Copy_to_user problems - device-driver

Question

I wrote module for a device, and I'm having problems in my read function:
ssize_t my_sys_read(struct file *f, char __user *buffer, size_t s, loff_t *off){
char * myBuffer = "ossec buffer";
size_t read_bytes;
if (s > ( sizeof(char) * 13 ) ) s = ( sizeof(char) * 13 );
if (!access_ok(%VERIFY_WRITE, (void *) buffer, s)) return -EFAULT;
read_bytes = copy_to_user((void *) buffer, (void *) myBuffer, s);
printk(KERN_INFO "myBuffer %s", myBuffer);
printk(KERN_INFO "buffer %s", buffer);
read_bytes = s - ( sizeof(char) * 13 );
return read_bytes;
}
I really don't know why but the copy it's not working, and the printed buffer has no sense for me.
[10038.885838] buffer \xffffff81\xffffffc3\xffffffcb\x1a
I guess the problem is in the copy because the program which use the device is simple.
int main(void)
{
int fd = open(device_name, O_RDONLY);
if(fd < 0)
{
printf("Error: Impossible to open device, action not permited.\n");
return 0;
}
char * buff;
int read_bytes;
read_bytes = read(fd, buff, (13 * sizeof(char) ) );
printf(" %s\n", buff);
}
Thanks!

Answer 1

char * buff;
int read_bytes;
read_bytes = read(fd, buff, (13 * sizeof(char) ) );
You are using the pointer buff without allocating memory for it, it is then a dangling pointer with a random value. You need to do
char *buff = new char(); or
char buff;
int read_bytes;
read_bytes = read(fd, &buff, (13 * sizeof(char)) );