[PATCH 2/4] Input: introduce ABS_MAX2/CNT2 and friends

[Peter Hutterer]

On Wed, Dec 18, 2013 at 04:05:37PM -0800, Dmitry Torokhov wrote:
> On Thu, Dec 19, 2013 at 09:55:04AM +1000, Peter Hutterer wrote:
> > On Wed, Dec 18, 2013 at 03:48:37PM -0800, Dmitry Torokhov wrote:
> > > On Thursday, December 19, 2013 09:40:09 AM Peter Hutterer wrote:
> > > > > +     memset(&abs, 0, sizeof(abs));
> > > > > +     for (i = valid_cnt; i < cnt; ++i)
> > > > > +             if (copy_to_user(&pinfo->info[i], &abs, sizeof(abs)))
> > > > > +                     return -EFAULT;
> > > > > +
> > > > > +     return 0;
> > > > 
> > > > why don't you return the number of valid copied axes to the user?
> > > > that seems better even than forcing the remainder to 0.
> > > 
> > > Well, if your program messed up buffers that it faulted we do not know
> > > for sure if data that did not cause fault ended up where it should have
> > > or if it smashed something else. This condition I think should be
> > > signaled early.
> > 
> > not 100% sure I understand but I wasn't proposing to remove the -EFAULT, i
> > was proposing to replace "return 0" with "return valid_cnt".
> 
> I understand what you were saying. Now consider: your program supplied
> buffer that is actually smaller than what it said to the kernel.
> Depending on the exact placement we may or may not fault when we get
> pass the buffer boundary, most likely not. We are likely to fault when
> we go way past the buffer boundary and wracked process' memory. If we
> return -EFAULT the program will at least notice that something wrong. If
> we return count it will try to resubmit the remainder of operation and
> not even know that there was something very bad happening.
> 
> IOW we should not treat fault condition as other partial read/write
> conditions.

I'm still not sure we're talking about the same thing :)
let me rephrase: why can't we use the behaviour bits_to_user() provides?
it limits the output to maxlen and returns that value (or -EFAULT), it's
only a small step from that to limit the output to min(maxbit, ABS_CNT2).

Cheers,
   Peter