Subject: Re: [PATCH 03/16] mm: simplify gup_fast_permitted

From: Christoph Hellwig <[email-masked]>
Date: Mon, 3 Jun 2019 09:41:21 +0200

On Sat, Jun 01, 2019 at 09:14:17AM -0700, Linus Torvalds wrote:
> On Sat, Jun 1, 2019 at 12:50 AM Christoph Hellwig <[email-masked]> wrote:
> >
> > Pass in the already calculated end value instead of recomputing it, and
> > leave the end > start check in the callers instead of duplicating them
> > in the arch code.
> 
> Good cleanup, except it's wrong.
> 
> > -       if (nr_pages <= 0)
> > +       if (end < start)
> >                 return 0;
> 
> You moved the overflow test to generic code - good.
> 
> You removed the sign and zero test on nr_pages - bad.

I only removed a duplicate of it.  The full (old) code in
get_user_pages_fast() looks like this:

	if (nr_pages <= 0)
		return 0;

	if (unlikely(!access_ok((void __user *)start, len)))
		return -EFAULT;

	if (gup_fast_permitted(start, nr_pages)) {