linux-kernel - Re: [PATCH] mm/gup.c: Handling ERR within unpin_user

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Message-ID: <20200914140814.GE1221970@ziepe.ca>
Date:   Mon, 14 Sep 2020 11:08:14 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Souptick Joarder <jrdr.linux@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH] mm/gup.c: Handling ERR within unpin_user_pages()

On Mon, Sep 14, 2020 at 07:20:34AM +0530, Souptick Joarder wrote:
> On Sun, Sep 13, 2020 at 8:25 PM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > On Sun, Sep 13, 2020 at 08:02:35PM +0530, Souptick Joarder wrote:
> > > It is possible that a buggy caller of unpin_user_pages()
> > > (specially in error handling path) may end up calling it with
> > > npages < 0 which is unnecessary.
> > > @@ -328,6 +328,9 @@ void unpin_user_pages(struct page **pages, unsigned long npages)
> > >  {
> > >       unsigned long index;
> > >
> > > +     if (WARN_ON_ONCE(npages < 0))
> > > +             return;
> >
> > But npages is unsigned long.  So it can't be less than zero.
> 
> Sorry, I missed it.
> 
> Then, it means if npages is assigned with -ERRNO by caller, unpin_user_pages()
> may end up calling a big loop, which is unnecessary.

How will a caller allocate memory of the right size and still manage
to call with the wrong npages? Do you have an example of a broken caller?

Jason