How to exit a chroot inside a perl script? - linux

Question

While writing a perl script intended to fully automate the setup of virtual machines (Xen pv) I hit a small maybe very simple problem.
Using perl's chroot function I do my things on the guest file system and then I need to get back to my initial real root. How the hell I do that?
Script example:
`mount $disk_image $mount_point`;
chdir($mount_point);
chroot($mount_point);
#[Do my things...]
#<Exit chroot wanted here>
`umount $mount_point`;
#[Post install things...]
I've tried exit; but obviously that exit the whole script.
Searching for a way to exit the chroot I've found a number of scripts who aim to exit an already setup chroot (privilege escalation). Since I do the chroot here theses methods do not aplies.
Tried some crazy things like:
opendir REAL_ROOT, "/";
chdir($mount_point);
chroot($mount_point);
chdir(*REAL_ROOT);
But no go.
UPDATE
Some points to consider:
I can't split the script in multiple files. (Silly reasons, but really, I can't)
The chrooted part involve using a lot of data gathered earlier by the script (before the chroot), enforcing the need of not lunching another script inside the chroot.
Using open, system or backticks is not good, I need to run commands and based on the output (not the exit code, the actual output) do other things.
Steps after the chroot depends on what was done inside the chroot, hence I need to have all the variables I defined or changed while inside, outside.
Fork is possible, but I don't know a good way to handle correctly the passing of informations from and to the child.

Answer 1

The chrooted process() cannot "unchroot" itself by exiting (which would just exit).
You have to spawn a children process, which will chroot.
Something along the lines of the following should do the trick:
if (fork())
{
# parent
wait;
}
else
{
# children
chroot("/path/to/somewhere/");
# do some Perl stuff inside the chroot...
exit;
}
# The parent can continue it's stuff after his chrooted children did some others stuff...
It stills lacks of some error checking thought.

Answer 2

You can't undo a chroot() on a process - that's the whole point of the system call.
You need a second process (a child process) to do the work in the chrooted environment. Fork, and have the child undergo the chroot and do its stuff and exit, leaving the parent to do the cleanup.

Answer 3

Try spawning a child process that does the chroot, e.g. with system or fork depending on your needs, and waiting for the child to return the main program continues.

Answer 4

This looks like it might be promising:
Breaking Out of a Chroot Jail Using PERL

Answer 5

Save the original root as the current working directory or as a file descriptor:
chdir "/";
chroot "/mnt";
# Do something
chroot ".";
OR
open DIR, "<", "/";
chroot "/mnt";
# Do something
chdir DIR;
chroot ".";
close DIR;