On Wed, Aug 11, 2021 at 08:04:01PM +0200, Pali Rohár wrote: > On Wednesday 11 August 2021 19:38:11 Guillaume Nault wrote: > > On Tue, Aug 10, 2021 at 02:11:11PM -0400, James Carlson wrote: > > > On 8/10/21 1:16 PM, Pali Rohár wrote: > > > > On Tuesday 10 August 2021 16:38:32 Chris Fowler wrote: > > > > > Isn't the UNIT ID the interface number? As in 'unit 100' will give me ppp100? > > > > > > > > If you do not specify pppd 'ifname' argument then pppd argument 'unit 100' > > > > will cause that interface name would be ppp100. > > > > > > > > But you are free to rename interface to any string which you like, even > > > > to "ppp99". > > > > > > > > But this ppp unit id is not interface number. Interface number is > > > > another number which has nothing with ppp unit id and is assigned to > > > > every network interface (even loopback). You can see them as the first > > > > number in 'ip -o l' output. Or you can retrieve it via if_nametoindex() > > > > function in C. > > > > > > Correct; completely unrelated to the notion of "interface index." > > > > > > > ... So if people are really using pppd's 'unit' argument then I think it > > > > really make sense to support it also in new rtnl interface. > > > > > > The pppd source base is old. It dates to the mid-80's. So it predates not > > > just rename-able interfaces in Linux but Linux itself. > > > > > > I recall supported platforms in the past (BSD-derived) that didn't support > > > allowing the user to specify the unit number. In general, on those > > > platforms, the option was accepted and just ignored, and there were either > > > release notes or man page updates (on that platform) that indicated that > > > "unit N" wouldn't work there. > > > > > > Are there users on Linux who make use of the "unit" option and who would > > > mourn its loss? Nobody really knows. It's an ancient feature that was > > > originally intended to deal with systems that couldn't rename interfaces > > > (where one had to make sure that the actual interface selected matched up > > > with pre-configured filtering rules or static routes or the like), and to > > > make life nice for administrators (e.g., making sure that serial port 1 maps > > > to ppp1, port 2 is ppp2, and so on). > > > > > > I would think and hope most users reach for the more-flexible "ifname" > > > option first, but I certainly can't guarantee it. It could be buried in a > > > script somewhere or (god forbid) some kind of GUI or "usability" tool. > > > > > > If I were back at Sun, I'd probably call it suitable only for a "Major" > > > release, as it removes a publicly documented feature. But I don't know what > > > the considerations are here. Maybe it's just a "don't really care." > > > > I'm pretty sure someone, somewhere, would hate us if we broke the > > "unit" option. The old PPP ioctl API has been there for so long, > > there certainly remains tons of old tools, scripts and config files > > that "just work" without anybody left to debug or upgrade them. > > > > We can't just say, "starting from kernel x.y.z the unit option is a > > noop, use ifname instead" as affected people surely won't get the > > message (and there are other tools beyond pppd that may use this > > kernel API). > > > > But for the netlink API, we don't have to repeat the same mistake. > > ifname is not atomic (first it creates ppp<id> interface and later it is > renamed) and have issues. Due to bug described here: > https://lore.kernel.org/netdev/20210807160050.17687-1-pali@xxxxxxxxxx/ > you may get your kernel into state in which it is not possible to create > a new ppp interface. And this issue does not happen when using "unit" > argument. This is specific to the ioctl api. Netlink doesn't have this problem. > To fix above issue it is needed to migrate pppd from ioctl API to rtnl. It would have helped a lot if you had explained that before. > But this would be possible only after rtnl API starts providing all > features, including specifying custom "unit" argument... You can already simulate the "unit" option by setting the interface name as "ppp${unit}" and retrieving the kernel assigned id with ioctl(PPPIOCGUNIT). What's wrong with that? > I hit above problem, so now I'm migrating all pppd setups from "ifname" > to "unit" option. Why did you write 3125f26c51482 ("ppp: Fix generating ppp unit id when ifname is not specified") then?
