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@kaizen_nagoya(Dr. Kiyoshi Ogawa)

Ethernetのsource codeを読む(linux)

Last updated at Posted at 2021-02-11

お正月休みにIEEE Ethernet規格(英文：個人利用無料)を読もう。１日１０００ページ目標。
https://qiita.com/kaizen_nagoya/items/e1a3155a950e799a17f5

だけじゃプログラマじゃないだろというお言葉をいただき、
Ethernetのsource codeを読む(linux)

という企画を立てます。

本当はBSDのehternetを読みたいのですが、読んだことがないのです。

無線が専門で、物理層は空中線の理論式を少し。
１級無線技術士の資格の勉強会や、マクスウェルの方程式の勉強会を企画したこともあります。

linux/net/ethernet/
https://github.com/torvalds/linux/tree/master/net/ethernet

linux/drivers/net/ethernet/
https://github.com/torvalds/linux/tree/master/drivers/net/ethernet

今から読むのは、eth.c。
https://github.com/torvalds/linux/blob/master/net/ethernet/eth.c

以下、第一日目。

<この項は書きかけです。順次追記します。>

1. Comment

まずは注釈(comment)。書いた人たちの名前が並ぶのはよいこと。プログラマの名前のないソースコードは決して信頼してはいけない。
できれば、想定する環境（OSの版）、コンパイラの版、コンパイルオプション、Compileスクリプトなども記載するとよい。

例：
https://researchmap.jp/blogs/blog_entries/view/82322/c65fbd44fb58b3d0946218a98a98eb75?frame_id=445675

eth.c
// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * INET		An implementation of the TCP/IP protocol suite for the LINUX
 *		operating system.  INET is implemented using the  BSD Socket
 *		interface as the means of communication with the user level.
 *
 *		Ethernet-type device handling.
 *
 * Version:	@(#)eth.c	1.0.7	05/25/93
 *
 * Authors:	Ross Biro
 *		Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
 *		Mark Evans, <evansmp@uhura.aston.ac.uk>
 *		Florian  La Roche, <rzsfl@rz.uni-sb.de>
 *		Alan Cox, <gw4pts@gw4pts.ampr.org>
 *
 * Fixes:
 *		Mr Linux	: Arp problems
 *		Alan Cox	: Generic queue tidyup (very tiny here)
 *		Alan Cox	: eth_header ntohs should be htons
 *		Alan Cox	: eth_rebuild_header missing an htons and
 *				  minor other things.
 *		Tegge		: Arp bug fixes.
 *		Florian		: Removed many unnecessary functions, code cleanup
 *				  and changes for new arp and skbuff.
 *		Alan Cox	: Redid header building to reflect new format.
 *		Alan Cox	: ARP only when compiled with CONFIG_INET
 *		Greg Page	: 802.2 and SNAP stuff.
 *		Alan Cox	: MAC layer pointers/new format.
 *		Paul Gortmaker	: eth_copy_and_sum shouldn't csum padding.
 *		Alan Cox	: Protect against forwarding explosions with
 *				  older network drivers and IFF_ALLMULTI.
 *	Christer Weinigel	: Better rebuild header message.
 *             Andrew Morton    : 26Feb01: kill ether_setup() - use netdev_boot_setup().
 */

2. include

C言語の便利なところは、ソースコードのまま読み込むことができる。

linux/eth.c
#include <linux/module.h>
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/string.h>
#include <linux/mm.h>
#include <linux/socket.h>
#include <linux/in.h>
#include <linux/inet.h>
#include <linux/ip.h>
#include <linux/netdevice.h>
#include <linux/nvmem-consumer.h>
#include <linux/etherdevice.h>
#include <linux/skbuff.h>
#include <linux/errno.h>
#include <linux/init.h>
#include <linux/if_ether.h>
#include <linux/of_net.h>
#include <linux/pci.h>
#include <net/dst.h>
#include <net/arp.h>
#include <net/sock.h>
#include <net/ipv6.h>
#include <net/ip.h>
#include <net/dsa.h>
#include <net/flow_dissector.h>
#include <linux/uaccess.h>
#include <net/pkt_sched.h>

includeファイルは、読み込む順番に無駄がないのがよい。
二重読みこみ防止機能をつけているはずである。

/linux/module.hではこんな感じ。
https://github.com/torvalds/linux/blob/master/include/linux/module.h

module.h
#ifndef _LINUX_MODULE_H
#define _LINUX_MODULE_H

3. declaration, data section

関数の宣言(declaration)はheaderファイルに書くのが基本。主要なデータもheaderファイルの方がよいかも。

linux/eth.c
__setup("ether=", netdev_boot_setup);

