Re: 32-bit bug in iovec iterator changes

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Jun 21, 2014 at 05:03:20PM -0700, James Bottomley wrote:

> > Anyway, does the following alone fix the problem you are seeing?
> > 
> > diff --git a/include/linux/uio.h b/include/linux/uio.h
> > index ddfdb53..dbb02d4 100644
> > --- a/include/linux/uio.h
> > +++ b/include/linux/uio.h
> > @@ -94,7 +94,7 @@ static inline size_t iov_iter_count(struct iov_iter *i)
> >  	return i->count;
> >  }
> >  
> > -static inline void iov_iter_truncate(struct iov_iter *i, size_t count)
> > +static inline void iov_iter_truncate(struct iov_iter *i, u64 count)
> >  {
> >  	if (i->count > count)
> >  		i->count = count;
> 
> Al, how can that work?  i->count is size_t, which is 32 bit, so we're
> going to get truncation errors.

No, we are not.  Look:
	* comparison promotes both operands to u64 here, so its result is
accurate, no matter how large count is.  They are compared as natural
numbers.
	* assignment converts count to size_t, which *would* truncate for
values that are greater than the maximal value representable by size_t.
But in that case it's by definition greater than i->count, so we do not
reach that assignment at all.