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.