how to make boot image

如何 make boot image

首先看看 boot image header 的定义：

typedef struct boot_img_hdr boot_img_hdr;

#define BOOT_MAGIC "ANDROID!"
#define BOOT_MAGIC_SIZE 8
#define BOOT_NAME_SIZE 16
#define BOOT_ARGS_SIZE 512

struct boot_img_hdr
{
    unsigned char magic[BOOT_MAGIC_SIZE];

    unsigned kernel_size;  /* size in bytes */
    unsigned kernel_addr;  /* physical load addr */

    unsigned ramdisk_size; /* size in bytes */
    unsigned ramdisk_addr; /* physical load addr */

    unsigned second_size;  /* size in bytes */
    unsigned second_addr;  /* physical load addr */

    unsigned tags_addr;    /* physical addr for kernel tags */
    unsigned page_size;    /* flash page size we assume */
    unsigned unused[2];    /* future expansion: should be 0 */

    unsigned char name[BOOT_NAME_SIZE]; /* asciiz product name */
    
    unsigned char cmdline[BOOT_ARGS_SIZE];

    unsigned id[8]; /* timestamp / checksum / sha1 / etc */
};

boot image 的结构

/*
** +-----------------+
** | boot header     | 1 page
** +-----------------+
** | kernel          | n pages  
** +-----------------+
** | ramdisk         | m pages  
** +-----------------+
** | second stage    | o pages
** +-----------------+
**
** n = (kernel_size + page_size - 1) / page_size
** m = (ramdisk_size + page_size - 1) / page_size
** o = (second_size + page_size - 1) / page_size
**
** 0. all entities are page_size aligned in flash
** 1. kernel and ramdisk are required (size != 0)
** 2. second is optional (second_size == 0 -> no second)
** 3. load each element (kernel, ramdisk, second) at
**    the specified physical address (kernel_addr, etc)
** 4. prepare tags at tag_addr.  kernel_args[] is
**    appended to the kernel commandline in the tags.
** 5. r0 = 0, r1 = MACHINE_TYPE, r2 = tags_addr
** 6. if second_size != 0: jump to second_addr
**    else: jump to kernel_addr
*/

make file 通过以下参数进行设置

INTERNAL_BOOTIMAGE_ARGS += --cmdline "$(BOARD_KERNEL_CMDLINE)"
INTERNAL_BOOTIMAGE_ARGS += --base $(BOARD_KERNEL_BASE)
INTERNAL_BOOTIMAGE_ARGS += --pagesize $(BOARD_KERNEL_PAGESIZE)
INTERNAL_BOOTIMAGE_ARGS += --ramdisk_offset $(BOARD_RAMDISK_OFFSET)

$(MKBOOTIMG) $(INTERNAL_BOOTIMAGE_ARGS) $(BOARD_MKBOOTIMG_ARGS) --output $@

 我们逐一看一下，这个文件其实比较简单

int main(int argc, char **argv)
{
    boot_img_hdr hdr;

    char *kernel_fn = 0;
    void *kernel_data = 0;
    char *ramdisk_fn = 0;
    void *ramdisk_data = 0;
    char *second_fn = 0;
    void *second_data = 0;
    char *cmdline = "";
    char *bootimg = 0;
    char *board = "";
    unsigned pagesize = 2048;
    int fd;
    SHA_CTX ctx;
    uint8_t* sha;
    unsigned base           = 0x10000000;   //set base address  256M
    unsigned kernel_offset  = 0x00008000;  //kernel storage offset
    unsigned ramdisk_offset = 0x01000000;
    unsigned second_offset  = 0x00f00000;
    unsigned tags_offset    = 0x00000100;

    argc--;
    argv++;

    memset(&hdr, 0, sizeof(hdr));

    //通过以下进行参数解析
    while(argc > 0){
        char *arg = argv[0];
        char *val = argv[1];
        if(argc < 2) {
            return usage();
        }
        argc -= 2;
        argv += 2;
        if(!strcmp(arg, "--output") || !strcmp(arg, "-o")) {
            bootimg = val;
        } else if(!strcmp(arg, "--kernel")) {
            kernel_fn = val;
        } else if(!strcmp(arg, "--ramdisk")) {
            ramdisk_fn = val;
        } else if(!strcmp(arg, "--second")) {
            second_fn = val;
        } else if(!strcmp(arg, "--cmdline")) {
            cmdline = val;
        } else if(!strcmp(arg, "--base")) {
            base = strtoul(val, 0, 16);
        } else if(!strcmp(arg, "--kernel_offset")) {
            kernel_offset = strtoul(val, 0, 16);
        } else if(!strcmp(arg, "--ramdisk_offset")) {
            ramdisk_offset = strtoul(val, 0, 16);
        } else if(!strcmp(arg, "--second_offset")) {
            second_offset = strtoul(val, 0, 16);
        } else if(!strcmp(arg, "--tags_offset")) {
            tags_offset = strtoul(val, 0, 16);
        } else if(!strcmp(arg, "--board")) {
            board = val;
        } else if(!strcmp(arg,"--pagesize")) {
            pagesize = strtoul(val, 0, 10);
            if ((pagesize != 2048) && (pagesize != 4096)
                && (pagesize != 8192) && (pagesize != 16384)) {
                fprintf(stderr,"error: unsupported page size %d\n", pagesize);
                return -1;
            }
        } else {
            return usage();
        }
    }

