Re: netdevice notifier and device private data

[Alexander Aring <aring@mojatatu.com>]

Hi,

On Sat, Jun 09, 2018 at 03:01:18PM -0400, Michael Richardson wrote:
> 
> Alexander Aring <aring@mojatatu.com> wrote:
>     > Futhermore user space programs e.g. radvd will do 6lowpan specific
>     > handling on 6lowpan dev->type, it will not work either on tun
>     > devices.
> 
>     > I know that wpantund from NestLabs do this switch, I am very
>     > curious about the reason but I think they do it because the name
>     > is 6LoWPAN. But wpantund is just a SLIP like protocol with
>     > additional radio/foo commands.
> 
> How do they change it then, and what does it do?

They change it with the ioctl() of tun characte device, see [0].

What it does, it just changing the interface type to something else,
also there is no check at all that Linux has this interface type.

User space software e.g. radvd [1] will evaluate this type and doing
specific handling. Obviously changing it to 6LoWPAN and using this code
will confuse everything, because the handling makes only sense for a
6LoWPAN Linux interface which actually also use the 6LoWPAN subsystem.

They just using tun as all other to feed a IPv6 stack on a remote
microcontroller e.g. openthread, contiki, riot. via slip. (wpantund also
allow some radio, foo configuration).

> It totally seems like broken behaviour.  Maybe it's not even intentional.
> Maybe they are just foobar.
> 

They simple don't know what they doing... somebody thought 6LoWPAN need
to be 6LoWPAN, but they actually don't use the 6LoWPAN handling inside
the kernel. _Except_ they doing out of tree stuff which I don't believe.

According to [0] it also works with tun default (I suppsoe raw IPv6),
because ifdef. And they should not change it because they don't use
in-kernel 6LoWPAN functionality.

I really think that this tun/tap feature makes a lot of trouble for some
type changes. I probably introduce lowpan_dev pointer to netdevice and
then check if it's really a 6LoPWAN interface, a dev->type will not
garantuee anymore you have a 6LoWPAN interface. At least in user space
it's not possible to have a check if you really have a 6LoWPAN interface.

- Alex

[0] https://github.com/openthread/wpantund/blob/master/src/util/tunnel.c#L180
[1] https://github.com/reubenhwk/radvd/blob/master/device-linux.c#L75