linux-kernel - Re: [PATCH] Don't hang user processes if Kerberos ticket for nfs4 mount expires

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Date:	Wed, 16 Nov 2011 20:31:19 -0500
From:	Jeff Layton <jlayton@...hat.com>
To:	Jim Rees <rees@...ch.edu>
Cc:	John Hughes <john@...vaedi.com>,
	Trond Myklebust <trond.myklebust@...app.com>,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Don't hang user processes if Kerberos ticket for nfs4
 mount expires

On Wed, 16 Nov 2011 18:44:34 -0500
Jim Rees <rees@...ch.edu> wrote:

> Jeff Layton wrote:
> 
>   Uhhh, no...EKEYEXPIRED was never passed to userland. The patchset that
>   added EKEYEXPIRED returns in this codepath also added the code to make
>   it hang. 
>   
>   This not a bug, or at least it's intentional behavior. When a krb5
>   ticket expires, we *want* the process to hang. Otherwise, people with
>   long running jobs will often find that their jobs error out
>   inexplicably when their ticket expires.
> 
> Who decided that?  This seems completely wrong to me.  If my credentials
> expire, I want to get permission denied, not a client hang.  In 20 years of
> using authenticated file systems I never once wished my process had hung
> when my ticket expired.
> 

I proposed it, we discussed it on the list, and Trond and Steve
committed the patches necessary to make it happen. This was back in
late 2009/early 2010 though, so my memory is a bit fuzzy...

> Why should this be any different from any other failure condition?  If you
> try to open a file that doesn't exist, do you want your process to hang
> instead of getting ENOENT, just in case the file magically appears at some
> point in the future?
>

That's different. Not renewing your credentials is often a temporary
situation. Kerberos is different than other authentication methods in
that you get a ticket only for a period of time, so expired credentials
are not a situation that's common with other authentication methods.

> This seems a recipe for disaster.  Suppose I have a cron job that fires once
> a minute, and all those jobs hang waiting for a ticket.  I come to work in
> the morning and discover I've got 10,000 hung processes.  Or not, because my
> computer has crashed from resource exhaustion.

The previous situation was also a recipe for disaster, and was often
cited as a primary reason why people didn't want to deploy kerberized
NFS. Having everything fall down and go boom when your ticket expires
is not desirable either.

I suppose we'll have to agree to disagree on this point. That said, I'm
open to sane suggestions however that don't regress the behavior for
those users who need to be able to cope with expired tickets.

-- 
Jeff Layton <jlayton@...hat.com>
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