linux-kernel - Re: + core_pattern-set-core-helpers-root-and-namespace-to-crashing-process .patch added to -mm tree

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Date:	Mon, 17 Dec 2012 17:04:08 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Neil Horman <nhorman@...driver.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Daniel Berrange <berrange@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: +
	core_pattern-set-core-helpers-root-and-namespace-to-crashing-process
	.patch added to -mm tree

On 12/17, Neil Horman wrote:
>
> On Mon, Dec 17, 2012 at 01:34:28PM +0100, Oleg Nesterov wrote:
> > @@ -455,6 +468,14 @@ static int umh_pipe_setup(struct subproc
> >  	/* and disallow core files too */
> >  	current->signal->rlim[RLIMIT_CORE] = (struct rlimit){1, 1};
> >
> > +
> > +	if (cp->switch_ns) {
> > +		get_fs_root(cp->cprocess->fs, &root);
> > +		set_fs_root(current->fs, &root);
> > +		switch_task_namespaces(current, cp->cprocess->nsproxy);
> >
> > How? You can't simply change ->nsproxy this way.
> >
> Why not?  This is exactly how fork, exit, and setns use this call.

No. exit() does switch_task_namespaces(NULL), this is different.
fork() doesn't do this, and unshare/setns carefully creates the new ns.

> > If nothing else this breaks sys_getpid(), no?
> >
> hmm, I think you're inferring here that there is a chance that a pid allocated
> in the init namespace might conflict with another process who holds the same pid
> in another namespace?

No, I meant that sys_getpid() should always return 0 after this
switch_task_namespaces() if the coredumping task is not from the root
namespace.

> Is there a way to switch all namespaces, except for the pid
> namespace?

Which exactly namespaces you want to change?

To be honest, I do not understand this patch at all. It seems that
you need to do something like sys_setns(). But if we do this, then
why we can't make core_pattern per-namespace?

Anyway, please ask Pavel and Eric, they should know better ;)

> > And a lot more problems, afaics. For example, this thread can continue
> > to run after, say, this cprocess->nsproxy->pid_ns was already destroyed.
> > zap_pid_ns_processes() obviously won't see this thread.
> >
> Hmm, I don't think so.  The crashing process won't exit until the pipe reader is
> done, so the reference on the namespace should never decrement to zero.
>
> Actually I take that back.  switch_task_namespaces doesn't add a ref count to
> the name space being switched to.  So if the pipe reader doesn't exit
> immediately after closing the pipe, it may live on after the namespace is
> destroyed.

Yes,

> It would seem a get_nsproxy call is needed here to hold an
> additional reference.  Or do you think more is necessecary?

This can only pin ->nsproxy itself, this is not enough iirc.

Note that the exiting sub-init assumes that nobody else can use
ns->proc_mnt after zap_pid_ns_processes().

Oleg.

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