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BOOTBOOT Bootable Disk Image Creator

See BOOTBOOT Protocol for common details.

This is an all-in-one, multiplatform, dependency-free disk image creator tool. You pass a disk configuration to it in a very flexible JSON, and it generates ESP FAT boot partition with the required loader files, GPT partitioning table, PMBR, etc. It also creates an initrd from a directory (currently cpio, tar, jamesm (James Molloy's initrd), echfs and FS/Z supported, but the code is written in a way that it is easily expandable). In contrast, partitions can be generated from directories for fat (with long file names), minix (Minix V3 with POSIX permissions and device files), tar, echfs, FS/Z filesystems.

The generated image was tested with fdisk, and with the verify function of gdisk. The FAT partition was tested with fsck.vfat and with TianoCore UEFI firmware and on Raspberry Pi. The ISO9660 part tested with iat (ISO9660 Analyzer Tool) and Linux mount.

Operating Modes

$ ./mkbootimg
BOOTBOOT mkbootimg utility - bztsrc@gitlab
 BOOTBOOT Copyright (c) bzt MIT https://gitlab.com/bztsrc/bootboot
 deflate 1.2.11 Copyright 1995-2017 Jean-loup Gailly and Mark Adler
 Raspbery Pi Firmware Copyright (c) Broadcom Corp, Raspberry Pi (Trading) Ltd

Validates ELF or PE executables for being BOOTBOOT compatible, otherwise
creates a bootable hybrid image or Option ROM image for your hobby OS.

Usage:
  ./mkbootimg check <kernel elf / pe>
  ./mkbootimg <configuration json> initrd.rom
  ./mkbootimg <configuration json> bootpart.bin
  ./mkbootimg <configuration json> <output disk image name>

Examples:
  ./mkbootimg check mykernel/mykernel.x86_64.elf
  ./mkbootimg myos.json initrd.rom
  ./mkbootimg myos.json bootpart.bin
  ./mkbootimg myos.json myos.img

If the first argument is check, then it's followed by a kernel filename. The utility will check the executable for BOOTBOOT compliance, and it will report all errors and if passed, which BOOTBOOT Protocol level it conforms to.

Otherwise the first argument is the configuration JSON file. If the second argument is initrd.rom, then it will generate a BIOS Option ROM image from the initrd directory. If that is bootpart.bin, then it saves the boot partition image (and only the partition image). Every other filename will make it generate a whole disk image with GPT.

The tool is multilingual. It will detect your operating system's language and if it has a dictionary for it, it will use that. You can override the autodetection from the command line by using the -l <lang> flag as the first argument (available for all operating modes). Language is given in two characters long code and fallbacks to en.

Configuration

The JSON is simple and flexible, accepts many variations. At the top level, you can define the output disk parameters.

Top Level

Field Type Description
diskguid GUID optional, the disk GUID. If not given, or full zeros, it will be generated
disksize integer optional, the size of the disk image in Megabytes. If not given, it is calculated
align integer optional, the partition alignment in Kilobytes. Zero gives sector alignment
iso9660 boolean optional, wether to generate ISO9660 Boot Catalog into the image. Defaults to false
config filename BOOTBOOT configuration file. It is parsed for the kernel filename
initrd struct the initial ramdisk's definition, see below
partitions array partition definitions, see below

Example:

{
    "diskguid": "00000000-0000-0000-0000-000000000000",
    "disksize": 128,
    "align": 1024,
    "iso9660": true,
    "config": "boot/sys/config",
    "initrd": { "type": "tar", "gzip": true, "directory": "boot" },
    "partitions": [
        { "type": "boot", "size": 16 },
        { "type": "ext4", "size": 128, "name": "Linux Exchange" },
        { "type": "ntfs", "size": 128, "name": "Windows Exchange" },
        { "type": "Microsoft basic data", "size": 32, "name": "MyOS usr", "file": "usrpart.bin" },
        { "type": "00000000-0000-0000-0000-000000000000", "size": 32, "name": "MyOS var", "file": "varpart.bin" }
    ]
}

Initrd

Field Type Description
gzip boolean optional, wether to compress the initrd image, defaults to true
type string format of the initrd image. When invalid value given, it lists the options
file filename the filename of the image file to be used
directory folder path to a folder, its contents will be used to generate the initrd
file array for multiarch images
directory array for multiarch images

The fields file and directory are mutually exclusive. They can be both strings (if there's only one architecture), or arrays (one array element for each architecture). Currently three architecture supported, which means there can be three strings in the arrays. Which architecture is used depends on the kernel's architecture in that folder or image file. Type is only mandatory for directory.

Examples:

    "initrd": { "file": "initrd.bin" },
    "initrd": { "type": "tar", "gzip": 0, "directory": "boot" },
    "initrd": { "gzip": true, "file": [ "initrd-x86.bin", "initrd-arm.bin", "initrd-rv64.bin" ] },
    "initrd": { "type": "cpio", "gzip": true, "directory": [ "boot/arm", "boot/x86", "boot/riscv64" ] },

Partitions

It is somewhat unusual, as the first array element is different than the rest. It specifies the boot partition, therefore it has different types, and file / directory and name are not interpreted because that partition image is always dinamically generated with the implicit name of "EFI System Partition". For the same reason, size is mandatory for the first (boot) partition.

