Undefined symbols in hello world C++ kernel module - linux

Question

I have added C++ support to the Linux kernel version 4.14.41, compiled it and booted using the kernel successfully. I can check the correctness of the C++ module by inserting a LKM. This is the module that I am trying to load:
#include<c++/begin_include.h>
#include<linux/module.h>
#include<linux/kernel.h>
#include<c++/end_include.h>
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("LKM in c++");
MODULE_AUTHOR("MOOL");
class hello
{
public:
hello();
void hi();
};
void hello::hi()
{
printk("Hello world!! \n");
}
hello::hello()
{
printk("Constructor is being called \n");
}
extern "C"
{
static int __init test_classes_init()
{
class hello obj;
obj.hi();
printk("Module inserted:\n");
return 0;
}
static void __exit test_classes_fini()
{
printk("Module removed:\n");
}
module_init(test_classes_init);
module_exit(test_classes_fini);
}
The Makefile:
obj-m = helloworld.o
KVERSION=$(shell uname -r)
all:
make -C /lib/modules/$(KVERSION)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(KVERSION)/build M=$(PWD) clean
When I enter the make command, the helloworld.ko is generated with the warnings
WARNING: "begin_fini" [/home/jai/Downloads/helloworld/helloworld.ko] undefined !
WARNING: "end_init" [/home/jai/Downloads/helloworld/helloworld.ko] undefined !
WARNING: "begin_init" [/home/jai/Downloads/helloworld/helloworld.ko] undefined !
But when I try to insert it using insmod helloworld.ko, the undefined symbol error occurs.
dmesg:
loading out-of-tree module taints kernel
Unknown symbol begin_init (err 0)
Unknown symbol end_init (err 0)
Unknown symbol begin_fini (err 0)
These begin_init, end_init and begin_fini are defined in lib/gcc/crtstuff.c (which was ported into the kernel). These functions are declared as extern in both crtstuff.c and linux/module.h. This module.h is being included in the helloworld module above, but still, those symbols become undefined. So, How can I make those functions defined?

Answer 1

Your kernel C++ implementation is incomplete. You will have to implement global constructor and destructor support (processing of .init_array and .fini_array sections properly), or stop using these C++ features in the source code. This needs cooperation from the kernel module loader. Changes to the startup code will not work because the startup code is not linked into kernel modules.