Undoing chroot?

Undoing chroot?

[Paul Powell]

Hello,

I have a linux bootable CD which executes a custom
init.  The job of init is to figure out on which
device the CD is located.  After finding the CD, init
mounts the device and executes a CHROOT to set the
root directory to the CD.

After I'm done I'd like to umount the CD and then
eject it by sending an IOCTL eject command.  But since
I executed a CHROOT I can't umount the CD, umount
complains that the device is busy.

So how do you reverse a CHROOT?

BTW, I use an initrd image and init is a C program,
not a script.

Thanks

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Re: Undoing chroot?

[alex]

On Fri, Jan 26, 2001 at 10:31:40AM -0800, Paul Powell wrote:
> I have a linux bootable CD which executes a custom
> init.  The job of init is to figure out on which
> device the CD is located.  After finding the CD, init
> mounts the device and executes a CHROOT to set the
> root directory to the CD.
> 
> After I'm done I'd like to umount the CD and then
> eject it by sending an IOCTL eject command.  But since
> I executed a CHROOT I can't umount the CD, umount
> complains that the device is busy.
> 
> So how do you reverse a CHROOT?

By definition, a chroot() is not undoable.  There are strong security reasons 
for this (chroot() is often used to create protected "jails" for untrusted 
code.  If the code could just undo the chroot() it wouldn't be very useful).  
However, chroot() is per-process, therefore what you probably want to do is 
fork(), then do a chroot() in the child and do whatever you want to do there, 
when you're done with the CD, exit the child, and have the parent eject it.

-alex
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Re: Undoing chroot?

[Chris Wedgwood]

On Fri, Jan 26, 2001 at 10:31:40AM -0800, Paul Powell wrote:

    So how do you reverse a CHROOT?

move the prison walls inside of your cell then walk away:

as root:

  mkdir("blah");
  chroot("blah");
  chdir("../../../../../../");

sort of thing



  --cw
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Re: Undoing chroot?

[Paul Menage]

In article <0C01A29FBAE24448A792F5C68F5EA47D04E323@nasdaq.ms.ensim.com>,
you write:
>
>So how do you reverse a CHROOT?
>

Assuming your process doesn't drop its root privileges, before you do
the initial chroot() you could do:

old_root = open("/", O_RDONLY);

Then later do

fchdir(oldroot);
chroot(".");

But the cleaner and more portable solution is probably to have a child
process executing inside the chroot, and have its parent eject the CD
when the child exists.

Paul
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