[gdm-list] /tmp and "GDM could not write to the authorization file"

[jdege <jdege jdege visi com>]

I'm running CentOS 5, with whichever version of GDM is current in that
distribution.

I've been trying to work out a reliable backup method, and I ran into an odd
problem.

I was using LVM snapshots, to get clean partition images, and using tar and
gzip, onto a removable HD.

After I untar'ed onto a clean disk, I'd get the message "GDM could not write
to the authorization file" when I tried to log in as an ordinary user. 
Console logins for ordinary users worked fine, and GDM logins for root
worked.  But ordinary user logins through GDM did not.

Google revealed that the cause of this was often a lack of disk space.  In
my case, none of the partitions had less than 40% free space.  A second
possibility was permissions or ownership.  I was pretty certain that running
tar with --preserve --xattrs would keep the permissions and ownership
correct.

But I still couldn't log in.  So I replaced tar with dump and restore. 
After a full level 0 backup - from read-only snapshots - and a full level 0
restore onto a newly formatted drive, when I booted off that new drive, I
got the same problem: "GDM could not write to the authorization file"

I had been keeping /tmp on a separate partition, and I'd not been including
it in my backups or restores.  But in an act of pure desperation, I added
/tmp to the backup.  And after the restore, I could log in fine.

So, my question.  What files or directories in /tmp is GDM relying upon,
when logging in an ordinary user?

And why, exactly, is it not reconstructing them, if they aren't present?  I
haven't got a problem with a program putting files into /tmp.  But if it's
depending upon their continuing to be there, from one boot to another, it's
broken.

I should not have to backup /tmp, to be able to restore a working system.  I
should be able to delete everything in /tmp, periodically, and still have a
working system.

The whole purpose of /tmp is to have a known place to keep transient files,
so that they don't get backed up, and so that they can be cleaned up on a
regular schedule.

Am I the only person to have run into this?

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