Accessing pointers inside struct passed as argument to ioctl calls

[Daniel.]

Hi all, I have a doubt about using pointers inside structs that are
passed (as pointers) to ioctl argument. Since pointers passed from
userspace can't be trusted, I need to copy they to kernel before
accessing they. In this case I have a pointer inside a struct that is
passed to the ioctl call also as pointer. So I need to copy the whole
struct to kernel space and only then dereference the pointer. If this
is true, two copy_from_user is needed right?

Suppose I have a struct like this

struct myioctl_arg {
    char *tx_buf;
    int tx_siz;
}

and this ioctl definition

#define MYIOCTL_TX _IOWR(MY_MAGIC, MY_BASE, struct myioctl_arg *);

At userspace program I do something like this:

struct myioctl_arg arg = { .tx_buf = somebuf, .tx_siz = 32 }
ioctl(fd, MYIOCTL_TX, &arg);

And in my ioclt implementation
static ioctl(struct file *fp, unsigned int cmd, unsigned long arg)
{
    struct myioctl_arg karg;
    char   kbuf[100];
    ...
    case MYIOCTL_TX:
          copy_from_user(&karg, arg, sizeof(karg));
          copy_from_user(kbuf, karg.tx_buf, karg.tx_siz);
          // now kbuf has the same contents of somebuf in userspace
   ...
}


I'm in doubt if this is the right way to access the userspace tx_buf.
Since the .tx_buf from arg is a userspace pointer, accessible from
userspace pointer arg, I can't type arg->tx_buf directly, doing this
is dereferencing a userspace pointer inside kernel. So I need to copy
the whole struct and dereference from the kernel space copy of it. So
this two calls of copy_from_user() is the right way to do it? They are
needed at all or is there better way of doing it?

Cheers,
-- 
"Do or do not. There is no try"
  Yoda Master