Re: [RFC PATCH] fs: call_usermodehelper_root helper introduced

[Stanislav Kinsbursky <skinsbursky@parallels.com>]

23.05.2013 15:56, Jeff Layton пишет:
> On Thu, 23 May 2013 15:38:17 +0400
> Stanislav Kinsbursky <skinsbursky@parallels.com> wrote:
>
>> 23.05.2013 15:31, Jeff Layton пишет:
>>> On Thu, 23 May 2013 14:35:53 +0400
>>> Stanislav Kinsbursky <skinsbursky@parallels.com> wrote:
>>>
>>>> 23.05.2013 14:00, Eric W. Biederman пишет:
>>>>> Stanislav Kinsbursky <skinsbursky@parallels.com> writes:
>>>>>
>>>>>> 22.05.2013 21:33, Eric W. Biederman пишет:
>>>>>>> Stanislav Kinsbursky <skinsbursky@parallels.com> writes:
>>>>>>>
>>>>>>>> Usermode helper executes all binaries in global "init" root context. This
>>>>>>>> doesn't allow to call a binary from other root context (for example in a
>>>>>>>> container).
>>>>>>>> Currently, both containerized NFS client and NFS server requires an ability to
>>>>>>>> execute a binary in a container's root context. Root swap can be done in
>>>>>>>> "init" callback, passed by UMH caller.
>>>>>>>> But since we have 2 callers already (and more of them are expected to appear
>>>>>>>> in future) and because set_fs_root() in not exported, it looks reasonable to
>>>>>>>> add one more generic UMH helper to generic fs code.
>>>>>>>> Root path reference must be hold by the caller, since it will be put on UMH
>>>>>>>> thread exit.
>>>>>>>
>>>>>>> Awesome.  With this patch as an uprivilieged user I get to pick which
>>>>>>> binary the kernel will execute.  At least if nfs and nfsd ever runs in a
>>>>>>> user namespace (something that looks like only matter of time).
>>>>>>>
>>>>>>
>>>>>> Not really. Only by using a kernel module to call the UMH.
>>>>>> And an unprivileged can't load a module as far a I know.
>>>>>> I.e. NFSd, for example, will use unprivileged user's root to perform this call.
>>>>>
>>>>> To help me understand the context which instances of call user mode
>>>>> helper are you expecting to use this facility?
>>>>>
>>>>
>>>> Ok. Here is how the NFSd uses UMH:
>>>> UMH is used on NFSd service to start user-space client tracker daemon
>>>> ("/sbin/nfsdcltarck"), which is used to store some per-client locks data on
>>>> persistent storage.
>>>>
>>>>>>> I think this is a seriously bad idea.
>>>>>>>
>>>>>>> Why can't we do this in userspace with setns as we do with the core dump
>>>>>>> helper?
>>>>>>>
>>>>>>
>>>>>> Could you, please, clarify, how setns can help here?
>>>>>
>>>>> setns can change the mount namespace, and chroot can change to root
>>>>> directory in the specified mount namespace.  Essentially you can enter
>>>>> into a containers complete context (pid, mnt, root, etc) comming from
>>>>> the outside.
>>>>>
>>>>
>>>> So, you are actually suggesting to move the binary start from the kernel to user-space.
>>>> IOW, you are suggesting to do not using UMH at all.
>>>> Am I right?
>>>> I don't know the reasons, why it was done by using UMH and not in userspace.
>>>> Could you clarify this, Jeff?
>>>>
>>>
>>> nfsdcltrack is a "one-shot" program for managing and querying the nfsd
>>> client tracking database. When knfsd needs to query or modify the
>>> db, it uses the UMH infrastructure to call this program that does
>>> what's requested and then exits.
>>>
>>> So, I'm not sure I really understand your question. It wasn't done in
>>> userspace since the whole purpose of this program is to handle upcalls
>>> from the kernel.
>>>
>>
>> The question is what was the reason to start this binary from kernel by UMH?
>
> Manipulating and querying the client tracking database is an infrequent
> event, so having a continuously running daemon is wasteful and means
> that the admin has to ensure that it's running. A UMH upcall is much
> simpler and generally "just works" if the program is present.
>
>> I.e. why it can't be started by some user-space process before or after NFSd start?
>> I don't familiar with this client tracking facility and that's the only reason why I'm asking.
>>
>
> This program is not a daemon that runs continuously. It's only called
> when the kernel needs to manipulate the database. Are you asking
> whether we could turn this into a continuously running daemon? If so
> then the answer is "yes", but that's not really a good idea either.
>
> In fact, we had that with the nfsdcld program, but no one liked it
> (including me) for the reasons I detailed above.
>

No, I'm just asking to understand.
Eric was, actually, asking the same. I.e. how does NFSd uses UMH and why this can't be done in userspace?
Thanks you for your answer.

-- 
Best regards,
Stanislav Kinsbursky