Your building steps work for me. I have been looking for such solutions for a while; yours is the simplest one. Thanks.

Now I am encountering an insmod problem:

insmod ./hell0_world.ko

ERROR: could not load module ./hell0_world.ko: No such file or directory

I am using buildroot master branch in my ubuntu host. My KDIR string is
/home/tclee/buildroot/output/build/linux-custom

tclee@tclee-Latitude-E6230:~/buildroot/output/build/linux-custom$ make kernelversion 6.12.20

My rpi3mb target is running a different kernel version:

Linux buildroot 6.12.20-v7 #1 SMP Tue Jun 3 20:03:35 PDT 2025 armv7l GNU/Linux

I am wondering if the kernel version difference is the cause of this failure.

I am resolving this problem; your comment is welcome.