Данные не отправляются в пространство пользователя.

        char *data = kstrdup ( "test", GFP_USER );
        copy_to_user ( buf, data, 5 ); 
        kfree ( data );

ssize_t scull_write ( struct file *file, const char __user *buf, size_t n, loff_t *lt )
{
        char *data = kstrdup ( "test", GFP_USER );
        copy_to_user ( buf, data, 4 ); 
        kfree ( data );
        lt += 4;

}

ssize_t scull_read ( struct file *file, char __user *buf, size_t count, loff_t *pos)
{
  char *data = kstrdup ( "test", GFP_USER );
  copy_to_user ( buf, data, 4 ); 
  kfree ( data );
  pos += 4;
  return 4;
}

pos += 4;

ssize_t scull_read ( struct file *file, char __user *buf, size_t count, loff_t *pos)
{

        char *data = kstrdup ( "test", GFP_USER );
        char *sdata = kstrdup ( "new", GFP_USER );
        copy_to_user ( buf, data, 4 ); 
        copy_to_user ( buf, sdata, 3 ); 
        kfree ( data );
        kfree ( sdata );
        return 7;

}

static char x[]="zdasdfasdfa";

copy_to_user(buf, z, sizeof(x));

KSTRDUP(9)                          Memory Management in Linux                         KSTRDUP(9)

NAME
       kstrdup - allocate space for and copy an existing string

SYNOPSIS
       char * kstrdup(const char * s, gfp_t gfp);

ARGUMENTS
       s
           the string to duplicate

       gfp
           the GFP mask used in the kmalloc call when allocating memory

COPYRIGHT
Kernel Hackers Manual 4.10                  April 2017                                 KSTRDUP(9)

        char data[255];
        char *p, *h;
        p = &data[0];

        struct task_struct *task;
        int i = 0;
        int len = 0;

        for ( task = current; task != &init_task; task = task->parent )
        {
                h = &task->comm[0];

                while ( *h != 0 )
                {
                        *p = *h;
                        p++;
                        h++;
                        len++;
                }

        }
        printk ( KERN_NOTICE "len %d\n", len );
        copy_to_user ( buf, data, len ); 
        return len;