Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <1908555.IiAGLGrh1Z@kreacher> <d1e90fa3-d978-90ae-a015-288139be3450@gmx.com>
 <CAHp75VcbSR1NSSPemg5dMyfp00uC4wkktVjKSFx4sjSgFC-_vQ@mail.gmail.com>
In-Reply-To: <CAHp75VcbSR1NSSPemg5dMyfp00uC4wkktVjKSFx4sjSgFC-_vQ@mail.gmail.com>
From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Mon, 3 Aug 2020 10:27:50 +0300
Message-ID: <CAHp75Ve0AozEbB_kc+S4qaZJxJJWf20toDK-T2QHYi1o2hfJBg@mail.gmail.com>
Subject: Re: [PATCH] kobject: Avoid premature parent object freeing in kobject_cleanup()
To:     Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc:     "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Heikki Krogerus <heikki.krogerus@linux.intel.com>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux ACPI <linux-acpi@vger.kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Guenter Roeck <guenter@roeck-us.net>,
        "linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Aug 3, 2020 at 10:25 AM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
> On Mon, Aug 3, 2020 at 9:47 AM Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:
> > On 2020/6/5 =E4=B8=8A=E5=8D=881:46, Rafael J. Wysocki wrote:

> > > +void kobject_del(struct kobject *kobj)
> > > +{
> > > +     struct kobject *parent =3D kobj->parent;
> > > +
> > > +     __kobject_del(kobj);
> > > +     kobject_put(parent);
> >
> > Could you please add an extra check on kobj before accessing kobj->pare=
nt?
>
> 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 =3D 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 woul=
d
> > be much better to keep such behavior.


--=20
With Best Regards,
Andy Shevchenko