linux-kernel - Re: [RFC] [PATCH] cpuset operations causes Badness at mm/slab.c:777 warning

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Date:	Fri, 1 Jun 2007 13:43:04 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Paul Jackson <pj@....com>
cc:	srinivasa@...ibm.com, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	vatsa@...ibm.com, dino@...ibm.com, simon.derr@...l.net,
	clameter@...ulhu.engr.sgi.com, rientjes@...gle.com
Subject: Re: [RFC] [PATCH] cpuset operations causes Badness at mm/slab.c:777
 warning

On Fri, 1 Jun 2007, Paul Jackson wrote:

> > There are no checks necessary. Your function worked fine so far for 
> > the case of zero objects with the pointer returned by kmalloc. If the 
> > code is correct then it will not dereference the pointer to the zero 
> > sized array. If not then we may find a bug and fix it.
> 
> I suspect you got lucky.  The check for a full pidarray[] in the routine
> pid_array_load() occurs -after- a pid is put in the array.  If a task
> showed up in this cpuset at the wrong time, we would fall over and die
> in the code:

Then you are deferencing an element in the pidarray that you did not 
allocate! This is a bug in cpuset code. So we would need to allocate at 
mininum one array element? Or would we need to allocate npids + 1 to be 
safe???


---
 kernel/cpuset.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6/kernel/cpuset.c
===================================================================
--- linux-2.6.orig/kernel/cpuset.c	2007-06-01 13:41:24.000000000 -0700
+++ linux-2.6/kernel/cpuset.c	2007-06-01 13:42:08.000000000 -0700
@@ -1741,7 +1741,7 @@ static int cpuset_tasks_open(struct inod
 	 * show up until sometime later on.
 	 */
 	npids = atomic_read(&cs->count);
-	pidarray = kmalloc(npids * sizeof(pid_t), GFP_KERNEL);
+	pidarray = kmalloc(max(1, npids) * sizeof(pid_t), GFP_KERNEL);
 	if (!pidarray)
 		goto err1;
 
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