Cross-Compilation of Linux Kernel and Debug with QEMU and GDB

In the following tutorial it uses LLVM=1 because I wanted to use LLVM to compile because I was using rust for linux.

Kernel Build

make LLVM=1 ARCH=x86_64 menuconfig

Kernel hacking → Compile the Kernel with debug info Kernel hacking → Provide GDB scripts for Kernel debugging

make LLVM=1 ARCH=x86_64 CROSS_COMPILE=x86_64-unknown-linux-gnu -j$(nproc)

Buildroot

git clone https://github.com/buildroot/buildroot
cd buildroot
make LLVM=1 ARCH=x86_64 menuconfig

Target Options → Target Architecture → x86_64

Filesystem images → ext2/3/4 root file system → ext4

make LLVM=1 ARCH=x86_64 CROSS_COMPILE=x86_64-unknown-linux-gnu -j$(nproc)

QEMU

Installing

sudo apt install qemu qemu-system

To boot the kernel using the root filesystem generated by Buildroot, run the following command:

BUILDROOT_PATH=../buildroot
qemu-system-x86_64 \
    -kernel arch/x86/boot/bzImage \
    -boot c \
    -m 2049M \
    -drive file=$BUILDROOT_PATH/output/images/rootfs.ext4,format=raw \
    -append "root=/dev/sda rw console=ttyS0,115200 acpi=off nokaslr" \
    -serial stdio \
    -display none \
    -s -S

-s: Opens a GDB server on port 1234.
-S: Stops QEMU execution, allowing GDB to connect.

Debugging with gdb-multiarch

Installing:

sudo apt-get install gdb-multiarch

Add the following line to your ~/.gdbinit file to load the GDB scripts provided by the kernel for enhanced debugging:

add-auto-load-safe-path <replace_kernel_path>/scripts/gdb/vmlinux-gdb.py

This enables GDB helper scripts that are useful for kernel debugging.

Debugging Session:

Start QEMU in Debug Mode

Ensure QEMU is running with the -s -S options so that it waits for GDB to connect.

Lauch GDB

In another terminal, start gdb-multiarch and load the kernel symbols from vmlinux:

gdb-multiarch vmlinux

In GDB, connect to QEMU by running target remote :1234:

(gdb) target remote :1234
Remote debugging using :1234
0x000000000000fff0 in exception_stacks ()

Now you can debug the kernel:

(gdb) hb start_kernel
Hardware assisted breakpoint 1 at 0xffffffff834174c3: file init/main.c, line 905.
(gdb) c
Continuing.

Breakpoint 1, start_kernel () at init/main.c:905
905		char *command_line;

References

https://medium.com/@depressedcoder/m1-mac-linux-kernel-development-environment-setup-748637131f92

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