[PATCH net-next 01/10] sysfs: add sysfs_file_change_owner()

[Christian Brauner <christian.brauner@ubuntu.com>]

Add a helper to change the owner of a sysfs file.
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 <christian.brauner@ubuntu.com>
---
 fs/sysfs/file.c       | 46 +++++++++++++++++++++++++++++++++++++++++++
 include/linux/sysfs.h |  7 +++++++
 2 files changed, 53 insertions(+)

diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 130fc6fbcc03..007b97ca8165 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -558,3 +558,49 @@ 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 - change owner of a file.
+ *	@kobj:	object.
+ *	@name:	name of the file to change.
+ *	        can be NULL to change current file.
+ */
+int sysfs_file_change_owner(struct kobject *kobj, const char *name)
+{
+	struct kernfs_node *kn;
+	int error;
+
+	if (!kobj->state_in_sysfs)
+		return -EINVAL;
+
+	if (name) {
+		kn = kernfs_find_and_get(kobj->sd, name);
+	} else {
+		kernfs_get(kobj->sd);
+		kn = kobj->sd;
+	}
+	if (!kn)
+		return -ENOENT;
+
+	error = internal_change_owner(kn, kobj);
+
+	kernfs_put(kn);
+
+	return error;
+}
+EXPORT_SYMBOL_GPL(sysfs_file_change_owner);
diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
index fa7ee503fb76..58fa71d47c7f 100644
--- a/include/linux/sysfs.h
+++ b/include/linux/sysfs.h
@@ -310,6 +310,8 @@ static inline void sysfs_enable_ns(struct kernfs_node *kn)
 	return kernfs_enable_ns(kn);
 }
 
+int sysfs_file_change_owner(struct kobject *kobj, const char *name);
+
 #else /* CONFIG_SYSFS */
 
 static inline int sysfs_create_dir_ns(struct kobject *kobj, const void *ns)
@@ -522,6 +524,11 @@ static inline void sysfs_enable_ns(struct kernfs_node *kn)
 {
 }
 
+static inline int sysfs_file_change_owner(struct kobject *kobj, const char *name)
+{
+	return 0;
+}
+
 #endif /* CONFIG_SYSFS */
 
 static inline int __must_check sysfs_create_file(struct kobject *kobj,
-- 
2.25.0