[RFD] reboot / shutdown of a container

Daniel Lezcano daniel.lezcano at free.fr
Thu Jan 13 13:32:19 PST 2011

On 01/13/2011 09:09 PM, Bruno Prémont wrote:
> On Thu, 13 January 2011 Daniel Lezcano<daniel.lezcano at free.fr>  wrote:
>> in the container implementation, we are facing the problem of a process
>> calling the sys_reboot syscall which of course makes the host to
>> poweroff/reboot.
>> If we drop the cap_sys_reboot capability, sys_reboot fails and the
>> container reach a shutdown state but the init process stay there, hence
>> the container becomes stuck waiting indefinitely the process '1' to exit.
>> The current implementation to make the shutdown / reboot of the
>> container to work is we watch, from a process outside of the container,
>> the<rootfs>/var/run/utmp file and check the runlevel each time the file
>> changes. When the 'reboot' or 'shutdown' level is detected, we wait for
>> a single remaining in the container and then we kill it.
>> That works but this is not efficient in case of a large number of
>> containers as we will have to watch a lot of utmp files. In addition,
>> the /var/run directory must *not* mounted as tmpfs in the distro.
>> Unfortunately, it is the default setup on most of the distros and tends
>> to generalize. That implies, the rootfs init's scripts must be modified
>> for the container when we put in place its rootfs and as /var/run is
>> supposed to be a tmpfs, most of the applications do not cleanup the
>> directory, so we need to add extra services to wipeout the files.
>> More problems arise when we do an upgrade of the distro inside the
>> container, because all the setup we made at creation time will be lost.
>> The upgrade overwrite the scripts, the fstab and so on.
>> We did what was possible to solve the problem from userspace but we
>> reach always a limit because there are different implementations of the
>> 'init' process and the init's scripts differ from a distro to another
>> and the same with the versions.
>> We think this problem can only be solved from the kernel.
>> The idea was to send a signal SIGPWR to the parent of the pid '1' of the
>> pid namespace when the sys_reboot is called. Of course that won't occur
>> for the init pid namespace.
> Wouldn't sending SIGKILL to the pid '1' process of the originating PID
> namespace be sufficient (that would trigger a SIGCHLD for the parent
> process in the outer PID namespace.

This is already the case. The question is : when do we send this signal ?
We have to wait for the container system shutdown before killing it.

> (as far as I remember the PID namespace is killed when its 'init' exits,
> if this is not the case all other processes in the given namespace would
> have to be killed as well)

Yes, absolutely but this is not the point, reaping the container is not 
a problem.

What we are trying to achieve is to shutdown properly the container from 
inside (from outside will be possible too with the setns syscall).

Assuming the process '1234' creates a new process in a new namespace set 
and wait for it.

The new process '1' will exec /sbin/init and the system will boot up. 
But, when the system is shutdown or rebooted, after the down scripts are 
executed the kill -15 -1 will be invoked, killing all the processes 
expect the process '1' and the caller. This one will then call 
'sys_reboot' and exit. Hence we still have the init process idle and its 
parent '1234' waiting for it to die.

If we are able to receive the information in the process '1234' : "the 
sys_reboot was called in the child pid namespace", we can take then kill 
our child pid.  If this information is raised via a signal sent by the 
kernel with the proper information in the siginfo_t (eg. si_code 
solution will be generic for all the shutdown/reboot of any kind of 
container and init version.

> Only issue is how to differentiate the various reboot() modes (restart,
> power-off/halt) from outside, though that one also exists with the SIGPWR
> signal.


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