implement custom system call by module way

Makefile

obj-m += project_mod.o

all:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

project_mod.c

#include <linux/linkage.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/path.h>
#include <linux/slab.h>

static void print_mem(struct task_struct *task) {
        struct mm_struct *mm;
        struct vm_area_struct *vma;
        struct path base_path;
        char* tpath = NULL;
        char* ret_ptr = NULL;

        mm = task->mm;
        // printk("\nThis mm_struct has %d vmas.\n", mm->map_count);

        for ( vma = mm->mmap ; vma ; vma = vma->vm_next ) {
                printk("0x%lx, 0x%lx", vma->vm_start, vma->vm_end);

                if ( vma->vm_file != NULL ) {
                        tpath = (char*)kmalloc(512, 0);
                        base_path = vma->vm_file->f_path;
                        ret_ptr = d_path(&base_path, tpath, 512);
                        printk(" %s", ret_ptr);
                }       
                
                printk("\n");
        }       
}       

static int mm_exp_load() {
        long pid = 1;
        struct task_struct *task;
        printk("\nprocess id: %d.\n", pid);

        for_each_process(task) {
                if ( task->pid == pid ) {
                        printk("process name: %s\n", task->comm, task->pid);
                        print_mem(task);
                }
        }
        return 0;
}

static void mm_exp_unload(void) {
        printk("\nPrint segment information module exiting.\n");
}

module_init(mm_exp_load);
module_exit(mm_exp_unload);