VFS: Add GPL_EXPORTED function vfs_kern_mount()

do_kern_mount() does not allow the kernel to use private mount interfaces
without exposing the same interfaces to userland. The problem is that the
filesystem is referenced by name, thus meaning that it and its mount
interface must be registered in the global filesystem list.

vfs_kern_mount() passes the struct file_system_type as an explicit
parameter in order to overcome this limitation.

Signed-off-by: Trond Myklebust <[e-mail]>

fs/super.c

···
 EXPORT_SYMBOL(get_sb_single);
 
 struct vfsmount *
-do_kern_mount(const char *fstype, int flags, const char *name, void *data)
+vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void *data)
 {
-	struct file_system_type *type = get_fs_type(fstype);
 	struct super_block *sb = ERR_PTR(-ENOMEM);
 	struct vfsmount *mnt;
 	int error;
 	char *secdata = NULL;
-
-	if (!type)
-		return ERR_PTR(-ENODEV);
 
 	mnt = alloc_vfsmnt(name);
 	if (!mnt)
···
 	mnt->mnt_parent = mnt;
 	up_write(&sb->s_umount);
 	free_secdata(secdata);
-	put_filesystem(type);
 	return mnt;
 out_sb:
 	up_write(&sb->s_umount);
···
 out_mnt:
 	free_vfsmnt(mnt);
 out:
-	put_filesystem(type);
 	return (struct vfsmount *)sb;
+}
+
+EXPORT_SYMBOL_GPL(vfs_kern_mount);
+
+struct vfsmount *
+do_kern_mount(const char *fstype, int flags, const char *name, void *data)
+{
+	struct file_system_type *type = get_fs_type(fstype);
+	struct vfsmount *mnt;
+	if (!type)
+		return ERR_PTR(-ENODEV);
+	mnt = vfs_kern_mount(type, flags, name, data);
+	put_filesystem(type);
+	return mnt;
 }
 
 EXPORT_SYMBOL_GPL(do_kern_mount);

include/linux/mount.h

···
 extern struct vfsmount *do_kern_mount(const char *fstype, int flags,
 				      const char *name, void *data);
 
+struct file_system_type;
+extern struct vfsmount *vfs_kern_mount(struct file_system_type *type,
+				      int flags, const char *name,
+				      void *data);
+
 struct nameidata;
 
 extern int do_add_mount(struct vfsmount *newmnt, struct nameidata *nd,