Internals of ext2fs

ext2fs Superblock

ext2_read_super also checks to make sure the FS is really an ext2fs:

        es = (struct ext2_super_block *) (((char *)bh->b_data) + offset);
        sb->s_magic = le16_to_cpu(es->s_magic);
        if (sb->s_magic != EXT2_SUPER_MAGIC) {
                if (!silent)
                        printk ("VFS: Can't find an ext2 filesystem on dev "
                                "%s.\n", bdevname(dev));
                ...
                return NULL;
        }

And that's how mount works