v4.19.13 snapshot.
diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
new file mode 100644
index 0000000..92682fc
--- /dev/null
+++ b/fs/sysfs/mount.c
@@ -0,0 +1,79 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * fs/sysfs/symlink.c - operations for initializing and mounting sysfs
+ *
+ * Copyright (c) 2001-3 Patrick Mochel
+ * Copyright (c) 2007 SUSE Linux Products GmbH
+ * Copyright (c) 2007 Tejun Heo <teheo@suse.de>
+ *
+ * Please see Documentation/filesystems/sysfs.txt for more information.
+ */
+
+#include <linux/fs.h>
+#include <linux/magic.h>
+#include <linux/mount.h>
+#include <linux/init.h>
+#include <linux/user_namespace.h>
+
+#include "sysfs.h"
+
+static struct kernfs_root *sysfs_root;
+struct kernfs_node *sysfs_root_kn;
+
+static struct dentry *sysfs_mount(struct file_system_type *fs_type,
+ int flags, const char *dev_name, void *data)
+{
+ struct dentry *root;
+ void *ns;
+ bool new_sb = false;
+
+ if (!(flags & SB_KERNMOUNT)) {
+ if (!kobj_ns_current_may_mount(KOBJ_NS_TYPE_NET))
+ return ERR_PTR(-EPERM);
+ }
+
+ ns = kobj_ns_grab_current(KOBJ_NS_TYPE_NET);
+ root = kernfs_mount_ns(fs_type, flags, sysfs_root,
+ SYSFS_MAGIC, &new_sb, ns);
+ if (!new_sb)
+ kobj_ns_drop(KOBJ_NS_TYPE_NET, ns);
+ else if (!IS_ERR(root))
+ root->d_sb->s_iflags |= SB_I_USERNS_VISIBLE;
+
+ return root;
+}
+
+static void sysfs_kill_sb(struct super_block *sb)
+{
+ void *ns = (void *)kernfs_super_ns(sb);
+
+ kernfs_kill_sb(sb);
+ kobj_ns_drop(KOBJ_NS_TYPE_NET, ns);
+}
+
+static struct file_system_type sysfs_fs_type = {
+ .name = "sysfs",
+ .mount = sysfs_mount,
+ .kill_sb = sysfs_kill_sb,
+ .fs_flags = FS_USERNS_MOUNT,
+};
+
+int __init sysfs_init(void)
+{
+ int err;
+
+ sysfs_root = kernfs_create_root(NULL, KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK,
+ NULL);
+ if (IS_ERR(sysfs_root))
+ return PTR_ERR(sysfs_root);
+
+ sysfs_root_kn = sysfs_root->kn;
+
+ err = register_filesystem(&sysfs_fs_type);
+ if (err) {
+ kernfs_destroy_root(sysfs_root);
+ return err;
+ }
+
+ return 0;
+}