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Subject: Re: [PATCH] kobject: Avoid premature parent object freeing in
 kobject_cleanup()
To:     Andy Shevchenko <andy.shevchenko@gmail.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>
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From:   Qu Wenruo <quwenruo.btrfs@gmx.com>
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Message-ID: <703338a4-4806-f919-9ed8-b06749b748b4@gmx.com>
Date:   Mon, 3 Aug 2020 15:33:58 +0800
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On 2020/8/3 =E4=B8=8B=E5=8D=883:27, Andy Shevchenko wrote:
> 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> wrot=
e:
>>> On 2020/6/5 =E4=B8=8A=E5=8D=881:46, Rafael J. Wysocki wrote:
>=20
>>>> +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->pa=
rent?
>>
>> I do not understand. Where do we access it?
>> kobject_put() is NULL-aware.
>=20
> Ah, I see, now.
>=20
> Should be something like
>     struct kobject *parent =3D kobj ? kobj->parent : NULL;

Exactly.

Thanks,
Qu

>=20
>>> 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 wo=
uld
>>> be much better to keep such behavior.
>=20
>=20


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