[BusyBox] 1.00-pre1 problem: can't umount oldroot after pivot_root&chroot

[Yang Xiaopeng]

Yang Xiaopeng wrote:

> Old /proc is unmounted when init restart, before executing my scripts,
> all user processes are also killed in that step. When my scripts run,
> proc filesystem is remounted to /tmp/proc before pivot_root&chroot to 
> /tmp.
> after chroot, its real mount point should be /proc in the new root.
>
> Here is the output of `mount` just after pivot_root&chroot to /tmp:
>
> /dev/root on /old_root type cramfs (rw)
> none on / type tmpfs (rw)
> none on /proc type proc (rw)
>
> and the `ps` output:
>
>  PID  Uid     VmSize Stat Command
>    1 root        616 S   /bin/sh sbin/flash.sh
>    2 root            SW  [keventd]
>    3 root            SWN [ksoftirqd_CPU0]
>    4 root            SW  [kswapd]
>    5 root            SW  [bdflush]
>    6 root            SW  [kupdated]
>    7 root            SW  [keyd]
>    8 root            RW  [ledtimed]
>    9 root            SW  [mtdblockd]
>   10 root            Z   [khubd]
>  181 root        676 R   ps
>
> I'm sure that no user process use old root now,  but when run "umount 
> /old_root", it says:
>    umount: /old_root: Device or resource busy
>
> I have tried to remount /proc within the new root *after* chroot, but 
> get the same result.
>
>
I found the problem, I said that no user process use old root when run 
my scripts, but
I'm wrong, actually there is a '3' fd open the file 
"/old_root/dev/console". By adding
debug message in init/init.c, I found the problem: when init restart(in 
exec_signal()),
before open the new terminal device, there is still a file opened(I 
don't know which file it is), so the
terminal device(stdin) get fd '1', and the first dup(0)(stdout) return 
'2', the second(stderr) return '3'.

I attach a simple patch to solve this problem.

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