dummy1: <BROADCAST,NOARP> mtu 100000 qdisc noop state DOWN mode
> > > DEFAULT group default qlen 1000
> > > link/ether 0a:61:6b:16:14:ce brd ff:ff:ff:ff:ff:ff
> > >
> > > This patch adds device MTU validation check.
> >
> > What is wrong with big MTU on dummy ?
>
> It looks like the "centralize MTU checking" series broke that, but
> only for changing the MTU on an existing dummy device. Commit
> a52ad514fdf3 defined min_mtu/max_mtu in ether_setup, which dummy uses,
> but there was no MTU check in dummy prior to that commit.
>
It looks like we accept big mtu on loopback, right ?
lpaa23:~# ifconfig lo mtu 100000
lpaa23:~# ifconfig lo
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:100000 Metric:1
RX packets:3823 errors:0 dropped:0 overruns:0 frame:0
TX packets:3823 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:759159 (759.1 KB) TX bytes:759159 (759.1 KB)
Also we accept very small MTU as well (although this automatically
removes IP addresses, as one would expect)
lpaa23:~# ifconfig lo mtu 50
lpaa23:~# ifconfig lo
lo Link encap:Local Loopback
UP LOOPBACK RUNNING MTU:50 Metric:1
RX packets:4052 errors:0 dropped:0 overruns:0 frame:0
TX packets:4052 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:806274 (806.2 KB) TX bytes:806274 (806.2 KB)
So, why dummy devices would not accept bit MTU ?
Do we have some fundamental assumption in the stack ?
If yes, we need to fix loopback urgently, it is more important than
dummy.
Thanks.
