When I am programming a loadable kernel module in Linux, I added a system call that traverses processes and stores them in a structure, then uses copy_to_user() to copy the structure from kernel space to user space. However, I encountered an error during the copy_to_user() process.
this is my struct:

struct process
{
    int pid;
    int depth;
    int have_brother;
    char name[16]; 
}

Below is my module code.

#define free_syscall_num    336             //free syscall number
#define syscall_table_address 0xffffffff8d200320    //syscall table address

long(*save_syscall)(void);                  //Save the original sys_ni_syscall();
unsigned long *syscall_table=0;             

struct process a[1000];         
int counter = 0;    

/*
*Recursively traverse the linux process tree
*/
void preorder_traversal_processtree(struct task_struct *p_task,int b)
{
    struct list_head *list;
        
    a[counter].pid = p_task -> pid;
    a[counter].depth = b;
    if(p_task -> sibling.next == &(p_task->parent->children))   
        {
            a[counter].have_brother=0;
        }
    else
        {
            a[counter].have_brother=1;
        }
    
    strcpy(a[counter].name,p_task -> comm);                     
    printk("pid: %dn", a[counter].pid);
    counter ++;

    list_for_each(list, &p_task->children)                      
    {
        printk("pstree is running, counter = %dn", counter);
        struct task_struct *t = list_entry(list, struct task_struct, sibling); 
        preorder_traversal_processtree(t,b+1);                                  
    }
    
}

/*
The body of the system call
*/
asmlinkage long my_syscall(char __user* buf)
{
    counter = 0;
    preorder_traversal_processtree(&init_task,0);                           
    if (!access_ok(buf, 1000 * sizeof(struct process))) 
    {
        printk("Invalid user space addressn");
    }

    if(copy_to_user((struct process *)buf, a, 1000*sizeof(struct process)))
    {
        printk("Efaultn");
        return -EFAULT;
        
    }
    else
    {
        printk("%lun",sizeof(a));
        return sizeof(a);
    }
}


module_init(mymod_init);    
module_exit(mymod_exit);    
MODULE_LICENSE("GPL");

this is my test code:

 
struct process a[1000];
int brother_reg[100]; 

int main()
{
    int i,j;
    int x = syscall(336, &a);   
    
    printf("%dn",a[0].pid); 
    return 0;
}

I tried testing with access_ok() and found that the address is invalid. When I tested with strace, syscall_0x150() returned -1 (bad address).Linux is Linux 5.15.0-105-generic.
Any help will great.