MODULE_LICENSE is defined but undetected in linux kernel 5.X - c

Question

I have a simple kernel module with multiple objects. When I compile it, it prompts me that MODULE_LICENSE is missing. It's defined in the main.o but somehow undetected by the compiler:
make -C /lib/modules/`uname -r`/build M=`pwd` modules
make[1]: Entering directory '/usr/src/kernels/5.14.0-105.el9.x86_64'
CC [M] /tmp/linux5-multi-files/util.o
LD [M] /tmp/linux5-multi-files/main.o
MODPOST /tmp/linux5-multi-files/Module.symvers
ERROR: modpost: missing MODULE_LICENSE() in /tmp/linux5-multi-files/main.o
make[2]: *** [scripts/Makefile.modpost:150: /tmp/linux5-multi-files/Module.symvers] Error 1
make[2]: *** Deleting file '/tmp/linux5-multi-files/Module.symvers'
make[1]: *** [Makefile:1792: modules] Error 2
make[1]: Leaving directory '/usr/src/kernels/5.14.0-105.el9.x86_64'
make: *** [Makefile:5: build] Error 2
On older Linux (centos 6-7) it works fine, but on CentOS 8 and CentOS 9, it no longer works. What was wrong? I've attached all the files.
-- attached all files --
Makefile
main-objs := util.o
obj-m += main.o
build:
make -C /lib/modules/`uname -r`/build M=`pwd` modules
clean:
make -C /lib/modules/`uname -r`/build M=`pwd` clean
main.c
#include "util.h"
static void __exit cleanup(void)
{
}
static int __init startup(void)
{
test();
return 0;
}
module_init(startup);
module_exit(cleanup);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Linux");
util.h
#ifndef UTIL_H
#define UTIL_H
#include <linux/module.h>
#include <linux/kernel.h>
void test(void);
#endif
util.c
#include "util.h"
void test()
{
printk(KERN_INFO "whatever\n");
}

Answer 1

To further debug this issue, I've added -v to the ccflags, and main.c is not even compiled on CentOS 8!
So far it looks like the module name can't be the same as the object name, so I have to modify the makefile to something like this:
xxx-objs := main.o util.o
obj-m += xxx.o
And the problem is gone.