From mboxrd@z Thu Jan  1 00:00:00 1970
From: Oren Laadan <orenl-eQaUEPhvms7ENvBUuze7eA@public.gmane.org>
Subject: Re: [PATCH 4/6] cr: checkpoint and restore task credentials
Date: Thu, 21 May 2009 10:02:20 -0400
Message-ID: <4A155EEC.9070509@cs.columbia.edu>
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To: "Serge E. Hallyn" <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
Cc: Linux Containers <containers-qjLDD68F18O7TbgM5vRIOg@public.gmane.org>, David Howells <dhowells-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
List-Id: containers.vger.kernel.org



Serge E. Hallyn wrote:
> Quoting Oren Laadan (orenl-eQaUEPhvms7ENvBUuze7eA@public.gmane.org):
>>>  /* dump the task_struct of a given task */
>>>  static int checkpoint_task_struct(struct ckpt_ctx *ctx, struct task_struct *t)
>>>  {
>>>  	struct ckpt_hdr_task *h;
>>>  	int ret;
>>> +	int realcred_ref, ecred_ref;
>>> +
>>> +	realcred_ref = checkpoint_obj(ctx, t->real_cred, CKPT_OBJ_CRED);
>>> +	if (realcred_ref < 0)
>>> +		return realcred_ref;
>>> +
>>> +	ecred_ref = checkpoint_obj(ctx, t->cred, CKPT_OBJ_CRED);
>>> +	if (ecred_ref < 0)
>>> +		return ecred_ref;
>> Is this safe even if the checkpointed task's state changes ?
>> (e.g. unfrozen - yes, I know there is a patch in the works to
>> prevent this; but if we ever want to checkpoint STOPPED tasks...
>> for instance).
>>
>> Would incrementing the refcount on t->{cred,real_cred} help ?
> 
> Doesn't checkpoint_obj already do that through obj_new?
> 

No, it does not. There is a (potentially long) window of opportunity
between the callback invoked from checkpoint_obj() - where the pointer
is used, and when checkpoint_obj() later takes the extra reference.

See for comparison checkpoint_mm_obj(), it safely grabs the task->mm
(with a reference) around the invocation of checkpoint_obj().

Oren.