Re: [PATCH] ppp: Add rtnl attribute IFLA_PPP_UNIT_ID for specifying ppp unit id

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On Thu, Aug 12, 2021 at 04:09:18PM +0200, Pali Rohár wrote:
> The problem is that ppp from rtnl is of the same class as ppp from
> ioctl. And if you want to use ppp, you still have to use lot of ioctl
> calls as rtnl does not implement them. And these ioctl calls use ppp
> unit id, not interface id / interface name.

Indeed, the netlink api only replaces ioctl(PPPIOCNEWUNIT). It's
technically feasible to implement other ppp unit ioctls with netlink,
but I didn't do that because:
  * some of them make no sense,
  * netlink wouldn't bring any advantage over ioctl() for these cases
    (and ppp is ioctl-centric anyway, so users will have to write
    ioctl() calls no matter what).

If there was a bright future for ppp in sight, I'd certainly work on a
new modern ioctl-less api. But in the current situation, that'd be just
feature duplication and code churn.

> So in the end you can use RTM_NEWLINK and then control ppp via ioctls.
> And for controlling you have to known that ppp unit id.
> If you are using ppp over serial devices, you can "simplify" it by
> forcing mapping that serial number device matches ppp unit id. And then
> you do not have to use dynamic ids (and need for call PPPIOCGUNIT).

How is dropping a single PPPIOCGUNIT call going to simplify the code
while you have to write netlink message handlers?

> With dynamic unit id allocation (which is currently the only option when
> creating ppp via rtnl) for single ppp connection you need to know:
> * id of serial tty device
> * id of channel bound to tty device
> * id of network interface
> * id of ppp unit bound to network interface

I see you're not working with L2TPv2 :). A few more things you'd need
to add to the list:
 * a UDP socket,
 * a tunnel id,
 * a session id,
 * a tunnel file descriptor,
 * a session file descriptor,
 * a new ioctl() to figure out which channel id is assigned to your
   L2TP session,
 * a bunch of setsockopt() to configure the whole thing,
 * ...
(and I'm not counting the ioctl() calls necessary to set up the channel,
which also apply to your use case).

Really, I'm sorry, but the possibility to drop a single PPPIOCGUNIT
call isn't going to simplify the ppp landscape.

> > As I already proposed, we can add an attribute to make the interface
> > name independant from the unit id.
> I agree, that above proposal with a new attribute which makes interface
> name independent from the ppp unit id is a good idea. Probably it should
> have been default rtnl behavior (but now it is too late for changing
> default behavior).

Well, it was the default, until a collegue complained and I accepted to
align the default device name with the original ioctl() behaviour. But
the beauty of netlink is that we can revise this behaviour without
breaking compatibility.

> But prior adding this attribute, we first need a way how to retrieve
> interface name of newly created interface. Which we agreed that

Yes, that'd be ideal. That might require quite some work though (I
haven't looked in detail). At last resort, if adding NLM_F_ECHO
support to rtnl proves too hard, it might be possible to add a call to
get the device name associated to a ppp unit file descriptor.

At least know I understand why we had this conversation about

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