[PATCH net v2 1/3] class: add class_has_file_ns() helper function

[PATCH net v2 1/3] class: add class_has_file_ns() helper function

[Taehee Yoo]

The new helper function is to check 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>
---

v1 -> v2:
 - Implement class_has_file_ns() instead of class_find_and_get_file_ns().
 - Change headline.
 - Add kernel documentation comment.

 drivers/base/class.c         | 22 ++++++++++++++++++++++
 include/linux/device/class.h |  3 ++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/drivers/base/class.c b/drivers/base/class.c
index bcd410e6d70a..a2f2787f6aa7 100644
--- a/drivers/base/class.c
+++ b/drivers/base/class.c
@@ -105,6 +105,28 @@ void class_remove_file_ns(struct class *cls, const struct class_attribute *attr,
 		sysfs_remove_file_ns(&cls->p->subsys.kobj, &attr->attr, ns);
 }
 
+/**
+ * class_has_file_ns - check whether file is existing or not
+ * @cls: the compatibility class
+ * @name: name to look for
+ * @ns: the namespace tag to use
+ */
+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;
+}
+EXPORT_SYMBOL_GPL(class_has_file_ns);
+
 static struct class *class_get(struct class *cls)
 {
 	if (cls)
diff --git a/include/linux/device/class.h b/include/linux/device/class.h
index e8d470c457d1..b3d43658b201 100644
--- a/include/linux/device/class.h
+++ b/include/linux/device/class.h
@@ -209,7 +209,8 @@ extern int __must_check class_create_file_ns(struct class *class,
 extern void class_remove_file_ns(struct class *class,
 				 const struct class_attribute *attr,
 				 const void *ns);
-
+bool class_has_file_ns(struct class *cls, const char *name,
+		       const void *ns);
 static inline int __must_check class_create_file(struct class *class,
 					const struct class_attribute *attr)
 {
-- 
2.17.1

Re: [PATCH net v2 1/3] class: add class_has_file_ns() helper function

[Greg KH]

On Sat, Apr 04, 2020 at 02:18:27PM +0000, Taehee Yoo wrote:
> The new helper function is to check 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>
> ---
> 
> v1 -> v2:
>  - Implement class_has_file_ns() instead of class_find_and_get_file_ns().
>  - Change headline.
>  - Add kernel documentation comment.
> 
>  drivers/base/class.c         | 22 ++++++++++++++++++++++
>  include/linux/device/class.h |  3 ++-
>  2 files changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/base/class.c b/drivers/base/class.c
> index bcd410e6d70a..a2f2787f6aa7 100644
> --- a/drivers/base/class.c
> +++ b/drivers/base/class.c
> @@ -105,6 +105,28 @@ void class_remove_file_ns(struct class *cls, const struct class_attribute *attr,
>  		sysfs_remove_file_ns(&cls->p->subsys.kobj, &attr->attr, ns);
>  }
>  
> +/**
> + * class_has_file_ns - check whether file is existing or not
> + * @cls: the compatibility class
> + * @name: name to look for
> + * @ns: the namespace tag to use
> + */
> +bool class_has_file_ns(struct class *cls, const char *name,
> +		       const void *ns)

Why would you use this?  And what happens if the file shows up, or goes
away, instantly after this call is made?

This feels very broken.

greg k-h

Re: [PATCH net v2 1/3] class: add class_has_file_ns() helper function

[Taehee Yoo]

On Sun, 5 Apr 2020 at 00:50, Greg KH <gregkh@linuxfoundation.org> wrote:
>

Hi Greg,
Thank you for your review!

> On Sat, Apr 04, 2020 at 02:18:27PM +0000, Taehee Yoo wrote:
> > The new helper function is to check 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>
> > ---
> >
> > v1 -> v2:
> >  - Implement class_has_file_ns() instead of class_find_and_get_file_ns().
> >  - Change headline.
> >  - Add kernel documentation comment.
> >
> >  drivers/base/class.c         | 22 ++++++++++++++++++++++
> >  include/linux/device/class.h |  3 ++-
> >  2 files changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/base/class.c b/drivers/base/class.c
> > index bcd410e6d70a..a2f2787f6aa7 100644
> > --- a/drivers/base/class.c
> > +++ b/drivers/base/class.c
> > @@ -105,6 +105,28 @@ void class_remove_file_ns(struct class *cls, const struct class_attribute *attr,
> >               sysfs_remove_file_ns(&cls->p->subsys.kobj, &attr->attr, ns);
> >  }
> >
> > +/**
> > + * class_has_file_ns - check whether file is existing or not
> > + * @cls: the compatibility class
> > + * @name: name to look for
> > + * @ns: the namespace tag to use
> > + */
> > +bool class_has_file_ns(struct class *cls, const char *name,
> > +                    const void *ns)
>
> Why would you use this?  And what happens if the file shows up, or goes
> away, instantly after this call is made?
>
> This feels very broken.
>

Ah, I missed considering other usescases.
If other users don't use locks, this function would return incorrect
information. The problem seems to become from that this function
calls kernfs_put().

Thanks a lot!
Taehee Yoo