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Date: Fri, 5 Feb 2016 16:08:11 -0500
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Subject: Re: Question about XID use in sunrpc
From: Trond Myklebust <trond.myklebust@primarydata.com>
To: Olga Kornievskaia <aglo@umich.edu>
Cc: linux-nfs <linux-nfs@vger.kernel.org>
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Sender: linux-nfs-owner@vger.kernel.org

On Fri, Feb 5, 2016 at 2:03 PM, Olga Kornievskaia <aglo@umich.edu> wrote:
> On Fri, Feb 5, 2016 at 1:31 PM, Trond Myklebust
> <trond.myklebust@primarydata.com> wrote:
>> On Fri, Feb 5, 2016 at 12:01 PM, Olga Kornievskaia <aglo@umich.edu> wrote:
>>> On Fri, Feb 5, 2016 at 11:44 AM, Trond Myklebust
>>> <trond.myklebust@primarydata.com> wrote:
>>>> On Fri, Feb 5, 2016 at 10:37 AM, Olga Kornievskaia <aglo@umich.edu> wrote:
>>>>> I have a question regarding the implementation of sunrpc use of XID
>>>>> when the client receives an AUTH_ERROR. The code (clnt.c line 1933)
>>>>> explicitly comments that a new XID should be acquired and releases the
>>>>> currently rpc task (and gets a new one). Why is that? Since the
>>>>> operation is "replayed" but with the new credentials, why shouldn't
>>>>> the same XID be used?
>>>>>
>>>>> The RPC RFC says that XID is used by the server to detect
>>>>> retransmissions. It's not clear if in the specs means "retransmission"
>>>>> == tcp retransmissions. If so then it explains why the client uses the
>>>>> same XID.
>>>>>
>>>>
>>>> The questions you are asking come under the header "RPC lore" rather
>>>> than "RPC law". The use of XIDs as a basis for replay caching is not
>>>> speced out in any RFC. The closest thing we have in the form of
>>>> documentation is Ric Werme's presentation at the 1996 Connectathon:
>>>> http://nfsv4bat.org/Documents/ConnectAThon/1996/werme1.pdf
>>>>
>>>> Basically, those comments are there in the Linux code to denote issues
>>>> found when interoperability testing with server implementations that
>>>> are probably now long dead, but might still be in use somewhere.
>>>
>>> Would you consider changing this to use the same XID in case of
>>> redoing the operation due to the AUTH_ERROR?
>>>
>>> The issue it causes (one of the) server's implementation is of the
>>> following nature:
>>> 1. client sends an operation to the server. the server process the
>>> operation but before replying back to the server has an issue and
>>> resets the connection.
>>> 2. client re-establishes the connection and replays the RPC. the
>>> server now fails with the AUTH_ERROR.
>>> 3. client establishes a new connection and replays the same NFS
>>> operation over the new XID. The server cached the operation but since
>>> the last operation arrives with the new XID it won't find the entry in
>>> the cache. It's problematic when the operation is like REMOVE.
>>>
>>> I realize this is why nfs4.1 session were introduce to solve these
>>> non-idenpotency issues but using the same XID seems like the right
>>> idea since it is the same operation.
>>>
>>> If you don't have objections to the change, I can ask on the IETF list
>>> to see if any servers will object to such change.
>>
>> What you describe is a clear and obvious server bug. It is not a
>> client bug, and is not something that I'd find acceptable as
>> justification for changing the client code.
>>
>> The server should not be replying AUTH_ERROR and then processing the
>> RPC anyway. That's not behaviour that is sanctioned by the RPC spec.
>
> Perhaps I wasn't clear let me try again. In the first step, the server
> processes request and does not reply with an AUTH_ERROR but instead
> resets a connection but it has already populated it's replay cache.
> Client reestablishes connection resends exactly the same bytes but
> gets back an AUTH_ERROR (server does not process the operation). It's
> the recovery from this error that's in question.
>

Hi Olga,

I understood what you said, but you cannot have multiple replies to
the same RPC call. It doesn't matter if it was a replay, if the server
replies AUTH_ERROR, then it is saying "I'm not executing this".

Cheers
  Trond