Is it necessary to copy int to kernel space before accessing

[bestwc]

Dear all,

I'm new to FreeBSD kernel programming, I've seen people talking about using copy to copy user space data to kernel space instead of accessing them directly. I tried copying strings with copyinstr(9) and there's no problem, but I seem can't copy an int value using copyin(9).

Then I realized that it might not even be necessary to do so, since I can just assign the int value to a kernel space variable.

/* The system call's arguments */
struct sysmonitor_args {
    int target;
    //char *target;
};

/* The system call function */
static int sysmonitor(struct thread *td, void *syscall_args)
{
    struct sysmonitor_args *args; /* local struct to receive syscall_args */
    args = (struct sysmonitor_args *)syscall_args; /* receive syscall_args with casting */

    /* Copy args to kernel space */
    int target = args->target;
    printf("target: %p\n", &target);
    printf("args target: %p\n", &args->target);

    return (0);
}

And I can see two different addresses. Not sure if this means it's okay to do such thing. Otherwise, I would like to know why my copyin(9) always return EFAULT.

int target;
size_t size = sizeof(int);
copyin(&args->target, &target, size);

Thank you very much.

Best Regards,
BestWC

 

[trasz@]

When your syscall runs, the arguments ('syscall_args' above) are already copied into the kernel; you can just use them directly. That's why copyin(9) failed - you gave it a pointer that was a kernel address, while it expected userspace address.

You need to use copyin(9) for things other than the automatically copied syscall_args. Take the string, for example - the pointer to the first char (syscall_args->target) was copied for you, but the data it points to wasn't. That's why you need to use copyinstr(9) in that case.