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Date:	Tue, 10 Nov 2009 20:06:31 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Bryan Donlan <bdonlan@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Americo Wang <xiyou.wangcong@...il.com>,
	Timo Sirainen <tss@....fi>,
	Ulrich Drepper <drepper@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org
Subject: Re: [PATCH v5] Added PR_SET_PROCTITLE_AREA option for prctl()

2009/11/10 Bryan Donlan <bdonlan@...il.com>:
> On Mon, Nov 9, 2009 at 5:47 PM, Andrew Morton <akpm@...ux-foundation.org> wrote:
>
>> What happens if userspace unmaps the memory after telling the kernel to
>> use it?
>>
>> Will processes which try to read the command line get an error reading
>> /proc?  If so, do all the commandline-reading programs in the world
>> handle this in an appropriate fashion?
>
> This case can already occur in the current code; the userspace process
> would have to munmap() the top of its stack, but it certainly can do
> so if it tries. In any case, access_process_vm() then returns 0
> because of the fault, and thus /proc/pid/cmdline is seen to have zero
> length. Since a zero-length /proc/pid/cmdline occurs with kernel
> threads as well, we know this isn't a problem.

Plus, ps can read under exiting process. In this case, task->mm is NULL and
proc_pid_cmdline return 0.

procps tools are already NUL safe since long time ago.
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