linux-kernel - Re: [RFC PATCH 3/4] kmod - add call_usermodehelper

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Date:	Wed, 26 Nov 2014 08:44:57 +0800
From:	Ian Kent <ikent@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Stanislav Kinsbursky <skinsbursky@...allels.com>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	David Howells <dhowells@...hat.com>,
	Benjamin Coddington <bcodding@...hat.com>,
	Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [RFC PATCH 3/4] kmod - add call_usermodehelper_ns() helper

On Wed, 2014-11-26 at 07:50 +0800, Ian Kent wrote:
> > 
> > If we are going to set this stuff up in the kernel we need a reference
> > process that we can create children of because what is possible with
> > respect to containers keeps changing, and it is extremely error prone to
> > figure out what all othe crazy little bits are, and to update everything
> > every time someone tweaks the kernel's capabilities.  We have kthreadd
> > because it was too error prone to scrub a userspace thread of all of the
> > userspace bits and make it the equivalent of what kthreadd is today.
> > 
> > Of course it is also rather nice to have something to hang everything
> > else on.
> > 
> > In summary we need a reference struct task that is all setup properly
> > so that we can create an appropriate kernel thread.
> 
> I'm having trouble understanding what your getting at here but I'm not
> that sharp so bear with me.
> 
> When call_usermodehelper() is called it's called from a process that is
> within the context within which the execution is required.

Umm .. OK, that's probably not quite right either ....

For nfsd I think it's OK but for nfs clients the context is probably
that of the caller ....

Whereas the helper to get a key info maybe does need to be called in the
context of the caller .....

> 
> So what information do we not have available for setup?
> 
> Are you saying that the problem is that when the user mode helper run
> thread is invoked we don't have the information available that was
> present when call_usermodehelper() was called and that's where the
> challenge lies?
> 
> Ian
> 


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