RealYukiSan/note.md

## note.md

      
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              note.md
            
          
    The idea came from Nir Lichtman video with the title: 'Making Simple Linux Distro from Scratch'
this gist just re-document it in the form of text
Requirement


compiled linux kernel
compiled static-linked busybox
installed qemu

Creating initramfs

install busybox to specific directory, I suggest to use /tmp/distro as working directory
and on the busybox source code directory:
make CONFIG_PREFIX=/tmp/distro/initramfs install
delete linuxrc file and replace it with init file
cat > ./initramfs/init << "EOF"
#!/bin/sh
/bin/sh
EOF
compress initramfs with cpio:
cd initramfs
find . | cpio -o -H newc > ../init.cpio
you can cross-check the content of init.cpio with cpio -v -t -F init.cpio
Creating boot image

on linux, you'll need to install dosfstools package to be able create FAT file system
dd if=/dev/zero of=boot.img bs=1M count=50
mkfs -t fat boot.img
mount boot.img /mnt
syslinux -i /mnt

Use a loopback device, which allows a disk file to be treated as a device. Using a loopback device you can create a three megabyte file on your hard disk and build the filesystem on it.

TIL, such file is called loop-back device.
Try to mount the image to /mnt/whatever and execute command findmnt -A to see the loop-back device, reference.
Run it on QEMU

qemu-system-x86_64 -drive format=raw,file=./boot.img,index=0,media=disk \
-initrd ./init.cpio -kernel ./bzImage \
-nographic -enable-kvm  -append "root=/ console=ttyS0"
tested on arch linux.
Alternative


You can use TigerVNC to connect to your guest OS
You can place bzImage and init.cpio to the boot image instead of pass it as parameter to qemu command

just mount the boot image, copy the file, and unmount
but you'll need to specify the path to bzImage later when running the guest, the prompt looks like this:
boot: <path-to-kernel-image>
boot: /bzImage -initrd=/init.cpio

see other kernel parameter option (initrd, console, root):
curl -L -s https://raw.githubusercontent.com/torvalds/linux/master/Documentation/admin-guide/kernel-parameters.txt | less

Note

instead of provide your custom init shell script, it is possible to work with default init by busybox. You just need to provide the /dev directory and other default standard linux directory for Virtual File System + /etc/fstab, see LFS chapter 7.3.
you can use extlinux instead of syslinux for bootloader on ext file system.
you can use truncate command instead of dd.
if you are not providing bootloader configuration, you'll need to specify the path of kernel image.
also, it's possible to build linux system without initramfs, so no need to create archived cpio file and instead, just install the busybox to the disk image or other drive.
when emulate the linux system on QEMU make sure the disk image have read and write permission and it is recommended to pass rw to kernel parameter.
Unanswered Question

Does the following statement from tldp:

When the kernel is completely loaded, it initializes device drivers and its internal data structures. Once it is completely initialized, it consults a special location in its image called the ramdisk word. This word tells it how and where to find its root filesystem. A root filesystem is simply a filesystem that will be mounted as ``/''. The kernel has to be told where to look for the root filesystem; if it cannot find a loadable image there, it halts.

have something to do with the Creating Boot Image and the kernel parameter root?
what's relation between firmware of BIOS/UEFI and the partition layout table such as MBR or GPT?
Link and References


stackoverflow.com turns out it's possible to running qemu without TigerVNC
unix.stackexchange.com - there's 2 possibilities if you encounter rootfs unknown block
unix.stackexchange.com - missing init can lead to rootfs unknown block error