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@kaizen_nagoya

Ethernetのsource codeを読む(linux)

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。

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。>

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kaizen_nagoya
I'm a network designer.I work on TOPPERS SmallestSetProfile Kernel,MISRA-C, STARC RTL Design StyleGuide (Verilog-HDL),HAZOP,ISO/IEC15504(AutomotiveSPICE),ISO26262. I was an editor on ISO/IEC 15504.

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