linux-kernel - Re: [RFC PATCH 3/8] kmod - teach call_usermodehelper() to use a namespace

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Message-ID: <20150216182435.GA7954@redhat.com>
Date:	Mon, 16 Feb 2015 19:24:35 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ian Kent <ikent@...hat.com>
Cc:	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Benjamin Coddington <bcodding@...hat.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	Jeff Layton <jeff.layton@...marydata.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [RFC PATCH 3/8] kmod - teach call_usermodehelper() to use a
	namespace

On 02/16, Oleg Nesterov wrote:
>
> On 02/16, Ian Kent wrote:
> >
> > On Tue, 2015-02-10 at 17:55 +0100, Oleg Nesterov wrote:
> > > On 02/10, Ian Kent wrote:
> > > >
> > > > On Mon, 2015-02-09 at 17:03 +0100, Oleg Nesterov wrote:
> > > > >
> > > > > I understand. but I still can't understand why we can't implement something
> > > > > like
> > > > > 	enter_ns(struct nsproxy *p)
> > > > > 	{
> > > > > 		new_nsproxy = create_new_namespaces(...);
> > > > >
> > > > > 		p->mnt_ns->ns->ops->install(new_nsproxy, ...);
> > > > > 		p->pid_ns_for_children->ns->ops->install(new_nsproxy, ...);
> > > > > 		...
> > > > >
> > > > > 		switch_task_namespaces(new_nsproxy);
> > > > > 	}
> > > > >
> > > > > Why we should abuse fs/proc ?
> > > >
> > > > That sounds like a much better approach.
> > > > Your saying just take a reference to the nsproxy from the located
> > > > process and use it instead, right?
> > >
> > > Yes,
> >
> > I'm still not sure if this can be done (at least without surgery to the
> > namespace implementation) and I think I've been here before which is
> > what lead to the file_open_root() approach.
> >
> > The difficulty is the second parameter to the install() call above, the
> > struct ns_common. In setns() it's obtained from the procfs file inode
> > and the file open is what's used to obtain each namespace type (in the
> > form of a struct ns_common) from a process context different from
> > current, current being the thread runner process.
> >
> > I might still be able to work out a (viable) way to obtain the
> > appropriate ns_common struct in each case without a file open but it's
> > hard to see how.
>
> Not sure I understand... Every "namespace" pointer "struct nsproxy" has
> the "struct ns_common ns" you need? So you can do (for example)
>
> 	p->mnt_ns->ns->ops->install(new_nsproxy, &p->mnt_ns->ns);
>
> or I missed something? (userns differs, you need cred->user_ns, of course).
>
>
> Perhaps I missed something, but this doesn't look like a problem...

And in case this wasn't clear we obviously need to pass nsproxy/cred to
sub_info->init().

There is nothing magical in /proc or I am totally confused. Look at ns_get_path(),
it (to simplify) does

	inode->i_private = ns_ops->get(task);

and every ->get() method returns &ns->ns.


But again, again:

> The real problem is that , let me repeat, is that pidns_install() does not
> and can't change active_pid_ns. So I think that kernel_thread_in_ns() probably
> make more sense.

Yes...

Oleg.

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