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

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Date:	Thu, 22 Oct 2009 22:30:01 -0700
From:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.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

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 ?

Sukadev
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