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From: Paul Menage <paul@paulmenage.org>
Date: Tue, 18 Oct 2011 21:57:47 -0700
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Subject: Re: patch] cpusets, cgroups: disallow attaching kthreadd
To: Mike Galbraith <efault@gmx.de>
Cc: LKML <linux-kernel@vger.kernel.org>, Tejun Heo <htejun@gmail.com>,
        Li Zefan <lizf@cn.fujitsu.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        David Rientjes <rientjes@google.com>
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On Tue, Oct 18, 2011 at 1:10 AM, Mike Galbraith <efault@gmx.de> wrote:
>
> Save the user some unexpected trouble, just say no.
>
> Signed-off-by: Mike Galbraith <efault@gmx.de>

Acked-by: Paul Menage <paul@paulmenage.org>

>
> ---
>  kernel/cpuset.c |    6 ++++--
>  kernel/sched.c  |   10 ++++++++++
>  2 files changed, 14 insertions(+), 2 deletions(-)
>
> Index: linux-3.0-tip/kernel/cpuset.c
> ===================================================================
> --- linux-3.0-tip.orig/kernel/cpuset.c
> +++ linux-3.0-tip/kernel/cpuset.c
> @@ -59,6 +59,7 @@
>  #include <linux/mutex.h>
>  #include <linux/workqueue.h>
>  #include <linux/cgroup.h>
> +#include <linux/kthread.h>
>
>  /*
>  * Workqueue for cpuset related tasks.
> @@ -1382,9 +1383,10 @@ static int cpuset_can_attach(struct cgro
>         * set of allowed nodes is unnecessary.  Thus, cpusets are not
>         * applicable for such threads.  This prevents checking for success of
>         * set_cpus_allowed_ptr() on all attached tasks before cpus_allowed may
> -        * be changed.
> +        * be changed.  We also disallow attaching kthreadd, to prevent it's
> +        * child from becoming trapped should it then acquire PF_THREAD_BOUND.
>         */
> -       if (tsk->flags & PF_THREAD_BOUND)
> +       if (tsk->flags & PF_THREAD_BOUND || tsk == kthreadd_task)
>                return -EINVAL;
>
>        return 0;
> Index: linux-3.0-tip/kernel/sched.c
> ===================================================================
> --- linux-3.0-tip.orig/kernel/sched.c
> +++ linux-3.0-tip/kernel/sched.c
> @@ -71,6 +71,7 @@
>  #include <linux/ctype.h>
>  #include <linux/ftrace.h>
>  #include <linux/slab.h>
> +#include <linux/kthread.h>
>
>  #include <asm/tlb.h>
>  #include <asm/irq_regs.h>
> @@ -9141,6 +9142,15 @@ cpu_cgroup_destroy(struct cgroup_subsys
>  static int
>  cpu_cgroup_can_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
>  {
> +       /*
> +        * kthreadd can fork workers for an RT workqueue in a cgroup
> +        * which may or may not have rt_runtime allocated.  Just say no,
> +        * as attaching a global resource to a non-root group  doesn't
> +        * make any sense anyway.
> +        */
> +       if (tsk == kthreadd_task)
> +               return -EINVAL;
> +
>  #ifdef CONFIG_RT_GROUP_SCHED
>        if (!sched_rt_can_attach(cgroup_tg(cgrp), tsk))
>                return -EINVAL;
>
>
>