kernel-debug.txt

How Linux Kernel Runs Executables ,
https://www.youtube.com/watch?v=ZlZDWeVL2LI&t=11s ,
https: //github.com/nir9/welcome ,

# building a kernel,
# which we can debug with gdb,

$ cd ~/cloned/linux

$ make defconfig

# or copy a existing running config

$ cp /boot/config-6.12.8-amd64 .

# enable debugging options:
# - kernel hacking, .. , Debug information,
# - Generic Kernel Debugging Instruments, KGDB: kernel debugger,

$ make menuconfig
$ make -j 4
$ make scripts_gdb

$ cd ~/fun/bash-5.2.37
$ ./configure --enable-static-link
$ make -j 4

# rename the output executable bash as init
$ cd ~/fun/
$ mv ~/fun/bash-5.2.37/bash init

$ cd ~/fun/
$ vi hello.c
#include <unistd.h>
void main(){
    write(1, "hello\n", 6);
    _exit(0xa4);
}

$ gcc -static -o hello --entry main hello.c
$ ldd hello

# make initramfs that we can use with newly built kernel

$ echo hello >> list
$ echo init >> list
$ cat list
hello
init
$

$ cat list | cpio -o -H newc > init.cpio
$


# https://docs.kernel.org/dev-tools/gdb-kernel-debugging.html ,

$ vi ~/.gdbinit
add-auto-load-safe-path ~/cloned/linux
$

boot the system that we just built.
use QEMU for this.
make QEMU to boot in debug mode so we can connect with GDB.

# target
# gdb server
# bzImage is compressed kernel

# https://www.qemu.org/docs/master/system/gdb.html#gdb-usage ,
# The -s option will make QEMU listen for an incoming connection from gdb on TCP port 1234,
# qemu option ` -s `, or -gdb tcp::1234 , default port 1234,

$ cd ~/cloned/linux
$ qemu-system-x86_64 -kernel ./arch/x86/boot/bzImage \
> -initrd ~/fun/init.cpio -s
$

# the new kernel is booting up and we drop into bash.

# host
# vmlinux is uncompressed kernel,
# it is with debug information like function name, line number,
# ` target remote :1234 `, connects gdb to the server,

$ cd ~/cloned/linux
$ gdb ./vmlinux
(gdb) target remote :1234
(gdb)
$