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Creating Custom Linux Images and Booting

Create custom Linux images and boot it with KRaBs. This example needs to be run on Linux.

  1. Build a vmlinux
  2. Create a initramfs
  3. Create a Disk Image
  4. Boot the custom linux image with KRaBs

Build a minimal vmlinux

  1. Get the Linux source code: 
    wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.8.3.tar.xz
tar xf linux-5.8.3.tar.xz
cd linux-5.8.3
  2. Configure your Linux build according the following: 
    make allnoconfig
make menuconfig
    64-bit kernel ---> yes
General setup ---> Initial RAM filesystem and RAM disk (initramfs/initrd) support ---> yes
General setup ---> Configure standard kernel features ---> Enable support for printk ---> yes
Executable file formats / Emulations ---> Kernel support for ELF binaries ---> yes
Executable file formats / Emulations ---> Kernel support for scripts starting with #! ---> yes
Enable the block layer ---> yes
Device Drivers ---> Generic Driver Options ---> Maintain a devtmpfs filesystem to mount at /dev ---> yes
Device Drivers ---> Generic Driver Options ---> Automount devtmpfs at /dev, after the kernel mounted the rootfs ---> yes
Device Drivers ---> Character devices ---> Enable TTY ---> yes
Device Drivers ---> Character devices ---> Serial drivers ---> 8250/16550 and compatible serial support ---> yes
Device Drivers ---> Character devices ---> Serial drivers ---> Console on 8250/16550 and compatible serial port ---> yes
Device Drivers ---> Block devices ---> yes
Device Drivers ---> PCI Support --> yes
Device Drivers ---> Serial ATA and Parallel ATA drivers (libata) ---> yes
Device Drivers ---> Serial ATA and Parallel ATA drivers (libata) ---> Intel ESB, ICH, PIIX3, PIIX4 PATA/SATA support ---> yes
Device Drivers ---> Serial ATA and Parallel ATA drivers (libata) ---> Generic ATA support ---> yes   
Device Drivers ---> SCSI device support ---> SCSI disk support
File systems ---> The Extended 4 (ext4) filesystem ---> yes
File systems ---> Pseudo filesystems ---> /proc file system support ---> yes
File systems ---> Pseudo filesystems ---> sysfs file system support ---> yes
File systems ---> DOS/FAT/EXFAT/NT Filesystems  ---> VFAT (Windows-95) fs support ---> yes
File systems ---> DOS/FAT/EXFAT/NT Filesystems  ---> Enable FAT UTF-8 option by default ---> yes
File systems ---> Native language support  ---> Codepage 437 (United States, Canada) ---> yes
File systems ---> Native language support  ---> Codepage 437 (United States, Canada) ---> yes
File systems ---> Native language support  ---> ASCII (United States) ---> yes
File systems ---> Native language support  ---> NLS ISO 8859-1  (Latin 1; Western European Languages) ---> yes
File systems ---> Native language support  ---> NLS UTF-8 ---> yes
File systems ---> UTF-8 normalization and casefolding support --- yes
    Or Copy the my recommended config to .config, then do the menuconfig for adjustments: 
    wget https://raw.githubusercontent.com/ellbrid/krabs/master/resources/.config -O .config
make menuconfig
  3. Build the vmlinux: 
    make vmlinux
cp vmlinux vmlinux-5.8.3
  4. You can find vmlinux under current directry as ./vmlinux-5.8.3.

Create a initramfs

  1. Start off by creating basic ./src/initramfs directory: 
    cd ..
mkdir --parents src/initramfs/{bin,dev,etc,lib,lib64,mnt/root,proc,root,sbin,sys}
  2. Copy basic device nodes(null, console, tty, sda1, sda2 ...) from the root filesystem to the initramfs example location: 
    sudo cp --archive /dev/{null,console,tty,tty[0-4],sda,sda[1-8],mem,kmsg,random,urandom,zero} src/initramfs/dev/
  3. Instead of using some core tools like sh and mount, we can get them from busybox: 
    curl -L 'https://www.busybox.net/downloads/binaries/1.31.0-defconfig-multiarch-musl/busybox-x86_64' > src/initramfs/bin/busybox
sudo chmod +x src/initramfs/bin/busybox
./src/initramfs/bin/busybox --list | sed 's:^:src/initramfs/bin/:' | xargs -n 1 ln -s busybox
  4. We'll also need an init script. example: 
    cat >> src/initramfs/init << EOF
#!/bin/sh

mount -t devtmpfs  devtmpfs  /dev
mount -t proc      proc      /proc
mount -t sysfs     sysfs     /sys
sleep 2
cat <<END


