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

[Greg KH <gregkh@linuxfoundation.org>]

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@syzkaller.appspotmail.com
> 
> Signed-off-by: José Pekkarinen <jose.pekkarinen@foxhound.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?

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

thanks,

greg k-h