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Old 06-18-2013, 02:48 PM   #1
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chroot into linux root FS and give them a way to self unmount

hi all,
i'm developing and android universal bootloader.
this thread should NOT be moved to android forums!
our project it's to make users able to boot any OS ( linux, Mac, Windows, android and so on ) from any arm device. [ wiki: ]
our root_chooser works as /init replacement.
it chroot to an user defined device and substitute itself with another user specified init program ( usually /sbin/init ).
my problem is that the booted OS cannot unmount itself.
we are mounting the user-defined blockdevice over /newroot, chroot into /newroot ( or a subfolder ) and execv() the /sbin/init program.
for example gentoo say "cannot umount /newroot - we are using /newroot" in the shutdown phase.
i'm asking your help because every time i reboot the first mount() on the user-defined blockdevice takes so long due to unclean filesystem.
is there anyway to chroot into another linux root with giving him a way to unmount himself?
many init programs checks that they are the 1'st a fork is not possible.
but i belive that there is a way since many initramfs do this.

thanks in advance for your help.

-- open source rocks!
Old 06-18-2013, 04:15 PM   #2
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I'm sure I'm missing something here..
Why not just unmount /newroot once chroot exits?

Or maybe you need pivot_root?

Last edited by Firerat; 06-18-2013 at 04:25 PM. Reason: pivot_root
Old 06-19-2013, 02:21 AM   #3
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Originally Posted by Firerat View Post
Why not just unmount /newroot once chroot exits?
thanks for your suggestions,
i can't unmount /newroot, i have to exec binaries that are into /newroot....

btw, pivot_root seems to be what i'm looking for
i'll try it today and let you known if it works.

thanks for your help.
Old 06-19-2013, 06:26 AM   #4
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pivot_root shouldn't be used by the initramfs,
you have to use switch_root instead.
BUT both require that the newroot it's a mountpoint.
our target it's to chroot inside a subdirectory of a device ( or a fs dd image ).
after looking around for pivot_root and switch_root i learned that mount accept the "move" option.
maybe this can help me.

but if i move the /newroot to /, and than chroot into a subfolder,
how can the spawned init unmount the real / ?

thanks in advance!
Old 06-19-2013, 08:51 AM   #5
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Moderator Response

Moved: This thread is more suitable in <Linux - Embedded & Single-board computer> and has been moved accordingly to help your thread/question get the exposure it deserves.


chroot, initramfs, mount, unmount

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