at/batch broken

[YONETANI Tomokazu]

Hello.
Original issue has been posted to kernel@
  http://leaf.dragonflybsd.org/mailarchive/kernel/2006-09/msg00028.html
but since the thread has turned into how you use macro or inline functions,
I'm going to raise this topic on bugs@ again :-)

The problem: at or batch command fails to accept a new schedule
because it fails to create a lock file /var/at/jobs/.lockfile.
The bug exists in 1.4/1.6 releases too.  It's not the permission or
configuration file:
  $ su
  # cat /var/at/at.allow
  qhwt
  # ls -l /usr/bin/at
  -r-sr-xr-x  4 root  wheel  19316 Jan  3 21:02 /usr/bin/at
  # exit
  $ id
  uid=1000(qhwt) gid=1000(qhwt) groups=1000(qhwt), 0(wheel)
  $ at 1:00 echo
  at: cannot open lockfile /var/at/jobs/.lockfile: Permission denied

The root cause is, as I posted in that thread, second call to setreuid()
fails if you don't close a file descriptor you opened right after the
first call to setreuid(real_uid, effective_uid) with the same argument
fails, whether you close the descriptor or not.  I'm attaching the sample
code again.  It fails on DragonFly, but succeeds on FreeBSD.  I looked
around in the kernel code but I couldn't find any description that it's
a security feature specific to DragonFly.

Regards.