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Subject: Re: [PATCH 00/11] fix memory leak while kset_register() fails
To:     Greg KH <gregkh@linuxfoundation.org>
CC:     Luben Tuikov <luben.tuikov@amd.com>,
        <linux-kernel@vger.kernel.org>, <qemu-devel@nongnu.org>,
        <linux-f2fs-devel@lists.sourceforge.net>,
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        <rafael@kernel.org>, <somlo@cmu.edu>, <mst@redhat.com>,
        <jaegeuk@kernel.org>, <chao@kernel.org>,
        <hsiangkao@linux.alibaba.com>, <huangjianan@oppo.com>,
        <mark@fasheh.com>, <jlbec@evilplan.org>,
        <joseph.qi@linux.alibaba.com>, <akpm@linux-foundation.org>,
        <alexander.deucher@amd.com>, <richard@nod.at>,
        <liushixin2@huawei.com>
References: <20221021022102.2231464-1-yangyingliang@huawei.com>
 <d559793a-0ce4-3384-e74e-19855aa31f31@amd.com> <Y1IwLOUGayjT9p6d@kroah.com>
 <0591e66f-731a-5f81-fc9d-3a6d80516c65@huawei.com>
 <Y1JZ9IUPL6jZIQ8E@kroah.com>
From:   Yang Yingliang <yangyingliang@huawei.com>
Message-ID: <1f3aa2ac-fba6-dc7a-d01d-7dd5331c8dc5@huawei.com>
Date:   Fri, 21 Oct 2022 17:12:37 +0800
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On 2022/10/21 16:36, Greg KH wrote:
> On Fri, Oct 21, 2022 at 04:24:23PM +0800, Yang Yingliang wrote:
>> On 2022/10/21 13:37, Greg KH wrote:
>>> On Fri, Oct 21, 2022 at 01:29:31AM -0400, Luben Tuikov wrote:
>>>> On 2022-10-20 22:20, Yang Yingliang wrote:
>>>>> The previous discussion link:
>>>>> https://lore.kernel.org/lkml/0db486eb-6927-927e-3629-958f8f211194@huawei.com/T/
>>>> The very first discussion on this was here:
>>>>
>>>> https://www.spinics.net/lists/dri-devel/msg368077.html
>>>>
>>>> Please use this link, and not the that one up there you which quoted above,
>>>> and whose commit description is taken verbatim from the this link.
>>>>
>>>>> kset_register() is currently used in some places without calling
>>>>> kset_put() in error path, because the callers think it should be
>>>>> kset internal thing to do, but the driver core can not know what
>>>>> caller doing with that memory at times. The memory could be freed
>>>>> both in kset_put() and error path of caller, if it is called in
>>>>> kset_register().
>>>> As I explained in the link above, the reason there's
>>>> a memory leak is that one cannot call kset_register() without
>>>> the kset->kobj.name being set--kobj_add_internal() returns -EINVAL,
>>>> in this case, i.e. kset_register() fails with -EINVAL.
>>>>
>>>> Thus, the most common usage is something like this:
>>>>
>>>> 	kobj_set_name(&kset->kobj, format, ...);
>>>> 	kset->kobj.kset = parent_kset;
>>>> 	kset->kobj.ktype = ktype;
>>>> 	res = kset_register(kset);
>>>>
>>>> So, what is being leaked, is the memory allocated in kobj_set_name(),
>>>> by the common idiom shown above. This needs to be mentioned in
>>>> the documentation, at least, in case, in the future this is absolved
>>>> in kset_register() redesign, etc.
>>> Based on this, can kset_register() just clean up from itself when an
>>> error happens?  Ideally that would be the case, as the odds of a kset
>>> being embedded in a larger structure is probably slim, but we would have
>>> to search the tree to make sure.
>> I have search the whole tree, the kset used in bus_register() - patch #3,
>> kset_create_and_add() - patch #4
>> __class_register() - patch #5,  fw_cfg_build_symlink() - patch #6 and
>> amdgpu_discovery.c - patch #10
>> is embedded in a larger structure. In these cases, we can not call
>> kset_put() in error path in kset_register()
> Yes you can as the kobject in the kset should NOT be controling the
> lifespan of those larger objects.
Read through the code the only leak in this case is the name, so can we 
free it
directly in kset_register():

--- a/lib/kobject.c
+++ b/lib/kobject.c
@@ -844,8 +844,11 @@ int kset_register(struct kset *k)

         kset_init(k);
         err = kobject_add_internal(&k->kobj);
-       if (err)
+       if (err) {
+               kfree_const(k->kobj.name);
+               k->kobj.name = NULL;
                 return err;
+       }
         kobject_uevent(&k->kobj, KOBJ_ADD);
         return 0;
  }

or unset ktype of kobject, then call kset_put():

--- a/lib/kobject.c
+++ b/lib/kobject.c
@@ -844,8 +844,11 @@ int kset_register(struct kset *k)

         kset_init(k);
         err = kobject_add_internal(&k->kobj);
-       if (err)
+       if (err) {
+               k->kobj.ktype = NULL;
+               kset_put(k);
                 return err;
+       }
         kobject_uevent(&k->kobj, KOBJ_ADD);
         return 0;
  }

>
> If it is, please point out the call chain here as I don't think that
> should be possible.
>
> Note all of this is a mess because the kobject name stuff was added much
> later, after the driver model had been created and running for a while.
> We missed this error path when adding the dynamic kobject name logic,
> thank for looking into this.
>
> If you could test the patch posted with your error injection systems,
> that could make this all much simpler to solve.
>
> thanks,
>
> greg k-h
> .