linux-kernel - Re: call_usermodehelper in containers

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Date:	Tue, 12 Nov 2013 17:02:36 +0400
From:	Stanislav Kinsbursky <skinsbursky@...allels.com>
To:	Jeff Layton <jlayton@...hat.com>,
	Greg KH <gregkh@...uxfoundation.org>
CC:	<linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-nfs@...r.kernel.org>, <devel@...nvz.org>,
	<ebiederm@...ssion.com>, <oleg@...hat.com>, <bfields@...ldses.org>,
	<bharrosh@...asas.com>
Subject: Re: call_usermodehelper in containers

12.11.2013 15:12, Jeff Layton пишет:
> On Mon, 11 Nov 2013 16:47:03 -0800
> Greg KH <gregkh@...uxfoundation.org> wrote:
>
>> On Mon, Nov 11, 2013 at 07:18:25AM -0500, Jeff Layton wrote:
>>> We have a bit of a problem wrt to upcalls that use call_usermodehelper
>>> with containers and I'd like to bring this to some sort of resolution...
>>>
>>> A particularly problematic case (though there are others) is the
>>> nfsdcltrack upcall. It basically uses call_usermodehelper to run a
>>> program in userland to track some information on stable storage for
>>> nfsd.
>>
>> I thought the discussion at the kernel summit about this issue was:
>> 	- don't do this.
>> 	- don't do it.
>> 	- if you really need to do this, fix nfsd
>>
>
> Sorry, I couldn't make the kernel summit so I missed that discussion. I
> guess LWN didn't cover it?
>
> In any case, I guess then that we'll either have to come up with some
> way to fix nfsd here, or simply ensure that nfsd can never be started
> unless root in the container has a full set of a full set of
> capabilities.
>
> One sort of Rube Goldberg possibility to fix nfsd is:
>
> - when we start nfsd in a container, fork off an extra kernel thread
>    that just sits idle. That thread would need to be a descendant of the
>    userland process that started nfsd, so we'd need to create it with
>    kernel_thread().
>
> - Have the kernel just start up the UMH program in the init_ns mount
>    namespace as it currently does, but also pass the pid of the idle
>    kernel thread to the UMH upcall.
>
> - The program will then use /proc/<pid>/root and /proc/<pid>/ns/* to set
>    itself up for doing things properly.
>
> Note that with this mechanism we can't actually run a different binary
> per container, but that's probably fine for most purposes.
>

Hmmm... Why we can't? We can go a bit further with userspace idea.

We use UMH some very limited number of user programs. For 2, actually:
1) /sbin/nfs_cache_getent
2) /sbin/nfsdcltrack

If we convert them into proxies, which use /proc/<pid>/root and /proc/<pid>/ns/*, this will allow us to lookup the right binary.
The only limitation here is presence of this "proxy" binaries on "host".

And we don't need any significant changes in kernel.

BTW, Jeff, could you remind me, please, why exactly we need to use UMH to run the binary?
What are this capabilities, which force us to do so?

-- 
Best regards,
Stanislav Kinsbursky
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