[Bug 1728055] Re: GDM3 hangs, if local home directory is not accessible

[Michal Kukuča]

We're using SSSD. LDAP does not use authentication for search.

In the mean time we already upgraded to 18.04 and I'm not sure if the
bug persists (but will try it again and post the results here - the
workaround is still in place). The issue seems to be with the home
directory being mounted from the remote host and GDM and other (local)
processes trying to read from that directory with root access. That
cannot work by default, as NFS mounts with the local root are mapped to
nfs_nobody. In order to circumvent this I tried setting the
no_root_squash option in the exports file, which should in theory enable
local root processes to access /home normally. Well - that ended with
another bug filed with nfs-utils, as no_root_squash does not work (it
didn't work with 17.10 - that was a known issue with that particular nfs
server version, but it seems to be still there with 18.04). I still have
one idea that I will try this weekend, but this has become a nice big
rabbit hole for me.

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https://bugs.launchpad.net/bugs/1728055

Title:
  GDM3 hangs, if local home directory is not accessible

Status in autofs package in Ubuntu:
  Incomplete

Bug description:
  In Ubuntu 17.10 with GDM 3.26.1-3ubuntu3, the greeter will hang, if
  local /home directory is not accessible.

  Our setup:
  - Ubuntu Server (16.04) with DNS, LDAP, Kerberos 5 and NFS servers. The home directories are present on the server and made available via NFS with krb5 authentication. LDAP provides the user data for clients.
  - Ubuntu client (17.10), bound to the server, uses AutoFS to automount remote home directories under /home. This makes the otherwise present local user directories unaccessible.

  Expected behaviour: After system startup, the greeter should provide a way for users to log in.
  Actual behaviour: The system hangs before the greeter displays the user list (but it does display the top menu bar and Ubuntu logo at the bottom).

  Additional remarks:
  If the settings for local users under /var/lib/AccountsService/users contain SystemAccount=true, the greeter will work as expected (while not displaying local users). This is a workaround, that I'm using right now. BUT: if the network user will log in and invoke the authentication dialog for system-wide settings (e.g. if he will try to add a new printer, or change the system update settings), the system will hang before displaying the expected dialog window. Also if a local user will try to log in by entering his login name and password, the system will hang (it should either produce an error message, or log in without a home directory).

  AutoFS uses this line to mount the remote home directories under /home:
  * -fstype=nfs4,rw,intr,sec=krb5 FQDN:/home/&

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