Field Type Description
size integer optional, the size of the partition in Megabytes. If not given, it is calculated
file filename optional, path to a partition image to be used
directory folder optional, path to a folder, its contents will be used to generate the partition
driver string optional, in case type can't specify the format without a doubt
type string format of the partition. When invalid value given, it lists the options
name string UTF-8 partition names, limited to UNICODE code points 32 to 65535 (BMP)

For the first entry, valid values for type are: boot (or explicit fat16 and fat32). Generates only 8+3 file names. The utility handles this comfortably, it tries to use FAT16 if possible to save storage space. There's a minimal size for the boot partition, 16 Megabytes. Although both the image creator and BOOTBOOT is capable of handling smaller sizes, some UEFI firmware incorrectly assumes FAT12 when there are too few clusters on the file system. If the partition size is bigger than 128 Megabytes, then it automatically switches to FAT32. If you don't use iso9660, then you can also set FAT32 for smaller images, but at least 33 Megabytes (that's a hard lower limit for FAT32). With iso9660, each cluster must be 2048 bytes aligned, which is achieved by 4 sectors per cluster. The same problem applies here, both the image creator and the BOOTBOOT loader capable of handling FAT32 with smaller cluster numbers, but some UEFI firmware don't, and falsely assumes FAT16. To guarantee the minimum number of clusters, with ISO9660 and FAT32 the boot partition's minimum size is 128 Megabytes (128*1024*1024/512/4 = 65536, just one larger than what fits in 16 bits).

For the other entries (starting from the second), type is either a GUID or one of a pre-defined file system aliases. Here fat will decide between FAT16 and FAT32 based on the number of clusters, and it can generate long file names. With an invalid string, the utility will list all possible values.

Example:

mkbootimg: partition #2 doesn't have a valid type. Accepted values:
  "65706154-4120-6372-6968-766520465320" / "tar"
  "5A2F534F-0000-5346-2F5A-000000000000" / "FS/Z"
  "6A898CC3-1DD2-11B2-99A6-080020736631" / "ZFS"
  "EBD0A0A2-B9E5-4433-87C0-68B6B72699C7" / "ntfs"
  "0FC63DAF-8483-4772-8E79-3D69D8477DE4" / "ext4"
  "516E7CB6-6ECF-11D6-8FF8-00022D09712B" / "ufs"
  "C91818F9-8025-47AF-89D2-F030D7000C2C" / "p9"
  "D3BFE2DE-3DAF-11DF-BA40-E3A556D89593" / "Intel Fast Flash"
  "21686148-6449-6E6F-744E-656564454649" / "BIOS boot"
     ...
  "77719A0C-A4A0-11E3-A47E-000C29745A24" / "VMware Virsto"
  "9198EFFC-31C0-11DB-8F78-000C2911D1B8" / "VMware Reserved"
  "824CC7A0-36A8-11E3-890A-952519AD3F61" / "OpenBSD data"
  "CEF5A9AD-73BC-4601-89F3-CDEEEEE321A1" / "QNX6 file system"
  "C91818F9-8025-47AF-89D2-F030D7000C2C" / "Plan 9 partition"
  "5B193300-FC78-40CD-8002-E86C45580B47" / "HiFive Unleashed FSBL"
  "2E54B353-1271-4842-806F-E436D6AF6985" / "HiFive Unleashed BBL"
  ...or any non-zero GUID in the form "%08X-%04X-%04X-%04X-%12X"

If file given, then the partition is filled with data from that file. If size is not given or smaller than the file's size, then the file's size will be the partition's size. If both given, and size is larger, then the difference is filled up with zeros. Partition sizes will always be multiple of align Kilobytes. Using 1024 as alignment gives you 1 Megabyte aligned partitions. For the first entry, only size is valid, file isn't. Alternatively to file, you might also be able to use directory to generate the partition image from the contents of a directory. This option is only available if the file system driver is implemented for type. Because there might be no one-to-one relation between partition types and file system types, you can use driver to explicily set the latter. This is only relevant when the directory directive is used. For example:

    { "type": "5A2F534F-8664-5346-2F5A-000075737200", "driver": "FS/Z",  "size": 32, "name": "usr",  "directory": "myusr" },
    { "type": "Linux home",                           "driver": "minix", "size": 32, "name": "home", "directory": "myhome" },
    { "type": "Microsoft basic data",                 "driver": "fat",   "size": 32, "name": "data", "directory": "mydata" },

Finally, name is just an UTF-8 string, name of the partition. Maximum length is 35 characters. Not valid for the first entry.

Adding More File Systems

Types are listed in the fs registry, in the file fs.h. You can freely add new file system types. For file systems that you want to use for generating partition images or initrd as well, you must implement three functions, like:

void somefs_open(gpt_t *gpt_entry);
void somefs_add(struct stat *st, char *name, unsigned char *content, int size);
void somefs_close();

The first, the "open" is called whenever a new file system is to be created. The gpt_entry is NULL when called for initrd creation. As the given directory is recursively parsed, for each directory entry an "add" call is made. This should add the file or directory to the file system image. Here st is the stat struct for the file, name is the filename with full path, content and size are the file's content, or in case of a symbolic link, the pointed path. Finally when the parsing is done, the "close" function is called to finalize the image. Only the "add" function is mandatory, the other two are optional.

These functions can use two global variables, fs_base and fs_len which holds the buffer for the filesystem image in memory (this implies that partitions are limited to few gigabytes, depending how much RAM you have). In case they want to report error, fs_no is the number of the partition the driver is generating for.

In lack of these functions, the file system still can be used in the partition's type field, but then only the GPT entry will be created, not the content of the partition. The driver field only accepts file system types which have these functions.

Keeping the built-in binaries up-to-date

To avoid dependencies, the image creator includes all the necessary binaries. If these are updated, then delete data.c and run make which will regenerate it. If there are missing files, then in the aarch64-rpi directory run make getfw, that will download the latest Raspberry Pi firmware files. Then make in this directory should run without problems.