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

[José Pekkarinen]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > Signed-off-by: José Pekkarinen <jose.pekkarinen@xxxxxxxxxxx>
> > ---
> >  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.