Add helpers to change owner of a sysfs files.
The ownership of a sysfs object is determined based on the ownership of
the corresponding kobject, i.e. only if the ownership of a kobject is
changed will this function change the ownership of the corresponding
sysfs entry.
This function will be used to correctly account for kobject ownership
changes, e.g. when moving network devices between network namespaces.

Signed-off-by: Christian Brauner <redacted>
---
/* v2 */
-  Greg Kroah-Hartman [off-list ref]:
   - Better naming for sysfs_file_change_owner() to reflect the fact that it
     can be used to change the owner of the kobject itself by passing NULL as
     argument.
- Christian Brauner [off-list ref]:
  - Split sysfs_file_change_owner() into two helpers sysfs_change_owner() and
    sysfs_change_owner_by_name(). The former changes the owner of the kobject
    itself, the latter the owner of the kobject looked up via the name
    argument.
---
 fs/sysfs/file.c       | 82 +++++++++++++++++++++++++++++++++++++++++++
 include/linux/sysfs.h | 14 ++++++++
 2 files changed, 96 insertions(+)

diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 130fc6fbcc03..8f2607de2456 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c

@@ -558,3 +558,85 @@ void sysfs_remove_bin_file(struct kobject *kobj,
 	kernfs_remove_by_name(kobj->sd, attr->attr.name);
 }
 EXPORT_SYMBOL_GPL(sysfs_remove_bin_file);
+
+static int internal_change_owner(struct kernfs_node *kn, struct kobject *kobj)
+{
+	kuid_t uid;
+	kgid_t gid;
+	struct iattr newattrs = {
+		.ia_valid = ATTR_UID | ATTR_GID,
+	};
+
+	kobject_get_ownership(kobj, &uid, &gid);
+	newattrs.ia_uid = uid;
+	newattrs.ia_gid = gid;
+
+	return kernfs_setattr(kn, &newattrs);
+}
+
+/**
+ *	sysfs_file_change_owner_by_name - change owner of a file.
+ *	@kobj:	object.
+ *	@name:	name of the file to change.
+ *
+ * To change the ownership of a sysfs object, the caller must first change the
+ * uid/gid of the kobject and then call this function.