[RESEND,net,1/3] class: add class_find_and_get_file_ns() helper function

The new helper function is to find and get a class file.
This function is useful for checking whether the class file is existing
or not. This function will be used by networking stack to
check "/sys/class/net/*" file.

Reported-by: syzbot+830c6dbfc71edc4f0b8f@syzkaller.appspotmail.com
Fixes: b76cdba9cdb2 ("[PATCH] bonding: add sysfs functionality to bonding (large)")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
---
 drivers/base/class.c         | 12 ++++++++++++
 include/linux/device/class.h |  4 +++-
 2 files changed, 15 insertions(+), 1 deletion(-)

On Tue, Mar 24, 2020 at 02:17:22PM +0000, Taehee Yoo wrote:
> The new helper function is to find and get a class file.
> This function is useful for checking whether the class file is existing
> or not. This function will be used by networking stack to
> check "/sys/class/net/*" file.
> 
> Reported-by: syzbot+830c6dbfc71edc4f0b8f@syzkaller.appspotmail.com
> Fixes: b76cdba9cdb2 ("[PATCH] bonding: add sysfs functionality to bonding (large)")
> Signed-off-by: Taehee Yoo <ap420073@gmail.com>
> ---
>  drivers/base/class.c         | 12 ++++++++++++
>  include/linux/device/class.h |  4 +++-
>  2 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/base/class.c b/drivers/base/class.c
> index bcd410e6d70a..dedf41f32f0d 100644
> --- a/drivers/base/class.c
> +++ b/drivers/base/class.c
> @@ -105,6 +105,17 @@ void class_remove_file_ns(struct class *cls, const struct class_attribute *attr,
>  		sysfs_remove_file_ns(&cls->p->subsys.kobj, &attr->attr, ns);
>  }
>  
> +struct kernfs_node *class_find_and_get_file_ns(struct class *cls,
> +					       const char *name,
> +					       const void *ns)
> +{
> +	struct kernfs_node *kn = NULL;
> +
> +	if (cls)
> +		kn = kernfs_find_and_get_ns(cls->p->subsys.kobj.sd, name, ns);
> +	return kn;
> +}
> +

You can put the EXPORT_SYMBOL_GPL() under here.

And can you document what this function actually is in some kerneldoc?

But, returning a kernfs_node from a driver core is _REALLY_ odd.  Why do
you need this and who cares about kernfs here?

thanks,

greg k-h

On Wed, 25 Mar 2020 at 00:44, Greg KH <gregkh@linuxfoundation.org> wrote:
>

Hi Greg,
Thank you for the review!

> On Tue, Mar 24, 2020 at 02:17:22PM +0000, Taehee Yoo wrote:
> > The new helper function is to find and get a class file.
> > This function is useful for checking whether the class file is existing
> > or not. This function will be used by networking stack to
> > check "/sys/class/net/*" file.
> >
> > Reported-by: syzbot+830c6dbfc71edc4f0b8f@syzkaller.appspotmail.com
> > Fixes: b76cdba9cdb2 ("[PATCH] bonding: add sysfs functionality to bonding (large)")
> > Signed-off-by: Taehee Yoo <ap420073@gmail.com>
> > ---
> >  drivers/base/class.c         | 12 ++++++++++++
> >  include/linux/device/class.h |  4 +++-
> >  2 files changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/base/class.c b/drivers/base/class.c
> > index bcd410e6d70a..dedf41f32f0d 100644
> > --- a/drivers/base/class.c
> > +++ b/drivers/base/class.c
> > @@ -105,6 +105,17 @@ void class_remove_file_ns(struct class *cls, const struct class_attribute *attr,
> >               sysfs_remove_file_ns(&cls->p->subsys.kobj, &attr->attr, ns);
> >  }
> >
> > +struct kernfs_node *class_find_and_get_file_ns(struct class *cls,
> > +                                            const char *name,
> > +                                            const void *ns)
> > +{
> > +     struct kernfs_node *kn = NULL;
> > +
> > +     if (cls)
> > +             kn = kernfs_find_and_get_ns(cls->p->subsys.kobj.sd, name, ns);
> > +     return kn;
> > +}
> > +
>
> You can put the EXPORT_SYMBOL_GPL() under here.
>

Okay, I will change this.

> And can you document what this function actually is in some kerneldoc?
>

Thanks, I will add some kernel-doc comment.

> But, returning a kernfs_node from a driver core is _REALLY_ odd.  Why do
> you need this and who cares about kernfs here?
>

I fully agree with that.
My previous version of this function was here:

bool class_has_file_ns(struct class *cls, const char *name,
                       const void *ns)
{
        struct kernfs_node *kn = NULL;

        if (cls) {
                kn = kernfs_find_and_get_ns(cls->p->subsys.kobj.sd, name, ns);
                if (kn) {
                        kernfs_put(kn);
                        return true;
                }
        }
        return false;
}

I wanted this function could be used in general cases.
But I thought this function couldn't be used in general cases.
So, I made class_find_and_get_file_ns() but I couldn't find
a more appropriate return type.
I think I'd rather to use class_has_file_ns() instead of
class_find_and_get_file_ns() because of an awkward return type.
How do you think about it?

> thanks,
>
> greg k-h

Thanks a lot,
Taehee Yoo