linux-kernel - Re: [RFC][v8][PATCH 0/10] Implement clone3() system call

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Message-ID: <20091020141625.GA3645@us.ibm.com>
Date:	Tue, 20 Oct 2009 09:16:25 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	randy.dunlap@...cle.com, arnd@...db.de, linux-api@...r.kernel.org,
	Containers <containers@...ts.linux-foundation.org>,
	Nathan Lynch <nathanl@...tin.ibm.com>,
	linux-kernel@...r.kernel.org, Louis.Rilling@...labs.com,
	kosaki.motohiro@...fujitsu.com, hpa@...or.com, mingo@...e.hu,
	torvalds@...ux-foundation.org,
	Alexey Dobriyan <adobriyan@...il.com>, roland@...hat.com,
	Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [RFC][v8][PATCH 0/10] Implement clone3() system call

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com> writes:
> The only real argument in favor of doing this in user space is greater
> flexibility.

Yyyyup.

> I can see checkpointing/restoring a single thread process
> without a pid namespace.  Anything more and you are just asking for
> trouble.
> 
> A design that weakens security.

How does it weaken security?  It requires CAP_SYS_ADMIN, and, when
targeted capabilities are added, it will require CAP_SYS_ADMIN to
only those pid namespaces in which we choose a pid.

> Increases maintenance costs.

Does it really?  It re-uses most of the existing code.  More so than
the only existing in-kernel restart code I've seen does.

> All for
> an unreliable result seems like a bad one to me.

I've personally always been on the fence as to whether we rebuild the
process tree in user-space or kernel.  And I'm still on the fence, as
nothing you've said has convinced me otherwise.

> > | The pid assignment code is currently ugly.  I asked that we just pass
> > | in the min max pid pids that already exist into the core pid
> > | assignment function and a constrained min/max that only admits a
> > | single pid when we are allocating a struct pid for restart.  That was
> > | not done and now we have a weird abortion with unnecessary special cases.
> >
> > I did post a version of the patch attemptint to implement that. As
> > pointed out in:
> >
> > 	http://lkml.org/lkml/2009/8/17/445
> >
> > we would need more checks in alloc_pidmap() to cover cases like min or max
> > being invalid or min being greater than max or max being greater than pid_max
> > etc. Those checks also made the code ugly (imo).
> 
> If you need more checks you are doing it wrong.  The code already has min
> and max values, and even a start value.  I was just strongly suggesting
> we generalize where we get the values from, and then we have not special
> cases. 

(I'm not sure whether this argument is a separate one - regarding the
implementation of choosing the pid - from the kernel-vs-userspace one
or not, so will wait for a response to my other email about your API)

thanks,
-serge
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