网络子系统代码定义于 net/ 目录和 drivers/net 目录下：

• net/ 目录包含偏协议栈上层的代码；
• 而 drivers/net/ 目录下的代码则更多的偏于协议栈下层，即数据链路层和物理层，因为和物理设备强绑定，所以放在 drivers/ 目录下面。

网络子系统注册在 VFS 里的文件系统叫做 sockfs，使得我们能够 read() 和 write() 一个 socket fd：

// net/socket.c
static struct file_system_type sock_fs_type = {
	.name =		"sockfs",
	.init_fs_context = sockfs_init_fs_context,
	.kill_sb =	kill_anon_super,
};

static const struct file_operations socket_file_ops = {
	.owner =	THIS_MODULE,
	.llseek =	no_llseek,
	.read_iter =	sock_read_iter,
	.write_iter =	sock_write_iter,
	.poll =		sock_poll,
	.unlocked_ioctl = sock_ioctl,
#ifdef CONFIG_COMPAT
	.compat_ioctl = compat_sock_ioctl,
#endif
	.uring_cmd =    io_uring_cmd_sock,
	.mmap =		sock_mmap,
	.release =	sock_close,
	.fasync =	sock_fasync,
	.splice_write = splice_to_socket,
	.splice_read =	sock_splice_read,
	.splice_eof =	sock_splice_eof,
	.show_fdinfo =	sock_show_fdinfo,
};