linux-kernel - Re: [-mm3 PATCH] (Retry) Check the return value of kobject

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Message-ID: <20070405170514.481b765f@gondolin.boeblingen.de.ibm.com>
Date:	Thu, 5 Apr 2007 17:05:14 +0200
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	WANG Cong <xiyou.wangcong@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	viro@...iv.linux.org.uk
Subject: Re: [-mm3 PATCH] (Retry) Check the return value of kobject_add and
 etc.

On Thu, 5 Apr 2007 22:44:09 +0800,
WANG Cong <xiyou.wangcong@...il.com> wrote:

> On Thu, Apr 05, 2007 at 11:11:42AM +0200, Cornelia Huck wrote:
> >On Thu, 5 Apr 2007 12:54:11 +0800,
> >WANG Cong <xiyou.wangcong@...il.com> wrote:
> >
> >> --- linux-2.6.21-rc5-mm3/fs/partitions/check.c.orig	2007-03-30 21:35:45.000000000 +0800
> >> +++ linux-2.6.21-rc5-mm3/fs/partitions/check.c	2007-04-02 21:29:02.000000000 +0800
> >> @@ -385,10 +385,18 @@ void add_partition(struct gendisk *disk,
> >>  	p->kobj.parent = &disk->kobj;
> >>  	p->kobj.ktype = &ktype_part;
> >>  	kobject_init(&p->kobj);
> >> -	kobject_add(&p->kobj);
> >> +	if (kobject_add(&p->kobj)) {
> >> +		kfree(p);
> >> +		return;
> >> +	}
> >>  	if (!disk->part_uevent_suppress)
> >>  		kobject_uevent(&p->kobj, KOBJ_ADD);
> >> -	sysfs_create_link(&p->kobj, &block_subsys.kset.kobj, "subsystem");
> >> +	if (sysfs_create_link(&p->kobj, &block_subsys.kset.kobj, "subsystem")) {
> >> +		kobject_uevent(&p->kobj, KOBJ_REMOVE);
> >> +		kobject_del(&p->kobj);
> >> +		kfree(p);
> >
> >You should use kobject_put instead of kfree, since someone could have
> >(theoretically) obtained a reference on the object after the
> >kobject_add. (Or just use kobject_unregister, if the delete uevent
> >isn't suppressed anyway.)
> >
> 
> I can't understand that. The memory pointed by 'p' is obtained via kmalloc(), _not_ kobject_get. How can we use kobject_put instead? 

The hd_struct *p embeds a kobject kobj (which, amongst other things,
provides reference counting for the hd_struct). The release function
for the kobject will free the embedding object (note that kobject_init
sets the reference count to 1). That means, if kobject_put(&p->kobj)
will drop the reference to 0, the release function (part_release) will
free p. (See also Documentation/kobject.txt.)
-
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