copy_to_user does not return expected data

Question

I have a ioctl in my kernel driver and that needs to return some data upon read. When I read it back in userspace, it's not showing expected results.

Userspace snippet:

typedef struct abc_T {
    int size;
    int addr;
    int data[32];
} abc_T;

// Read
int abc_read(int addr, int size, int *data) {
     abc_T abc = {};
     int fd, retval;
     abc.size = size;
     abc.addr = addr;
     fd = open("/dev/"ABC_DEV, O_RDWR);

     if (fd >=0) {
     retval = ioctl(fd, READ_ABC, &abc);
     if (retval == 0)
         memcpy(data, abc.data, size);

     }
return retval;
}

Kernel:

static int ABC_ioctl(struct file * file, uint cmd, ulong arg)
{
ABC_T abc;
void __user * arg_ptr;
arg_ptr = (void *)arg;
int retval;

if (!access_ok(VERIFY_READ, arg_ptr, sizeof(ABC_T))) {

    return -EACCES;
}

if (copy_from_user(&abc, arg_ptr,
                        sizeof(ABC_T)) != 0) {
    return -EFAULT;
}

switch(cmd) {
case READ_ABC:
retval = read_func(&abc);
if (retval == 0) {
    if (copy_to_user((void __user *)arg, &abc,
                     sizeof(ABC_T) != 0)) {
                retval = -EFAULT;
    } else {
        retval = 0;
    }
}
return retval;
}

I have printed the results (i.e. data array) in the read_func and can see that the read is expected value , but when after copy_to_user, when I print data in the userspace, I see all zeros. Is there something wrong with this snippet?

Answer 1

Your parens are off on the call to copy_to_user. I think you mean to test if the response of copy_to_user is != 0. Instead, you're putting sizeof(ABC_T) != 0 as the last argument. Since sizeof(ABC_T) is non-zero, your call ends up being copy_to_user((void __user *)arg, &abc, true).

Fix your parens and see if you get better results:

if (copy_to_user((void __user *)arg, &abc, sizeof(ABC_T)) != 0) {
    // ...
}