use musl-cross-make, If you use a 64 bit target, make sure to build your kernel with 64 bit support.
At the root of the busybox source tree run:
make allnoconfig
make menuconfigI enabled the following set of utilities, but you can add less or more:
-
Settings -> Include busybox applet
-
Settings -> Build static library
-
Settings -> Use -static-libc
-
Settings -> Command line editing
-
Settings -> Tab completion
-
Coreutils -> cat
-
Coreutils -> echo
-
Coreutils -> cp
-
Coreutils -> ls
-
Editors -> vi
-
Linux System Utilities -> mount
-
Shells -> ash
-
Shells -> ash -> Echo Builtin
To cross compile with musl, Set Settings -> Cross compiler prefix to /path/to/musl-cross-make/build/local/x68_64-linux-musl/output/bin/x86_64-linux-musl- and Settings -> Path to sysroot /path/to/musl-cross-make/build/local/x86_64-linux-musl/output.
Finally, run make -j$(nproc) to compile busybox.
You can use readelf -a busybox to confirm that the binary is static linked
With my configuration (64 bit) the binary was 200K.
make tinyconfig is a good starting point for kernel configuration, but you will need to enable some things.
make tinyconfig
make menuconfig
- General Setup -> Initial RAM filesystem and RAM disk (initramfs/initrd) support = y
- General Setup -> Initial RAM filesystem and RAM disk (initramfs/initrd) support -> Support initial ramdisk/ramfs compressed using gzip = y
- General Setup -> Configure standard kernel features (expert users) -> Enable support for printk = y
- Executable file formats -> Kernel support for ELF binaries = y
- Executable file formats -> Kernel support for scripts starting with #! = y
- Device Drivers -> Character devices -> Virtual terminal = y
- Device Drivers -> Character devices -> Support for console on virtual terminal = y
- Device Drivers -> Graphics support -> Support for Frame buffer Devices = y
- Device Drivers -> Graphics support -> Support for Frame buffer Devices -> Simple framebuffer support = y
- Device Drivers -> Graphics support -> Firmware Drivers -> Mark VGA/VBE/EFI FB as generic system framebuffer = y
To build it just run
make -j$(nproc)
The resultant kernel on my system (64 bit) was 688K
First create 5A8B required structure and copy busybox.
mkdir root
mkdir root/{bin,proc,dev,sys}
cp /path/to/busybox/binary root/bin
ln -sr root/bin/sh root/bin/busyboxThen create root/init, A basic one follows (remember to chmod a+x root/init).
#!/bin/sh
# Put anything you want to run on startup here
/bin/shFinally you can run this one liner to create an initramfs:
find root -printf "%P\0" | cpio --null -o -H newc -D root | gzip > initramfs.cpio.gz
You will need SYSLINUX for both of these, prebuilt is fine. https://mirrors.edge.kernel.org/pub/linux/utils/boot/syslinux/
First create a direcory for the iso fs:
mkdir isoroot
Then copy the kernel and initramfs:
mkdir isoroot/boot
cp initramfs.cpio.gz isoroot/boot/ramfs.img
cp path/to/linux/kernel isoroot/boot/vmlinuz
The ISOLINUX/SYSLINUX files also have to be copied:
cp path/to/syslinux/bios/core/isolinux.bin isoroot
cp path/to/syslinux/bios/com32/elflink/ldlinux/ldlinux.c32 isoroot
ISOLINUX needs to be configured, create the file isoroot/isolinux.cfg, with the following:
default 1
label 1
kernel boot/vmlinuz
initrd boot/ramfs.img
append quiet
finally you can run:
mkisofs -no-pad -o tinyroot.iso -b isolinux.bin -no-emul-boot -boot-load-size 4 -boot-info-table isoroot
The iso should be at tinyroot.iso
You can test it with QEMU: qemu-system-x68_64 -cdrom tinyroot.iso