Re: copy_from_user returns a positive value?

[Eli Carter <eli.carter@inet.com>]

Ben Greear wrote:
> 
> I have an IOCTL defined something like this:
> 
>         _IOWR (0xfe, (30<<3 + 0), __u8 [696])
> 
> I'm really passing in a structure of size 696 (does that matter)?
> 
> When I make the copy from user call:
> 
>        if ((ret = copy_from_user(&reqconf, arg, sizeof(reqconf)))) {
>           printk("ERROR: copy_from_user returned: %i, sizeof(reqconf): %i\n",
>                  ret, sizeof(reqconf));
>           return ret;
>        }
> 
> I see this printed out:
> 
> ERROR: copy_from_user returned: 696, sizeof(reqconf): 696
> 
> According to some docs I saw on the web, it should return 0, or the
> number it has left to copy.  So, why does it have 696 bytes left
> to copy??

Because it couldn't copy any of the data?  The code I have seen
generally returns -EFAULT in that case.
Could you be trying to copy data from somewhere that the user does not
have permission to read? Can you verify that both pointers are valid? 
&reqconf should be in the kernel's memory space and arg should be a
pointer provided by the user-space app pointing to memory in userland.

You might want to get the Linux Device Drivers book... the 2nd ed. is
out.

HTH,

Eli
--------------------. "If it ain't broke now,
Eli Carter           \                  it will be soon." -- crypto-gram
eli.carter(a)inet.com `-------------------------------------------------