Providing permissions for a root user in a kernel module - c

Question

Linux Kernel 5.0.0-37
I'm writing a function to manage permissions to be provided as a function pointer to the struct inode_operations. Here is a simplified stub implementation:
#include <linux/cred.h>
int pfsw_permission(struct inode *ino, int op){
if(uid_eq(INIT_USER->uid, get_current_user()->uid)){
printk(KERN_INFO "Current user is root\n");
} else {
printk(KERN_INFO "Current user is not root\n");
}
if(op & (MAY_READ | MAY_WRITE)){
return 0;
}
return -EACCES;
}
when compiling a kernel module containing this function and trying to load it dmesg shows the following error:
Unknown symbol root_user (err -2)
This I think is due to INIT_USER macro coming from include/linux/sched/user.h which is defined as
extern struct user_struct root_user;
#define INIT_USER (&root_user)
QUESTION: Why is the symbol root_user declared, but not defined? How to use INIT_USER correctly?

Answer 1

root_user does not seem to be exported using EXPORT_SYMBOL by the linux kernel; therefore you cannot use it from a module.
Looking at its definition we can see that the uid value is set to GLOBAL_ROOT_UID. This is a macro defined in include/linux/uidgid.h and is basically just a type cast of 0 to kuid_t so you can use just that macro if all you need is the UID.
So ...
Why is the symbol root_user declared, but not defined?
The symbol is defined. It's not exported though so you cannot use it from a module.
How to use INIT_USER correctly?
"Not". You cannot use it.