linux-kernel - Re: Attaching PID 0 to a cgroup

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Message-ID: <20080701094734.GE3925@linux.vnet.ibm.com>
Date:	Tue, 1 Jul 2008 15:17:34 +0530
From:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>
To:	Paul Menage <menage@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <balbir@...ibm.com>,
	Sudhir Kumar <skumar@...ux.vnet.ibm.com>,
	lkml <linux-kernel@...r.kernel.org>, containers@...ts.osdl.org
Subject: Re: Attaching PID 0 to a cgroup

[put in the wrong alias for containers list correcting it.]

On Tue, Jul 01, 2008 at 03:15:45PM +0530, Dhaval Giani wrote:
> Hi Paul,
> 
> Attaching PID 0 to a cgroup caused the current task to be attached to
> the cgroup. Looking at the code,
> 
>         if (pid) {
>                 rcu_read_lock();
>                 tsk = find_task_by_vpid(pid);
>                 if (!tsk || tsk->flags & PF_EXITING) {
>                         rcu_read_unlock();
>                         return -ESRCH;
>                 }
>                 get_task_struct(tsk);
>                 rcu_read_unlock();
> 
>                 if ((current->euid) && (current->euid != tsk->uid)
>                     && (current->euid != tsk->suid)) {
>                         put_task_struct(tsk);
>                         return -EACCES;
>                 }
>         } else {
>                 tsk = current;
>                 get_task_struct(tsk);
>         }
> 
> I was wondering, why this was done. It seems to be unexpected behavior.
> Wouldn't something like the following be a better response? (I've used
> EINVAL, but I can change it to ESRCH if that is better.)
> 
> ---
> cgroups: Don't allow PID 0 to be attached to a group
> 
> Currently when one trys to attach PID 0 to a cgroup, it attaches
> the current task. That is not expected behavior. It should return
> an error instead.
> 
> Signed-off-by: Dhaval Giani <dhaval@...ux.vnet.ibm.com>
> 
> Index: linux-2.6/kernel/cgroup.c
> ===================================================================
> --- linux-2.6.orig/kernel/cgroup.c
> +++ linux-2.6/kernel/cgroup.c
> @@ -1309,8 +1309,7 @@ static int attach_task_by_pid(struct cgr
>  			return -EACCES;
>  		}
>  	} else {
> -		tsk = current;
> -		get_task_struct(tsk);
> +			return -EINVAL;
>  	}
>  
>  	ret = cgroup_attach_task(cgrp, tsk);
> -- 
> regards,
> Dhaval

-- 
regards,
Dhaval
--
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