linux-kernel - Re: Signals to cinit

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Message-ID: <20081110232735.GA20891@us.ibm.com>
Date:	Mon, 10 Nov 2008 15:27:35 -0800
From:	sukadev@...ux.vnet.ibm.com
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...nvz.org>, daniel@...ac.com,
	Nadia Derbey <Nadia.Derbey@...l.net>, serue@...ibm.com,
	clg@...ibm.com, Containers <containers@...ts.osdl.org>,
	sukadev@...ibm.com, linux-kernel@...r.kernel.org
Subject: Re: Signals to cinit

Oleg Nesterov [oleg@...hat.com] wrote:
| (lkml cced because containers list's archive is not useable)

Hmm. what do you mean by not usable ? I see your email here:
https://lists.linux-foundation.org/pipermail/containers/2008-November/014152.html

| 
| On 11/10, Oleg Nesterov wrote:
| >
| > On 11/01, sukadev@...ux.vnet.ibm.com wrote:
| > >
| > > Other approaches to try ?
| >
| > I think we should try to do something simple, even if not perfect. Because
| > most users do not care about this problem since they do not use containers
| > at all. It would be very sad to add intrusive changes to the code.
| >
| > I think we should fix another problem first. send_signal()->copy_siginfo()
| > path must be changed anyway, when the signal comes from the parent ns we
| > report the "wrong" si_code/si_pid, yes? So, somehow send_signal() must
| > have "bool from_parent_ns" (or whatever) annyway.
| >
| > Now, let's forget forget for a moment that send_signal()->__sigqueue_alloc()
| > can fail.
| >
| > I think we should encode this "from_parent_ns" into "struct siginfo". I do
| > not think it is good idea to extend this structure, I think we can introduce
| > SI_FROM_PARENT_NS or we perhaps can use "SI_FROMUSER(info) && info->si_pid == 0".

Yes, afaics, we just need to pass one extra bit of information per signal
(whether or not sender is in ancestor-ns) from sender to receiver.

| > Or something. yes, sys_rt_sigqueueinfo() is problematic...

Yes, if user-space sets si_pid to 0.

Can we change sys_rt_sigqueueinfo() to:

	if (!info->si_pid)
		info->si_pid = getpid();

or would that change semantics adversely ? How about putting this
under CONFIG_PID_NS or your CONFIG_I_DO_CARE_ABOUT_NAMESPACES ;)

| >
| > Now, copy_process(CLONE_NEWPID) sets child->signal |= SIGNAL_UNKILLABLE, this
| > protects cinit from unwanted signals. Then we change get_signal_to_deliver()
| >
| > 	-	if (unlikely(signal->flags & SIGNAL_UNKILLABLE) &&
| > 	+	if (unlikely(signal->flags & SIGNAL_UNKILLABLE) && !siginfo_from_parent_ns(info)
| >
| > and now we can kill cinit from parent ns. This needs more checks if we want
| > to stop/strace it, but perhaps this is enough for the start. Note that we
| > do not need to change complete_signal(), at least for now, the code under
| > "if (sig_fatal(p, sig)" is just optimization.
| >
| >
| > So, afaics, the only real problem is how we can handle the case when
| > __sigqueue_alloc() fails. I think for the start we can just return
| > -ENOMEM in this case (when from_parent_ns == T). Then we can improve
| > this behaviour. We can change complete_signal() to ensure that the
| > fatal signal from the upper ns always kills cinit, and in this case
| > we ignore the the failed __sigqueue_alloc(). This way at least SIGKILL
| > always works.
| >
| > Yes, this is not perfect, and it is very possible I missed something
| > else. But simple.

I agree 
| 
| But how can send_signal() know that the signal comes from the upper ns?
| This is not trivial, we can't blindly use current to check. The signal
| can be sent from irq/workqueue/etc.

You mean the in_interrupt() check we had in earlier patchset would
not be enough ?

| 
| Perhaps we can start with something like the patch below. Not that I like
| it very much though. We should really place this code under
| CONFIG_I_DO_CARE_ABOUT_NAMESPACES ;)

CONFIG_PID_NS ?
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