Boot took $(cut -d' ' -f1 /proc/uptime) seconds
                                             
_____           _        __    _             
|   __|___ ___ _| |_ _   |  |  |_|___ _ _ _ _ 
|__   | .'|   | . | | |  |  |__| |   | | |_'_|
|_____|__,|_|_|___|_  |  |_____|_|_|_|___|_,_|
                  |___|                       


Welcome to Sandy Linux



END
exec sh
EOF
    sudo chmod +x src/initramfs/init
  5. Now to create the the initramfs: 
    cd src/initramfs
find . | cpio -o -H newc | gzip > ../../initramfs.cpio.gz

Create a Disk Image

  1. Create an image file with qemu: 
    qemu-img create disk.img 512M
  2. Partition with fdisk: 
    gdisk disk.img 
...
Creating new GPT entries in memory.

Command (? for help): n
Partition number (1-128, default 1): 
First sector (34-1048542, default = 2048) or {+-}size{KMGTP}: 
Last sector (2048-1048542, default = 1048542) or {+-}size{KMGTP}: +1M
Current type is 8300 (Linux filesystem)
Hex code or GUID (L to show codes, Enter = 8300): EF02
Changed type of partition to 'BIOS boot partition'

Command (? for help): n
Partition number (2-128, default 2): 
First sector (34-1048542, default = 4096) or {+-}size{KMGTP}: 
Last sector (4096-1048542, default = 1048542) or {+-}size{KMGTP}: +50M
Current type is 8300 (Linux filesystem)
Hex code or GUID (L to show codes, Enter = 8300): EF00
Changed type of partition to 'EFI system partition'

Command (? for help): n
Partition number (3-128, default 3): 
First sector (34-1048542, default = 106496) or {+-}size{KMGTP}: 
Last sector (106496-1048542, default = 1048542) or {+-}size{KMGTP}: 
Current type is 8300 (Linux filesystem)
Hex code or GUID (L to show codes, Enter = 8300): 
Changed type of partition to 'Linux filesystem'

Command (? for help): w
  3. Create FAT32 on the second partition, ext4 on the 3rd partition. 
    sudo kpartx -av disk.img 
sudo mkfs.fat -F 32 /dev/mapper/loop0p2
sudo mkfs.ext4 /dev/mapper/loop0p3

Set FAT32 EFI System Partition

  1. Copy vmlinux and initrd into EFI System Partition 
    $ sudo mount /dev/mapper/loop0p2 /mnt
$ sudo cp path/to/vmlinux-5.8.3 /mnt/
$ sudo cp path/to/initramfs.cpio.gz /mnt/
  2. Set the CONFIG.TXT. it is a simple matrix-oriented text file. It is described as follows: 
    $ sudo vi /mnt/CONFIG.txt
main.kernel vmlinux-5.8.3
main.initrd initramfs.cpio.gz
main.cmdlin clocksource=tsc
  3. umount 
    sudo umount /mnt
sudo kpartx -d disk.img

Boot the custom linux image with KRaBs

Just run cargo; the -w option writes to disk and the -e option launches QEMU after the write.

$ pwd
path/to/krabs
$ cargo build
$ cargo run -- -w disk.img -e

Working Example

bootlin

DiskSpace layout

$ gdisk -l disk.img 
GPT fdisk (gdisk) version 1.0.5

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.
Disk disk.img: 1048576 sectors, 512.0 MiB
Sector size (logical): 512 bytes
Disk identifier (GUID): D35B348E-F03D-4B3C-B934-BD9FA055B967
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 1048542
Partitions will be aligned on 2048-sector boundaries
Total free space is 2014 sectors (1007.0 KiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048            4095   1024.0 KiB  EF02  BIOS boot partition
   2            4096          106495   50.0 MiB    EF00  EFI system partition
   3          106496         1048542   460.0 MiB   8300  Linux filesystem