Re: [PATCH 0/2] Send a SIGCHLD to the init's pid namespace parent when reboot

Serge E. Hallyn
2011-08-15T17:39:14Z

Quoting Oleg Nesterov (oleg@redhat.com):
> On 08/14, Serge E. Hallyn wrote:
> >
> > Quoting Oleg Nesterov (oleg@redhat.com):
> > > On 08/11, Daniel Lezcano wrote:
> > > >
> > > > In the case of a VPS, when we shutdown/halt/reboot the container, the
> > > > reboot utility will invoke the sys_reboot syscall which has the bad
> > > > effect to reboot the host.
> > >
> > > Stupid question. Can't sys_reboot() simply kill init (and thus the whole
> > > pid_ns) in this case?
> >
> > The goal is to be able to distinguish a request for reboot from shutdown.
> > If we just kill the init, then the parent of init (the container monitor)
> > cannot restart the container to emulate reboot.
>
> OK, thanks.
>
> What if init reports the reason it was killed?
>
> Ignoring LINUX_REBOOT_CMD_CAD_/etc, I mean, roughly,
>
> - add "int reboot_cmd" into struct pid_namespace
>
> - sys_reboot(cmd) does
>
> if (!global_namespace) {
> task_active_pid_ns(current)->reboot_cmd = cmd;
> sigkill_my_init();
> }
>
> - change zap_pid_ns_processes() to do
>
> if (pid_ns->reboot_cmd) // approximately
> current->exit_state = pid_ns->reboot_cmd;
>
> Then its parent can look at status after wait(&status).

That looks good to me. Daniel, is there a reason this wouldn't work?

> Not that I think this is very nice, but signals are not reliable.
> And once again, SIGCHLD doesn't queue. And, perhaps this doesn't
> matter, but sys_reboot() sends SIGCHLD and returns -EPERM, this
> can confuse the container.

I think you're looking at an older (Aug 11) version. Daniel's newer
patch should do the right thing (but I don't seem to have it in front
of me atm)

> In any case. If you want to send a signal, please do not introduce
> SA_CLDREBOOT. Please do not play with ptrace or __wake_up_parent, this
> is meaningless. Just fill siginfo and send SIGCHLD unconditionally.
>
> Oleg.

Thanks for the help, Oleg.

-serge