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From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: klibc
Date: Fri, 9 Jun 2006 15:48:43 -0700 (PDT)
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Followup to:  <bda6d13a0606091528h4e85265du8651818c73827b7d@mail.gmail.com>
By author:    "Joshua Hudson" <joshudson@gmail.com>
In newsgroup: linux.dev.kernel
> 
> Once again. Loopback mount requires a clean unmount of root and
> host filesystem. After remounting root read-only, host is still read-write
> and cannot be remounted read-only.
> 
> It is necessary to provide access to the rootfs tree somewhere else
> or use pivot_root, like the initrd solution below:
> 
> initrd: /linuxrc
> #!/bin/sh
> mount /dev/hda1 -o rw -t ntfs /host
> mount /host/linux/root.img -o loop,ro -t ext3 /root
> pivot_root /root /root/initrd
> exec /initrd/bin/init
> 
> root:/etc/rc.d/rc.halt:
> #!/bin/sh
> pivot_root /initrd /initrd/root
> cd /
> exec /stop $RUNLEVEL
> 
> initrd:/stop
> #!/bin/sh
> kill -SIGUSR1 1
> umount /root
> umount /host
> case $1 in
> 0) poweroff -f ;;
> *) reboot -f ;;
> esac
> 
> This requires static binaries of init, sh, mount, umount, an extant /etc, and a
> few nodes in /dev.

Another solution is to leave a process with its cwd parked in the
rootfs.  Look at run_linuxrc() in usr/kinit/initrd.c of any klibc tree
to see how this can be used.  (That is there to support old-style
/linuxrc, but should be applicable here, too.)

	-hpa