linux-kernel - Re: [PATCH] kobject: Avoid premature parent object freeing in kobject

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Date:   Mon, 3 Aug 2020 10:27:50 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Qu Wenruo <quwenruo.btrfs@....com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Guenter Roeck <guenter@...ck-us.net>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>
Subject: Re: [PATCH] kobject: Avoid premature parent object freeing in kobject_cleanup()

On Mon, Aug 3, 2020 at 10:25 AM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
> On Mon, Aug 3, 2020 at 9:47 AM Qu Wenruo <quwenruo.btrfs@....com> wrote:
> > On 2020/6/5 上午1:46, Rafael J. Wysocki wrote:

> > > +void kobject_del(struct kobject *kobj)
> > > +{
> > > +     struct kobject *parent = kobj->parent;
> > > +
> > > +     __kobject_del(kobj);
> > > +     kobject_put(parent);
> >
> > Could you please add an extra check on kobj before accessing kobj->parent?
>
> I do not understand. Where do we access it?
> kobject_put() is NULL-aware.

Ah, I see, now.

Should be something like
    struct kobject *parent = kobj ? kobj->parent : NULL;

> > This patch in fact removes the ability to call kobject_del() on NULL
> > pointer while not cause anything wrong.
> >
> > I know this is not a big deal, but such behavior change has already
> > caused some problem for the incoming btrfs code.
> > (Now I feels guilty just by looking into the old
> > kobject_del()/kobject_put() and utilize that feature in btrfs)
> >
> > Since the old kobject_del() accepts NULL pointer intentionally, it would
> > be much better to keep such behavior.


-- 
With Best Regards,
Andy Shevchenko