[Bug 1707645] Re: system with high numbered uids has huge sparse /var/log/lastlog

[Oliver O'Boyle]

This is affecting Ubuntu 20.04 with sssd integration to AD. I am seeing
massive lastlog files because of what appears to be the use of very high
UID's being assigned to AD users when they log in.

-rw-rw-r--   1 root      utmp            88943346292 Jan  6 16:50
lastlog

Oliver

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Title:
  system with high numbered uids has huge sparse /var/log/lastlog

Status in pam package in Ubuntu:
  Confirmed
Status in shadow package in Ubuntu:
  Confirmed
Status in util-linux package in Ubuntu:
  Confirmed

Bug description:
  I was investigating the use of a single high UID user (ie, 2000000000)
  and discovered that /var/log/lastlog grew to an enormously large
  sparse file:

  $ ls -lh /var/log/lastlog 
  -rw-rw-r-- 1 root utmp 544G Jul 27 12:35 /var/log/lastlog

  The file is actually quite small though:
  $ ls -lh --size /var/log/lastlog 
  56K -rw-rw-r-- 1 root utmp 544G Jul 27 12:35 /var/log/lastlog

  On a standalone system, this is possibly not a problem since it is
  highly unlikely that a system will have 2 billion users, but this is
  confirmed to wreak havoc on rsync backups.

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