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

[Taehee Yoo <ap420073@gmail.com>]

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