[RFD] reboot / shutdown of a container

Daniel Lezcano daniel.lezcano at free.fr
Thu Jan 13 14:09:15 PST 2011

On 01/13/2011 10:50 PM, Bruno Prémont wrote:
> On Thu, 13 January 2011 Daniel Lezcano<daniel.lezcano at free.fr>  wrote:
>> 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.
> I meant that sys_reboot() would kill the namespace's init if it's not
> called from boot namespace.
> See below
>>> (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.
> This call to sys_reboot() would kill "new process '1'" instead of trying to
> operate on the HW box.
> This also has the advantage that a container would not require an informed
> parent "monitoring" it from outside (though it would not be restarted even if
> requested without such informed outside parent).

Oh, ok. Sorry I misunderstood.

Yes, that could be better than crossing the namespace boundaries.

>> 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.
> Could this be passed for a SIGCHLD? (when namespace is reaped, and received
> by 1234 from above example assuming sys_reboot() kills the "new process '1'")

Yes, that sounds a good idea.

> Looks like yes, but with the need to define new values for si_code (reusing
> LINUX_REBOOT_CMD_* would certainly clash, no matter which signal is choosen).

CLD_REBOOT_CMD_RESTART2 (what about the cmd buffer, shall we ignore it ?)
CLD_REBOOT_CMD_SW_SUSPEND (useful for the future checkpoint/restart)

for a non-init pid namespace, no ?

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