linux-kernel - Re: [PATCH] drivers core: lookup sysfs power group before removal

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Date:   Fri, 03 Nov 2023 19:49:39 +0200
From:   José Pekkarinen <jose.pekkarinen@...hound.fi>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     rafael@...nel.org, len.brown@...el.com, pavel@....cz,
        skhan@...uxfoundation.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+95f2e2439b97575ec3c0@...kaller.appspotmail.com
Subject: Re: [PATCH] drivers core: lookup sysfs power group before removal

On 2023-11-01 19:54, Greg KH wrote:
> On Wed, Nov 01, 2023 at 07:36:27PM +0200, José Pekkarinen wrote:
>> Hinted by syzboot, there is a few cases where the sysfs power group 
>> may
>> not be there, like the failure while adding it, or adding its runtime
>> group, or when the sysfs firmware loader fallback fail to populate. In
>> the last case, the device_del function will be called leading to 
>> attempt
>> to remove the sysfs group. This patch will lookup for it in advance to
>> grant that it is effectively there before cleaning it up.
>> 
>> Reported-by: syzbot+95f2e2439b97575ec3c0@...kaller.appspotmail.com
>> 
>> Signed-off-by: José Pekkarinen <jose.pekkarinen@...hound.fi>
>> ---
>>  drivers/base/power/sysfs.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/base/power/sysfs.c b/drivers/base/power/sysfs.c
>> index a1474fb67db9..6601729c4698 100644
>> --- a/drivers/base/power/sysfs.c
>> +++ b/drivers/base/power/sysfs.c
>> @@ -834,5 +834,7 @@ void dpm_sysfs_remove(struct device *dev)
>>  	dev_pm_qos_constraints_destroy(dev);
>>  	rpm_sysfs_remove(dev);
>>  	sysfs_unmerge_group(&dev->kobj, &pm_wakeup_attr_group);
>> -	sysfs_remove_group(&dev->kobj, &pm_attr_group);
>> +
>> +	if (kernfs_find_and_get((&dev->kobj)->sd, pm_attr_group.name))
>> +		sysfs_remove_group(&dev->kobj, &pm_attr_group);
> 
> What's to keep it from going away right after finding it?
> 
> In other words, what is wrong with removing a group that is not there?
> What error happens?  It should be fine, or are you seeing real code
> failures somewhere?

     No, this is just hitting a warning that sysbot complains about by
setting panic on warning, no big deal, though it can be a wrong 
behaviour
in ueagle-atm driver, since it defines to disconnect the device if the
firmware is not there, no matter the sysfs fallback.

> Also, I think you just leaked a reference count here, how was this
> tested?

     Both by setting up a local vm following syzkaller instructions and
using the syzkaller itself to test it. You can take a look in the link
if you feel like it.

     José.