Re: 32-bit bug in iovec iterator changes

James Bottomley
2014-06-22T00:32:48Z

On Sun, 2014-06-22 at 01:26 +0100, Al Viro wrote:
> 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.

True ... figured this out 10 seconds after sending the email.

> * 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.

OK, so what I still don't get is why isn't the compiler warning when we
truncate a u64 to a u32? We should get that warning in your new code,
and we should have got that warning in fs/block_dev.c where it would
have pinpointed the actual problem.

James