How to use copy_to_user

Question

I'm trying to add a custom system call into the linux kernel. Here is a simple code:

#include <linux/mysyscall.h>
#include <linux/kernel.h>
#include <asm/uaccess.h>
#include <asm/system.h>

asmlinkage int sys_mysyscall(int *data){

    int a = 3;

    cli();
    copy_to_user(data, &a, 1);
    sti();

    printk(KERN_EMERG "Called with %d\n", a);

    return a;
}

I can compile a kernel with mysyscall added and when I try to access it with a user program like:

#include <linux/mysyscall.h>

int main(void){

    int *data;
    int r;
    int a = 0;
    data = &a;

    r = mysyscall(data);

    printf("r is %d and data is %d", r, *data);
}

*data does not equal to 3 it equals to 0.

How should I use copy_to_user to fix it?

Answer 1

The copy to user line of code copies only one byte from 'a'. In case of little endian systems it is going to be 0. Copy all the 4 bytes to get the correct result.