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

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Date:	Thu, 22 Oct 2009 22:44:19 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Cc:	Matt Helsley <matthltc@...ibm.com>,
	Oren Laadan <orenl@...rato.com>,
	Daniel Lezcano <daniel.lezcano@...e.fr>,
	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

Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com> writes:

> Eric W. Biederman [ebiederm@...ssion.com] wrote:
> | > | +	if (target < RESERVED_PIDS)
> | >
> | > Should we replace RESERVED_PIDS with 0 ? We currently allow new
> | > containers to have pids 1..32K in the first pass and in subsequent
> | > passes assign starting at RESERVED_PIDS.
> | 
> | If it is a preexisting namespace pid namespace removing the RESERVED_PIDS
> | check removes most if not all of the point of RESERVED_PIDS.
> | 
> | In a new fresh pid namespace I have no problem with not performing
> | the RESERVED_PIDS check.
>
> In that case can we do this
>
> 	if (target_pid < RESERVED_PIDS && !pid_ns->level)
> 		return -EINVAL;
>
> instead ?
> | 
> | So I guess that makes the check.
> | 
> | if ((target < RESERVED_PIDS) && pid_ns->last_pid >= RESERVED_PIDS)
> |    return -EINVAL;
>
> I am just wondering if there is a small corner case where C/R would randomly
> fail because of this sequence:
>
> 	- C/R code calls clone() or clone3() say about RESERVED_PIDS-1
> 	  times and ->last_pid == RESERVED_PIDS-1.
>
> 	- C/R code calls normal fork()/alloc_pidmap() for a short-lived
> 	  child - its pid == ->last_pid == RESERVED_PIDS
>
> 	- C/R code then calls clone3()/set_pidmap() to set the pid of
> 	  a new child to RESERVED_PID but fails (i.e it fails to restore
> 	  a pid even when the pid is not in use).
>
> We could argue that mixing alloc_pidmap() and set_pidmap() during restart
> is bad since set_pidmap() may fail.
>
> The C/R developer could argue that we are forcing them to specify a pid
> even for a short lived process that they wait()s on and thus ensure that
> pid is not in use.
>
> Anyway, is RESERVED_PIDS meant for initial kernel-threads/daemons - if so
> would it be ok enforce it only in init_pid_ns ?

It is mean for initial user space daemons, things that start on boot.

I don't know how much the protection matters at this date, but we have it.

Eric
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