linux-kernel - Re: [PATCHSET 0/4] kmod: Optional timeout on the wait in call_usermodehelper

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Date:	Wed, 21 Mar 2012 17:18:01 -0700
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Greg KH <gregkh@...uxfoundation.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, <keyrings@...ux-nfs.org>,
	<linux-security-module@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	NFS list <linux-nfs@...r.kernel.org>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	"Bhamare, Sachin" <sbhamare@...asas.com>,
	David Howells <dhowells@...hat.com>,
	Eric Paris <eparis@...hat.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Kay Sievers <kay.sievers@...y.org>,
	James Morris <jmorris@...ei.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: [PATCHSET 0/4] kmod: Optional timeout on the wait in call_usermodehelper_exec

On 03/21/2012 08:35 AM, Greg KH wrote:
> On Tue, Mar 20, 2012 at 04:18:49PM -0700, Boaz Harrosh wrote:
>> Andrew Hi
>>
>> I'm picking on you because I don't have any one else to pick on.
>> The 3 first patches here, are just good for today. Please see if
>> you would like to take them? or tell me who should take them?
>>
>> The 4th patch is an RFC, which got me looking into this.
>>
>> My motivation is that I added yet another Kernel dependency on the
>> call_usermodehelper() function and am not completely happy with the
>> error case of having the user-mode program stuck forever. In such
>> case I would like the Kernel part to timeout and properly error recover
>> and clean up. So therefor the proposed 4th patch.
> 
> What is this new use of call_usermodhelper that you are doing this work
> for?  Ideally, you never want to make this call, as it's slow and messy,
> as you have found out.  Is there an in-kernel user that you have
> recently added?
> 
> thanks,
> 
> greg k-h

I agree hence my comment in the 4th patch:
	"In the blasphemous occasions that a the Kernel must call a user-mode program"

I have added a new caller, to the nfs/objectlayoutdriver.ko that uses this
facility for auto-login into osd-targets (iscsi-targets) when new are requested
by the filesystem. This auto-login facility is mandated by the pnfs-objects
standard because in a large cluster filesystems for which pnfs was invented, storage
devices break and changed everyday, and a manual login by every client is not
feasible.

You can see this patch as posted to the mailing list here:
	http://article.gmane.org/gmane.linux.nfs/48024/match=login
	[title: pnfs-obj: autologin: Add support for protocol autologin]

It works very well and was heavily tested, with all error scenarios, but
the theoretical possibility that the user-mode program can be stuck forever
bothers me and I would like to do something about it. With this patch the
Kernel can recover cleanly and continue. I have actually tested this part
and it works as expected.

Thanks
Boaz
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