    // 填充 boot image 的 header

    hdr.page_size = pagesize;

    hdr.kernel_addr =  base + kernel_offset;
    hdr.ramdisk_addr = base + ramdisk_offset;
    hdr.second_addr =  base + second_offset;
    hdr.tags_addr =    base + tags_offset;

    // 检测错误的参数输入
    if(bootimg == 0) {
        fprintf(stderr,"error: no output filename specified\n");
        return usage();
    }

    if(kernel_fn == 0) {
        fprintf(stderr,"error: no kernel image specified\n");
        return usage();
    }

    if(ramdisk_fn == 0) {
        fprintf(stderr,"error: no ramdisk image specified\n");
        return usage();
    }

    if(strlen(board) >= BOOT_NAME_SIZE) {
        fprintf(stderr,"error: board name too large\n");
        return usage();
    }

    strcpy(hdr.name, board);

    memcpy(hdr.magic, BOOT_MAGIC, BOOT_MAGIC_SIZE);

    if(strlen(cmdline) > (BOOT_ARGS_SIZE - 1)) {
        fprintf(stderr,"error: kernel commandline too large\n");
        return 1;
    }
    strcpy((char*)hdr.cmdline, cmdline);

    kernel_data = load_file(kernel_fn, &hdr.kernel_size);
    if(kernel_data == 0) {
        fprintf(stderr,"error: could not load kernel '%s'\n", kernel_fn);
        return 1;
    }

    if(!strcmp(ramdisk_fn,"NONE")) {
        ramdisk_data = 0;
        hdr.ramdisk_size = 0;
    } else {
        ramdisk_data = load_file(ramdisk_fn, &hdr.ramdisk_size);
        if(ramdisk_data == 0) {
            fprintf(stderr,"error: could not load ramdisk '%s'\n", ramdisk_fn);
            return 1;
        }
    }

    if(second_fn) {
        second_data = load_file(second_fn, &hdr.second_size);
        if(second_data == 0) {
            fprintf(stderr,"error: could not load secondstage '%s'\n", second_fn);
            return 1;
        }
    }

    进行hash 计算，

    /* put a hash of the contents in the header so boot images can be
     * differentiated based on their first 2k.
     */
    SHA_init(&ctx);
    SHA_update(&ctx, kernel_data, hdr.kernel_size);
    SHA_update(&ctx, &hdr.kernel_size, sizeof(hdr.kernel_size));
    SHA_update(&ctx, ramdisk_data, hdr.ramdisk_size);
    SHA_update(&ctx, &hdr.ramdisk_size, sizeof(hdr.ramdisk_size));
    SHA_update(&ctx, second_data, hdr.second_size);
    SHA_update(&ctx, &hdr.second_size, sizeof(hdr.second_size));
    sha = SHA_final(&ctx);
    memcpy(hdr.id, sha,
           SHA_DIGEST_SIZE > sizeof(hdr.id) ? sizeof(hdr.id) : SHA_DIGEST_SIZE);

    fd = open(bootimg, O_CREAT | O_TRUNC | O_WRONLY, 0644);
    if(fd < 0) {
        fprintf(stderr,"error: could not create '%s'\n", bootimg);
        return 1;
    }

    //写入header, 并且填充0进行页对齐
    if(write(fd, &hdr, sizeof(hdr)) != sizeof(hdr)) goto fail;
    if(write_padding(fd, pagesize, sizeof(hdr))) goto fail;

    // 写入kernel 部分
    if(write(fd, kernel_data, hdr.kernel_size) != hdr.kernel_size) goto fail;
    if(write_padding(fd, pagesize, hdr.kernel_size)) goto fail;

   // 写入ramdisk 部分
    if(write(fd, ramdisk_data, hdr.ramdisk_size) != hdr.ramdisk_size) goto fail;
    if(write_padding(fd, pagesize, hdr.ramdisk_size)) goto fail;

    if(second_data) {
        if(write(fd, second_data, hdr.second_size) != hdr.second_size) goto fail;
        if(write_padding(fd, pagesize, hdr.ramdisk_size)) goto fail;
    }

    return 0;

fail:
    unlink(bootimg);
    close(fd);
    fprintf(stderr,"error: failed writing '%s': %s\n", bootimg,
            strerror(errno));
    return 1;
}