How to set up QEMU to debug the kernel using gdb

Install packages that we need. For Ubuntu this includes:

 apt-install git build-essential ncurses-dev xz-utils libssl-dev \
             bc flex libelf-dev bison qemu-system-x86 gdb

Get the Linux source. (Or use your Linux distribution's package manager to fetch it.)

 mkdir ~/src
 cd ~/src
 # Choose the tag for the version you want...
 git clone --depth 1 --branch v6.8 \
     git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
 cd linux

Choose debugging parameters for the kernel. (You can use your Linux distribution's config from /boot but it probably has a lot of features enabled and therefore take a long time to compile.)
```
 make defconfig
 ./scripts/config -d DEBUG_INFO_NONE \
                  -e DEBUG_INFO \
                  -e DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
 make oldconfig
```

Compile the kernel.

 # Choose a number of parallel processes based on the number of
 # processors in your CPU.
 make -j $(nproc)

Make initramfs. In this example we'll use ~/src/linux/my_test_binary as the "init" process. If you want to boot a full system then it will be more complicated!

 cat << EOF > initramfs
 dir /dev 755 0 0
 nod /dev/console 644 0 0 c 5 1
 nod /dev/loop0 644 0 0 b 7 0
 dir /bin 755 1000 1000
 dir /proc 755 0 0
 dir /sys 755 0 0
 dir /mnt 755 0 0
 file /init my_test_binary 755 0 0
 EOF
 ./usr/gen_initramfs.sh initramfs | gzip -9 > cpio

Start QEMU. This will wait for you to attach GDB in the next step. qemu-system-x86_64 -kernel ~/src/linux/arch/x86_64/boot/bzImage
-initrd /boot/initrd.img-6.8.0-51-generic
-append "console=ttyS0 nokaslr"
-s -S -nographic
In another terminal, start GDB.
```
 cd ~/src/linux
 gdb ./vmlinux
```

In GDB, attach to the kernel. Happy debugging!

 target remote :1234

 # You can set breakpoints
 hb load_elf_binary
 continue

References

https://kernelnewbies.org/KernelBuild https://www.youtube.com/watch?v=2VcA5Wj7IvU https://nickdesaulniers.github.io/blog/2018/10/24/booting-a-custom-linux-kernel-in-qemu-and-debugging-it-with-gdb/ https://bmeneg.com/post/kernel-debugging-with-qemu/ https://trac.gateworks.com/wiki/linux/initramfs

ben-cohen/DebugLinuxWithQEMU.md

How to set up QEMU to debug the kernel using gdb

References