マクロ定義を展開してみると分かるかも。

#define __setup_param(str, unique_id, fn, early)			\
	static const char __setup_str_##unique_id[] __initconst		\
		__aligned(1) = str; 					\
	static struct obs_kernel_param __setup_##unique_id		\
		__used __section(.init.setup)				\
		__attribute__((aligned((sizeof(long)))))		\
		= { __setup_str_##unique_id, fn, early }
#define __setup(str, fn)						\
	__setup_param(str, fn, fn, 0)

4. eth_header

/**
 * eth_header - create the Ethernet header
 * @skb:	buffer to alter
 * @dev:	source device
 * @type:	Ethernet type field
 * @daddr: destination address (NULL leave destination address)
 * @saddr: source address (NULL use device source address)
 * @len:   packet length (<= skb->len)
 *
 *
 * Set the protocol type. For a packet of type ETH_P_802_3/2 we put the length
 * in here instead.
 */
int eth_header(struct sk_buff *skb, struct net_device *dev,
	       unsigned short type,
	       const void *daddr, const void *saddr, unsigned int len)
{
	struct ethhdr *eth = skb_push(skb, ETH_HLEN);
	if (type != ETH_P_802_3 && type != ETH_P_802_2)
		eth->h_proto = htons(type);
	else
		eth->h_proto = htons(len);
	/*
	 *      Set the source hardware address.
	 */
	if (!saddr)
		saddr = dev->dev_addr;
	memcpy(eth->h_source, saddr, ETH_ALEN);
	if (daddr) {
		memcpy(eth->h_dest, daddr, ETH_ALEN);
		return ETH_HLEN;
	}
	/*
	 *      Anyway, the loopback-device should never use this function...
	 */
	if (dev->flags & (IFF_LOOPBACK | IFF_NOARP)) {
		eth_zero_addr(eth->h_dest);
		return ETH_HLEN;
	}
	return -ETH_HLEN;
}1
EXPORT_SYMBOL(eth_header);

この定義はheader にあった方がよいかどうかは意見が分かれるかもしれない。

5. eth_type_trans

関数の先頭には注釈があるとよい。

/**
 * eth_type_trans - determine the packet's protocol ID.
 * @skb: received socket data
 * @dev: receiving network device
 *
 * The rule here is that we
 * assume 802.3 if the type field is short enough to be a length.
 * This is normal practice and works for any 'now in use' protocol.
 */
__be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
{
	unsigned short _service_access_point;
	const unsigned short *sap;
	const struct ethhdr *eth;

	skb->dev = dev;
	skb_reset_mac_header(skb);

	eth = (struct ethhdr *)skb->data;
	skb_pull_inline(skb, ETH_HLEN);

	if (unlikely(!ether_addr_equal_64bits(eth->h_dest,
					      dev->dev_addr))) {
		if (unlikely(is_multicast_ether_addr_64bits(eth->h_dest))) {
			if (ether_addr_equal_64bits(eth->h_dest, dev->broadcast))
				skb->pkt_type = PACKET_BROADCAST;
			else
				skb->pkt_type = PACKET_MULTICAST;
		} else {
			skb->pkt_type = PACKET_OTHERHOST;
		}
	}

	/*
	 * Some variants of DSA tagging don't have an ethertype field
	 * at all, so we check here whether one of those tagging
	 * variants has been configured on the receiving interface,
	 * and if so, set skb->protocol without looking at the packet.
	 * The DSA tagging protocol may be able to decode some but not all
	 * traffic (for example only for management). In that case give it the
	 * option to filter the packets from which it can decode source port
	 * information.
	 */
	if (unlikely(netdev_uses_dsa(dev)) && dsa_can_decode(skb, dev))
		return htons(ETH_P_XDSA);

	if (likely(eth_proto_is_802_3(eth->h_proto)))
		return eth->h_proto;

