linux-kernel - [patch 1/2] cpusets: disallow attaching kthreadd

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Message-ID: <alpine.DEB.2.00.1204050013440.9466@chino.kir.corp.google.com>
Date:	Thu, 5 Apr 2012 00:14:52 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Mike Galbraith <mgalbraith@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>
cc:	Tejun Heo <tj@...nel.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Menage <paul@...lmenage.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Li Zefan <lizefan@...wei.com>
Subject: [patch 1/2] cpusets: disallow attaching kthreadd

From: Mike Galbraith <efault@....de>

A worker thread spawned by kthreadd may acquire PF_THREAD_BOUND and never
be able to leave a cpuset, rending the cpuset immortal.  Save the user
some unexpected trouble, just say no.

Acked-by: Paul Menage <paul@...lmenage.org>
Signed-off-by: Mike Galbraith <efault@....de>
Signed-off-by: David Rientjes <rientjes@...gle.com>
---
 kernel/cpuset.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -53,6 +53,7 @@
 #include <linux/time.h>
 #include <linux/backing-dev.h>
 #include <linux/sort.h>
+#include <linux/kthread.h>
 
 #include <asm/uaccess.h>
 #include <linux/atomic.h>
@@ -1389,9 +1390,11 @@ static int cpuset_can_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
 		 * 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.
+		 * cpus_allowed may be changed.  We also disallow attaching
+		 * kthreadd to prevent a child from becoming trapped should it
+		 * later acquire PF_THREAD_BOUND.
 		 */
-		if (task->flags & PF_THREAD_BOUND)
+		if (task->flags & PF_THREAD_BOUND || task == kthreadd_task)
 			return -EINVAL;
 		if ((ret = security_task_setscheduler(task)))
 			return ret;
--
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