lightdm whitelist support for specifying list of users to be SHOWN in greeter

[Rob]

Referred from thread: http://lists.freedesktop.org/archives/lightdm/2015-July/000830.html

 > If you think the feature should be provided by LightDM please file a bug
 > here: https://bugs.launchpad.net/lightdm

I think that's a very helpful feature, one quite familiar for a long while to existing KDE KDM users, at least, and since lightdm already

 ADDS all users, minus a user specified BLACKLIST

the alternative of using a WHTELIST, if specified, does not seem conceptually much of a stretch at all, and it seems like most of the code pieces shold be 'in there' anyway.

> I'm upgrading to KDE 'Next Gen' (plasma5 + KF5* + etc ).
>
> With this change, the default greeter switches from KDM to SDDM; tho, KDM
> still works -- for now.
>
> KDM's had the option -- for ages -- to specify a list of
> non-continugous-UID users to show in the greeter.
>
> So, e.g.
>
> ShowUsers=0,1000,1001
>
> or
>
> ShowUsers=1000,1006
>
> Afaict, lightdm's and sddm's greeters only provide an option to specify a
> list of users to HIDE.
>
> I raised the issue @ #sddm of whether the ShowUsers= option exists, or
> could be added. Ended up with a debate as to why the option to specify
> users is needed, and that it's not really the users choice ... No thanks
> to that.
>
> In #kde I was told that lightdm is a more flexible option, works with KDE,
> but was speculated that it does not -- in the currently available greeters
> -- have the ShowUsers= option.
>
> Could this feature just be added, and made available to all the greeters?

The LightDM daemon only provides a hint to the greeters if users should be
displayed or not. It is up to each greeter to decide if wants to show a
subset of users. Unity Greeter provides an option to hide users. The GTK+
greeter doesn't provide an option. I'm not sure what the KDE greeters
provide.

We could add a user whitelist to the LightDM configuration and let the
greeters use that or you could just implement it in each greeter as
appropriate. Note that LightDM just provides hints to each greeter so the
feature would still need to be implemented in each greeter (though we would
probably add a helper function in liblightdm).

[Robert Ancell (robert-ancell)]

Note lightdm doesn't currently specify a blacklist, just if a user list should be shown (if not a greeter should require usernames to be entered manually).

[Robert Ancell (robert-ancell)]

Could you give a use case for this?

[Rob (robert-devanna)]

Sure.

The use case is from above ... 2 parts

(1) KDM users have had this capability for a LONG time.
(2) I want my greeter to ONLY show

  ShowUsers=0,1000,1001

(e.g., me-as-myuser, me-as-root, and my admin assistant)

or

  ShowUsers=1000,1006

I.e., NO other users displayed -- JUST the ones specified.

What users I wish to display, and why is really quite irrelevant -- and varies in argument from security to convenience, to lack of clutter, and likely points in between.

I'm rather befuddled why a use case apparently exists for

(1) showing ALL users
(2) showing NO users
(3) showing ALL users - a BLACKLIST

as all of those features are already there.

but additional rationale/justification for the

(4) show NO users + a WHITELIST

seems to be needed.

IMO, i.e. my admittedly own $0.02, NONE of (1) - (3) are realistic use cases, whereas (4) is much more typical -- and is the way all 500+ of our machines have been configured for years.