mini-strace

Goal

Setup

Host OS: Debian Linux 11

Configure Host OS

# Download and install ARM toolchain
$ arm-none-linux-gnueabihf-gcc --version
$ sudo apt install bc

QEMU

$ sudo apt install qemu-system-arm
$ qemu-system-arm --version
QEMU emulator version 9.2.2 (v9.2.2)
Copyright (c) 2003-2024 Fabrice Bellard and the QEMU Project developers

Choose a model

# List of supported boards
$ qemu-system-arm -machine help

I'll choose a virt with cortex-a7 CPU

$ qemu-system-arm -M virt -cpu cortex-a7 ...

Build a Linux Kernel

# Requirement
$ sudo apt install -y build-essential libncurses-dev bison flex libssl-dev libelf-dev bc
# Download linux kernel
$ git clone https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
# Move downloaded directory
$ cd linux
# Configure
$ make ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabihf- vexpress_defconfig
# Build 
$ make ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabihf- -j$(nproc)
# Output directory
$ ls arch/arm/boot
# Check zImage
$ file arch/arm/boot/zImage

BusyBox

# Downlaod busybox 1.36.1
$ wget https://busybox.net/downloads/busybox-1.36.1.tar.bz2
# Decompress
$ tar xvjf busybox-1.36.1.tar.bz2
# Move
$ cd busybox-1.36.1
# configure
$ make ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabihf- defconfig
# make
$ make ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabihf- -j$(nproc)
# make install
$ make ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabihf- install

NOTE: busybox-1.37.0 has an error

_install directory includes output files.

Then, create the init file

#!/bin/sh
mount -t proc none /proc
mount -t sysfs none /sys
exec /bin/sh

$ chmod +x init
$ find . | cpio -o --format=newc > ../initramfs.cpio
$ ls ../ | grep initramfs
initramfs.cpio

$ file initramfs.cpio
initramfs.cpio: ASCII cpio archive (SVR4 with no CRC)

Run QEMU

$ qemu-system-arm -M virt -m 256M -cpu cortex-a7 -kernel zImage -initrd initramfs.cpio -append "console=ttyAMA0 rdinit=/init" -nographic

Test program: Simple hello world.

#include <stdio.h>
int main(void) { 
  printf("hello world\n");
}

$ arm-none-linux-gnueabihf-gcc -static -o main main.c
$ cp main busy-box/_install/bin

Then, create initramfs.cpio again.

Run QEMU with new rootfs.

System Calls Manual

# introduction to system calls
$ man 2 intro
# 
$ man 2 syscalls

ptrace system call

Technical Notes

What is zImage?

Relation busybox with rootfs

What is syscall?

About ptrace