	/*
	 *      This is a magic hack to spot IPX packets. Older Novell breaks
	 *      the protocol design and runs IPX over 802.3 without an 802.2 LLC
	 *      layer. We look for FFFF which isn't a used 802.2 SSAP/DSAP. This
	 *      won't work for fault tolerant netware but does for the rest.
	 */
	sap = skb_header_pointer(skb, 0, sizeof(*sap), &_service_access_point);
	if (sap && *sap == 0xFFFF)
		return htons(ETH_P_802_3);

	/*
	 *      Real 802.2 LLC
	 */
	return htons(ETH_P_802_2);
}
EXPORT_SYMBOL(eth_type_trans);

__be16: big endian 16 bit.

6. eth_header_parse

/**
 * eth_header_parse - extract hardware address from packet
 * @skb: packet to extract header from
 * @haddr: destination buffer
 */
int eth_header_parse(const struct sk_buff *skb, unsigned char *haddr)
{
	const struct ethhdr *eth = eth_hdr(skb);
	memcpy(haddr, eth->h_source, ETH_ALEN);
	return ETH_ALEN;
}
EXPORT_SYMBOL(eth_header_parse);

ETH_ALEN

#define ETH_ALEN        6

短い。実質３行のプログラム。

7. eth_header_cache

/**
 * eth_header_cache - fill cache entry from neighbour
 * @neigh: source neighbour
 * @hh: destination cache entry
 * @type: Ethernet type field
 *
 * Create an Ethernet header template from the neighbour.
 */
int eth_header_cache(const struct neighbour *neigh, struct hh_cache *hh, __be16 type)
{
	struct ethhdr *eth;
	const struct net_device *dev = neigh->dev;

	eth = (struct ethhdr *)
	    (((u8 *) hh->hh_data) + (HH_DATA_OFF(sizeof(*eth))));

	if (type == htons(ETH_P_802_3))
		return -1;

	eth->h_proto = type;
	memcpy(eth->h_source, dev->dev_addr, ETH_ALEN);
	memcpy(eth->h_dest, neigh->ha, ETH_ALEN);

	/* Pairs with READ_ONCE() in neigh_resolve_output(),
	 * neigh_hh_output() and neigh_update_hhs().
	 */
	smp_store_release(&hh->hh_len, ETH_HLEN);

	return 0;
}
EXPORT_SYMBOL(eth_header_cache);

struct ethhdr

struct ethhdr {
   unsigned char   h_dest[ETH_ALEN];   /* destination eth addr */
   unsigned char   h_source[ETH_ALEN]; /* source ether addr    */
   __be16      h_proto;        /* packet type ID field */
} __attribute__((packed));

8.eth_header_cache_update

/***
 * eth_header_cache_update - update cache entry
 * @hh: destination cache entry
 * @dev: network device
 * @haddr: new hardware address
 *
 * Called by Address Resolution module to notify changes in address.
 */
void eth_header_cache_update(struct hh_cache *hh,
			     const struct net_device *dev,
			     const unsigned char *haddr)
{
	memcpy(((u8 *) hh->hh_data) + HH_DATA_OFF(sizeof(struct ethhdr)),
	       haddr, ETH_ALEN);
}
EXPORT_SYMBOL(eth_header_cache_update);

9.eth_header_parse_protocol

/**
 * eth_header_parse_protocol - extract protocol from L2 header
 * @skb: packet to extract protocol from
 */
__be16 eth_header_parse_protocol(const struct sk_buff *skb)
{
	const struct ethhdr *eth = eth_hdr(skb);

	return eth->h_proto;
}
EXPORT_SYMBOL(eth_header_parse_protocol);

もっと短い。実質１行のプログラム。

こんな感じでぼちぼち１日10関数未満程度読んでいけば大丈夫。

博論では、端末間経路選択のための片方向遅延差測定方式は、無線機器で実験をしていました。最初はPHSを使っていましたが、博論発表日の前日から、徹夜でGPSのプログラムを作って、提案した片方向遅延差測定方式が有効なデータを取ることができたことがあります。

＜この記事は個人の過去の経験に基づく個人の感想です。現在所属する組織、業務とは関係がありません。＞

文書履歴(document history)

ver. 0.01 初稿 　20210221

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