mount rootfs

主要用到的命令为pivot_root，可man 8 pivot_root了解用法。

1. pivot_root - change the root filesystem

pivot_root new_root put_old

pivot_root  moves  the  root  file  system of the current process to the directory put_old and makes new_root the new root file system.

Note  that, depending on the implementation of pivot_root, root and cwd of the caller may or may not
change. The following is a sequence for invoking pivot_root that works in either case, assuming that
pivot_root and chroot are in the current PATH:

       cd new_root
       pivot_root . put_old
       exec chroot . command

Note that chroot must be available under the old root and under the new root, because pivot_root may
or may not have implicitly changed the root directory of the shell.

Note that exec chroot changes the running executable, which is necessary if the old  root  directory
should be unmounted afterwards.  Also note that standard input, output, and error may still point to
a device on the old root file system, keeping it busy. They can  easily  be  changed  when  invoking chroot. 

2. 应用

pivot() { # <new_root> <old_root>
    /bin/mount -o noatime,move /proc $1/proc && 
    pivot_root $1 $1$2 && {
        /bin/mount -o noatime,move $2/dev /dev
        /bin/mount -o noatime,move $2/tmp /tmp
        /bin/mount -o noatime,move $2/sys /sys 2>&-
        /bin/mount -o noatime,move $2/overlay /overlay 2>&-
        return 0
    }   
}