undefined reference to `ioctl_tty'

[valdis.kletnieks at vt.edu]

On Wed, 06 Mar 2019 01:58:41 -0500, Jeffrey Walton said:

>        TIOCEXCL  void
>               Put the terminal into exclusive mode.  No further open(2)
>               operations on the terminal are permitted.  (They fail with
>               EBUSY, except for a process with the CAP_SYS_ADMIN
>               capability.)

Note that this is still slightly racy - if two processes both try to do an open
and an ioctl, this can happen:

Process 1	open()
		Process 2 open()
Process 1 ioctl(TIOEXCL)
		Process 2 ioctl()

But at this point, the second process has already succeeded in opening the
device, so "no further opens" doesn't save you.

Usually, this sort of race would be really hard to hit, especially if the
ioctl() happens right after the open() - except that many systems will use cron
or similar to launch stuff "once per hour" or similar.  And they end up
batching together two processes that each try to do this. Suddenly, it's a lot
easier to hit that hole when cron launches two processes at 12:54:17 PM.....

If there's one thing I've learned from almost 4 decades of sysadmin'ing on all
sorts of systems, saying "Oh that timing hole is so tiny it won't ever get hit
in production" guarantees that the Clock Gods will smite you for your hubris.

:)