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[John Bramley <877766@xxxxxxxxxxxxxxxxxx>]

I have the same problem: logging in as a user with NFS home directory
fails

permissions on users home directories: rwx------  (default as created by useradd)
NFS (v3) filesystem is not exported with root permission for client machine (i.e. no 'no_root_squash' option)

User enters username and password on login screen and then screen goes
black and login window reappears.

Giving root access on the users home directory allows logins to work:

    chmod o+x <user home directory>

or if ACLs enabled for the filesystem:
    setfacl -m user:65534:x  <user home directory>

without changes /var/log/lightdm/lightdm.log shows:
[+16.20s] WARNING: Failed to change to home directory /npdisks/home/jb: Permission denied

relevant lines in lightdm.log:
[+15.91s] DEBUG: Authenticate result for user jb: Success
[+15.91s] DEBUG: User jb authorized
[+15.91s] DEBUG: Wrote 24 bytes to greeter
[+15.95s] DEBUG: Read 8 bytes from greeter
[+15.95s] DEBUG: Read 10 bytes from greeter
[+15.95s] DEBUG: Greeter requests session ubuntu
[+15.95s] DEBUG: Stopping greeter
[+15.95s] DEBUG: Dropping privileges to uid 106
[+15.95s] DEBUG: Removing session authority from /var/lib/lightdm/.Xauthority
[+15.99s] DEBUG: Restoring privileges
[+15.99s] DEBUG: Sending signal 15 to process 7053
[+16.00s] DEBUG: Process 7053 exited with return value 0
[+16.00s] DEBUG: pam_close_session(0xc5a840) -> 0 (Success)
[+16.00s] DEBUG: pam_setcred(0xc5a840, PAM_DELETE_CRED) -> 0 (Success)
[+16.00s] DEBUG: pam_end(0xc5a840) -> 0
[+16.00s] DEBUG: Ending ConsoleKit session 137061b491bb03a23bfe54c90000029c-1319111371.735096-2139365219
[+16.09s] DEBUG: Greeter quit
[+16.09s] DEBUG: Starting user session
[+16.11s] DEBUG: Dropping privileges to uid 6057
[+16.11s] DEBUG: Writing /npdisks/home/jb/.dmrc
[+16.11s] DEBUG: Restoring privileges
[+16.15s] DEBUG: Starting session ubuntu as user logging to /npdisks/home/jb/.xsession-errors
[+16.15s] DEBUG: Launching session
[+16.15s] DEBUG: pam_set_item(0xc6a9f0, 3, ":0") -> 0 (Success)
[+16.16s] DEBUG: pam_open_session(0xc6a9f0, 0) -> 0 (Success)
[+16.19s] DEBUG: Opened ConsoleKit session 137061b491bb03a23bfe54c90000029c-1319111387.433106-1241632904
[+16.19s] DEBUG: Dropping privileges to uid 6057
[+16.19s] DEBUG: Adding session authority to /npdisks/home/jb/.Xauthority
[+16.20s] DEBUG: Restoring privileges
[+16.20s] DEBUG: Launching process 7148: /usr/sbin/lightdm-session 'gnome-session --session=ubuntu'
[+16.20s] WARNING: Failed to change to home directory /npdisks/home/jb: Permission denied
[+16.20s] DEBUG: Registering session with bus path /org/freedesktop/DisplayManager/Session0
[+16.20s] DEBUG: Process 7148 exited with return value 1
[+16.21s] DEBUG: pam_close_session(0xc6a9f0) -> 0 (Success)
[+16.21s] DEBUG: pam_setcred(0xc6a9f0, PAM_DELETE_CRED) -> 0 (Success)
[+16.21s] DEBUG: pam_end(0xc6a9f0) -> 0
[+16.21s] DEBUG: Ending ConsoleKit session 137061b491bb03a23bfe54c90000029c-1319111387.433106-1241632904
[+16.24s] DEBUG: User session quit

having a brief look at the source for lightdm, src/session.c :

    /* Change working directory */
    if (chdir (user_get_home_directory (user)) != 0)
    {
        g_warning ("Failed to change to home directory %s: %s", user_get_home_directory (user), strerror (errno));
        _exit (EXIT_FAILURE);
    }

    /* Change to this user */
    if (getuid () == 0)
    {
        if (initgroups (user_get_name (user), user_get_gid (user)) < 0)
        {
            g_warning ("Failed to initialize supplementary groups for %s: %s", user_get_name (user), strerror (errno));
            _exit (EXIT_FAILURE);
        }

        if (setgid (user_get_gid (user)) != 0)
        {
            g_warning ("Failed to set group ID to %d: %s", user_get_gid (user), strerror (errno));
            _exit (EXIT_FAILURE);
        }

        if (setuid (user_get_uid (user)) != 0)
        {
            g_warning ("Failed to set user ID to %d: %s", user_get_uid (user), strerror (errno));
            _exit (EXIT_FAILURE);
        }
    }

I think it should be a simple case of moving the section:

    /* Change working directory */
    if (chdir (user_get_home_directory (user)) != 0)
    {
        g_warning ("Failed to change to home directory %s: %s", user_get_home_directory (user), strerror (errno));
        _exit (EXIT_FAILURE);
    }

to after the /* Change to this user */ section.
i.e.:

.
.
       if (setuid (user_get_uid (user)) != 0)
        {
            g_warning ("Failed to set user ID to %d: %s", user_get_uid (user), strerror (errno));
            _exit (EXIT_FAILURE);
        }
    }

    /* Change working directory */
    if (chdir (user_get_home_directory (user)) != 0)
    {
        g_warning ("Failed to change to home directory %s: %s", user_get_home_directory (user), strerror (errno));
        _exit (EXIT_FAILURE);
    }

-- 
You received this bug notification because you are a member of Desktop
Packages, which is subscribed to lightdm in Ubuntu.
https://bugs.launchpad.net/bugs/877766

Title:
  lightdm login fails with NFS home

Status in “lightdm” package in Ubuntu:
  New

Bug description:
  Lightdm appears not to be able to log in a user if all of the
  following are true:

  1.  They've got an NFS-mounted home directory
  2.  NFS is configured in the usual manner; ie, root privileges do not allow access to arbitrary remote files
  3.  The user's home directory permissions are such that "other" cannot access the directory (ie, mode 0700)

  In this situation, the password is accepted and the screen clears.
  Rather than bringing up the desktop as expected, a couple of seconds
  pass and we're returned to the lightdm screen.  This yielded a message
  in one of the /var/log/lightdm logs which I've just discovered was
  overwritten by subsequent successful logins.  I'll recreate this and
  update the bug tonight.

  Workaround was to chmod the user's homedir to 0711 which allowed
  lightdm to work properly.

  Release is Ubuntu 11.10 with all updates as of 17 Oct 2011 at roughly
  10pm EDT.  Lightdm package is 1.0.1-0ubuntu6.

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