linux-kernel - Re: [PATCH 1/1][V5] Add reboot_pid

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Date:	Fri, 3 Feb 2012 09:47:17 -0600
From:	Serge Hallyn <serge.hallyn@...onical.com>
To:	Daniel Lezcano <daniel.lezcano@...e.fr>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, oleg@...hat.com,
	containers@...ts.linux-foundation.org, gkurz@...ibm.com,
	linux-kernel@...r.kernel.org, mtk.manpages@...il.com
Subject: Re: [PATCH 1/1][V5] Add reboot_pid_ns to handle the reboot syscall

Quoting Daniel Lezcano (daniel.lezcano@...e.fr):
> On 02/03/2012 01:10 AM, Andrew Morton wrote:
> >On Thu,  5 Jan 2012 10:06:50 +0100
> >Daniel Lezcano<daniel.lezcano@...e.fr>  wrote:
> >
> >>In the case of a child pid namespace, rebooting the system does not
> >>really makes sense. When the pid namespace is used in conjunction
> >>with the other namespaces in order to create a linux container, the
> >>reboot syscall leads to some problems.
> >>
> >>A container can reboot the host. That can be fixed by dropping
> >>the sys_reboot capability but we are unable to correctly to poweroff/
> >>halt/reboot a container and the container stays stuck at the shutdown
> >>time with the container's init process waiting indefinitively.
> >>
> >>After several attempts, no solution from userspace was found to reliabily
> >>handle the shutdown from a container.
> >>
> >>This patch propose to make the init process of the child pid namespace to
> >>exit with a signal status set to : SIGINT if the child pid namespace called
> >>"halt/poweroff" and SIGHUP if the child pid namespace called "reboot".
> >>When the reboot syscall is called and we are not in the initial
> >>pid namespace, we kill the pid namespace for "HALT", "POWEROFF", "RESTART",
> >>and "RESTART2". Otherwise we return EINVAL.
> >>
> >>Returning EINVAL is also an easy way to check if this feature is supported
> >>by the kernel when invoking another 'reboot' option like CAD.
> >>
> >>By this way the parent process of the child pid namespace knows if
> >>it rebooted or not and can take the right decision.
> >Looks OK, although the comments need help.  Is the below still true?
> 
> Yes, thanks for fixing this.
> 
> >
> >Do you think it would be feasible to put your testcase into
> >tools/testing/selftests?  I'm thinking "no", because running the test
> >needs elevated permissions and might reboot the user's machine(!).
> 
> Yes, right. I don't think the user will be happy with that.
> Unfortunately, I don't see how to test this feature without falling
> into a reboot on failure. On the other side, this very specific
> feature is used in the container environment and if it fails that
> will be spotted immediately and fixed. So I don't think that does
> make sense to add this test in tools/testing/selftests.

In fact I can add Daniel's testcase to my lxc testsuite to run
separately, apart from lxc.  I haven't yet hooked it up into our
jenkins qa rig, but that's planned.

> [ ... ]
> 
> >  	gid_t pid_gid;
> >  	int hide_pid;
> >+	int reboot;
> >  };
> >This was particuarly distressing.  The field was poorly named and other
> >people forgotting to document their data structures doesn't mean that
> >we should continue to do this!
> 
> Thanks again for adding the description. I will take care next time
> to add a simple description when the field name is not self-explicit
> or ambiguous.
> 
>   -- Daniel
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