linux-kernel - Re: [PATCH] Mark kobjects as unitialized

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Date:	Sun, 9 Mar 2008 12:51:15 +0530
From:	Balaji Rao <balajirrao@...il.com>
To:	Greg KH <gregkh@...e.de>
Cc:	Mikael Pettersson <mikpe@...uu.se>,
	kvm-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Mark kobjects as unitialized

On Sunday 09 March 2008 12:33:07 pm Greg KH wrote:
<snip>
> > > > Hi,
> > > > 
> > > > This patch does not fix it all! The problem is in fact more involved. I also get these BUG reports when I reload 
> > kvm-intel.
> > > > 
> > > > BUG kmalloc-8: Object already free
> > > > [  74.696570] -----------------------------------------------------------------------------
> > > > [   74.696596] 
> > > > [   74.697310] INFO: Allocated in strndup_user+0x30/0x62 age=587 cpu=2 pid=1439
> > > > [   74.697845] INFO: Freed in kobject_set_name_vargs+0x29/0x32 age=559 cpu=3 pid=1439
> > > > [   74.698008] INFO: Slab 0xc16f93a0 used=10 fp=0xf7c9d2d8 flags=0x10000c3
> > > > [   74.698008] INFO: Object 0xf7c9d1f8 @offset=504 fp=0xf7c9d578
> > > > 
> > > > This happens because, sysdev_class_register assigns a name to the
> > > > kobject, and kfrees the old name if any. The poisoned 'name' object
> > > > persists in case of statically allocated kobjects and as its passed to
> > > > kfree again when re registered, we get the above warning.
> > > > 
> > > > So, AFAICS the best way to solve this is by fixing the kobject users
> > > > (kvm, oprofile etc.) to use dynamic kobjects instead of static ones or
> > > > memset the kobject to zero before passing it to sysdev_register.
> > > 
> > > I like the memset idea, how about this patch instead?
> > > 
> > > thanks,
> > > 
> > > greg k-h
> > > 
> > > --- a/drivers/base/sys.c
> > > +++ b/drivers/base/sys.c
> > > @@ -133,6 +133,7 @@ int sysdev_class_register(struct sysdev_
> > >  	pr_debug("Registering sysdev class '%s'\n",
> > >  		 kobject_name(&cls->kset.kobj));
> > >  	INIT_LIST_HEAD(&cls->drivers);
> > > +	memset(&cls->kset.kobj, 0x00, sizeof(struct kobject));
> > >  	cls->kset.kobj.parent = &system_kset->kobj;
> > >  	cls->kset.kobj.ktype = &ktype_sysdev_class;
> > >  	cls->kset.kobj.kset = system_kset;
> > > 
> > 
> > This should work.But I am afraid if we zeroed it, the kfree poison
> > test wouldn't catch even the genuine cases. Isn't it ?  CMIIW.
> 
> What "genuine cases"?  kobjects should always be initialized to zero
> before they are registered.
> 
The cases in which a freed kobject is passed to the register function ? Probably they will be caught when do a memset..
> > A better fix according to me would be to zero the kobject in the
> > places where we know it is being re-registered (kvm, oprofile) etc.
> > This should do for now. But we should fix the sys_device users later,
> > for the next cycle.
> 
> Are you sure you know all of the sysdev_class objects that can be
> re-registered?
> 
Hmm.. Right. No, I only know that KVM and Oprofile use it.
> Can you test this patch out?
> 
Yes the idea works. One more memset is needed in sysdev_register. Here's the final patch.

diff --git a/drivers/base/sys.c b/drivers/base/sys.c
index 2f79c55..7c839d9 100644
--- a/drivers/base/sys.c
+++ b/drivers/base/sys.c
@@ -133,6 +133,7 @@ int sysdev_class_register(struct sysdev_class * cls)
        pr_debug("Registering sysdev class '%s'\n",
                 kobject_name(&cls->kset.kobj));
        INIT_LIST_HEAD(&cls->drivers);
+       memset(&cls->kset.kobj, 0x00, sizeof(struct kobject));
        cls->kset.kobj.parent = &system_kset->kobj;
        cls->kset.kobj.ktype = &ktype_sysdev_class;
        cls->kset.kobj.kset = system_kset;
@@ -227,6 +228,7 @@ int sysdev_register(struct sys_device * sysdev)
 
        pr_debug("Registering sys device '%s'\n", kobject_name(&sysdev->kobj));
 
+       memset(&sysdev->kobj, 0x00, sizeof(struct kobject));
        /* Make sure the kset is set */
        sysdev->kobj.kset = &cls->kset;

-- 
regards,
balaji rao
3rd year,
Dept. of Mechanical Engineering,
National Institute of Technology, Karnataka
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