From: Peter Staubach <staubach@redhat.com>
Subject: Re: Should truncated READDIR replies return -EIO?
Date: Fri, 08 Feb 2008 10:39:50 -0500
Message-ID: <47AC77C6.80503@redhat.com>
References: <1202483082-5334-1-git-send-email-jlayton@redhat.com> <1202483596.8914.13.camel@heimdal.trondhjem.org>
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Cc: Jeff Layton <jlayton@redhat.com>, linux-nfs@vger.kernel.org
To: Trond Myklebust <trond.myklebust@fys.uio.no>
In-Reply-To: <1202483596.8914.13.camel-rJ7iovZKK19ZJLDQqaL3InhyD016LWXt@public.gmane.org>
Sender: linux-nfs-owner@vger.kernel.org

Trond Myklebust wrote:
> On Fri, 2008-02-08 at 10:04 -0500, Jeff Layton wrote:
>   
>> Recently, I ran across a server-side bug that caused the server to send
>> truncated READDIR replies. The server would send a valid RPC response to
>> a READDIR call, but the contents of it were basically missing
>> (everything after the status).
>>
>> The server problem had long been patched in mainline kernels, but the
>> interesting bit was that clients didn't return an error in this
>> situation. The XDR decoders for readdir calls are supposed to check the
>> validity of the response, but in this situation it just fudges the
>> contents of the pagecache to make it look like a completely empty
>> directory.
>>
>> Shouldn't the client return an error in this situation? The response
>> obviously isn't valid so it seems like it shouldn't pretend that it is.
>> If so, would something like the following patch make sense?
>>     
>
> It is quite valid (though silly!) for a server to return a READDIR reply
> with no entries. AFAICR there were servers that actually did this at one
> point (though I shall refrain from naming and shaming).
>
> So whereas I agree that it might be correct to flag a READDIR reply that
> contains no entries due to XDR encoding bugs, I'm not sure that we
> should be flagging errors in the case where the XDR is correct.

In this case, I believe that the response was malformed.  Pretty
much everything after the status was missing, including the EOF
indicator.  I would agree that it would be silly to return a
response with no error indicated, no entries, and the eof
indication set to false.

This really boils down to how do we handle malformed responses?
Is there a general policy to retransmit the request?  This would
seem to be the right thing because a malformed response would
result from many things including the TCP connection getting
dropped in the middle of receiving the response from a timeout
and other things.  However, in this situation, retransmitting
the request would just have resulted in the same, broken response
from the server.  This was due to a server bug, which has since
been fixed, but exists still out in nature.

    Thanx...

       ps