kernle 2.6 build.c analysis

/*
 *  $Id: build.c,v 1.5 1997/05/19 12:29:58 mj Exp $
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *  Copyright (C) 1997 Martin Mares
 */

/*
 * This file builds a disk-image from three different files:
 *
 * - bootsect: exactly 512 bytes of 8086 machine code, loads the rest
 * - setup: 8086 machine code, sets up system parm
 * - system: 80386 code for actual system
 *
 * It does some checking that all files are of the correct type, and
 * just writes the result to stdout, removing headers and padding to
 * the right amount. It also writes some system data to stderr.
 */

/*
 * Changes by tytso to allow root device specification
 * High loaded stuff by Hans Lermen & Werner Almesberger, Feb. 1996
 * Cross compiling fixes by Gertjan van Wingerde, July 1996
 * Rewritten by Martin Mares, April 1997
 */

#include <stdio.h>     /*this file was complied to generate execute file build*/
#include <string.h>    /* which was used to construct the bzImage*/
#include <stdlib.h>    /* all the headers are stand userspace`s header files*/
#include <stdarg.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/sysmacros.h>
#include <unistd.h>
#include <fcntl.h>
#include <asm/boot.h>

typedef unsigned char byte;
typedef unsigned short word;
typedef unsigned long u32;

#define DEFAULT_MAJOR_ROOT 0
#define DEFAULT_MINOR_ROOT 0

/* Minimal number of setup sectors (see also bootsect.S) */
#define SETUP_SECTS 4

byte buf[1024];
int fd;
int is_big_kernel;

void die(const char * str, ...)
{
    va_list args;
    va_start(args, str);
    vfprintf(stderr, str, args);
    fputc('\n', stderr);
    exit(1);
}

void file_open(const char *name)
{
    if ((fd = open(name, O_RDONLY, 0)) < 0)
        die("Unable to open `%s': %m", name);
}

void usage(void)
{
    die("Usage: build [-b] bootsect setup system [rootdev] [> image]");
}

int main(int argc, char ** argv)
{
    unsigned int i, c, sz, setup_sectors;
    u32 sys_size;
    byte major_root, minor_root;
    struct stat sb;

    if (argc > 2 && !strcmp(argv[1], "-b"))
      {
        is_big_kernel = 1;
        argc--, argv++;
      }
    if ((argc < 4) || (argc > 5))
        usage();
    if (argc > 4) {
        if (!strcmp(argv[4], "CURRENT")) {
            if (stat("/", &sb)) {
                perror("/");
                die("Couldn't stat /");
            }
            major_root = major(sb.st_dev);
            minor_root = minor(sb.st_dev);
        } else if (strcmp(argv[4], "FLOPPY")) {
            if (stat(argv[4], &sb)) {
                perror(argv[4]);
                die("Couldn't stat root device.");
            }
            major_root = major(sb.st_rdev);
            minor_root = minor(sb.st_rdev);
        } else {
            major_root = 0;
            minor_root = 0;
        }
    } else {
        major_root = DEFAULT_MAJOR_ROOT;
        minor_root = DEFAULT_MINOR_ROOT;
    }
    fprintf(stderr, "Root device is (%d, %d)\n", major_root, minor_root);

    file_open(argv[1]);
    i = read(fd, buf, sizeof(buf));
    fprintf(stderr,"Boot sector %d bytes.\n",i);
    if (i != 512)
        die("Boot block must be exactly 512 bytes");
    if (buf[510] != 0x55 || buf[511] != 0xaa)
        die("Boot block hasn't got boot flag (0xAA55)");
    buf[508] = minor_root;
    buf[509] = major_root;
    if (write(1, buf, 512) != 512)
        die("Write call failed");
    close (fd);

    file_open(argv[2]);                    /* Copy the setup code */
    for (i=0 ; (c=read(fd, buf, sizeof(buf)))>0 ; i+=c )
        if (write(1, buf, c) != c)
            die("Write call failed");
    if (c != 0)
        die("read-error on `setup'");
    close (fd);

    setup_sectors = (i + 511) / 512;    /* Pad unused space with zeros */
    /* for compatibility with ancient versions of LILO. */
    if (setup_sectors < SETUP_SECTS)
        setup_sectors = SETUP_SECTS;
    fprintf(stderr, "Setup is %d bytes.\n", i);
    memset(buf, 0, sizeof(buf));
    while (i < setup_sectors * 512) {
        c = setup_sectors * 512 - i;
        if (c > sizeof(buf))
            c = sizeof(buf);
        if (write(1, buf, c) != c)
            die("Write call failed");
        i += c;
    }

    file_open(argv[3]);
    if (fstat (fd, &sb))
        die("Unable to stat `%s': %m", argv[3]);
    sz = sb.st_size;
    fprintf (stderr, "System is %d kB\n", sz/1024);
    sys_size = (sz + 15) / 16;
    if (!is_big_kernel && sys_size > DEF_SYSSIZE)
        die("System is too big. Try using bzImage or modules.");
    while (sz > 0) {
        int l, n;

        l = (sz > sizeof(buf)) ? sizeof(buf) : sz;
        if ((n=read(fd, buf, l)) != l) {
            if (n < 0)
                die("Error reading %s: %m", argv[3]);
            else
                die("%s: Unexpected EOF", argv[3]);
        }
        if (write(1, buf, l) != l)
            die("Write failed");
        sz -= l;
    }
    close(fd);

    if (lseek(1, 497, SEEK_SET) != 497)            /* Write sizes to the bootsector */
        die("Output: seek failed");
    buf[0] = setup_sectors;
    if (write(1, buf, 1) != 1)
        die("Write of setup sector count failed");
    if (lseek(1, 500, SEEK_SET) != 500)
        die("Output: seek failed");
    buf[0] = (sys_size & 0xff);
    buf[1] = ((sys_size >> 8) & 0xff);
    if (write(1, buf, 2) != 2)
        die("Write of image length failed");

    return 0;                        /* Everything is OK */
}

用可执行文件build创建bzImage时的命令如下 ：
arch/i386/boot/tools/build \
    -b arch/i386/boot/bootsect \
    arch/i386/boot/setup \
    arch/i386/boot/vmlinux.bin\
    CURRENT \
    > arch/i386/boot/bzImage

其功能为：将boosect，setup，vmlinux.bin三个文件依次顺序的存放到bzImage文件中去，
并修改boosect部分内容，以及保证setup占用整数个sector，
，然后将 mlinux.bin追加到文件之后

步驟如下：
1：判断bootsect是否为有效的引导扇区（bzImage[508,509]={minor_root,major_root]）；
2：填充setup部分，将不足一个扇区的部分用0填充；
3：复制vmlinux.bin到setup后面即可；
4：按照setup，vmlinux.bin实际的所占扇区，及大小更改boosect中相应的指示位置
    bzImage[497]=bootsect；
    bzImage[500,501]=sys_size;