linux-kernel - Re: [PATCH v7 03/19] gup: Turn fault_in_pages_{readable,writeable} into fault_in

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Message-ID: <YVNE4HGKPb7bw+En@casper.infradead.org>
Date:   Tue, 28 Sep 2021 17:37:52 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Andreas Gruenbacher <agruenba@...hat.com>
Cc:     fdmanana@...il.com, Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        cluster-devel <cluster-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ocfs2-devel@....oracle.com
Subject: Re: [PATCH v7 03/19] gup: Turn fault_in_pages_{readable,writeable}
 into fault_in_{readable,writeable}

On Tue, Sep 28, 2021 at 05:02:43PM +0200, Andreas Gruenbacher wrote:
> On Fri, Sep 3, 2021 at 4:57 PM Filipe Manana <fdmanana@...il.com> wrote:
> > On Fri, Aug 27, 2021 at 5:52 PM Andreas Gruenbacher <agruenba@...hat.com> wrote:
> > > +size_t fault_in_writeable(char __user *uaddr, size_t size)
> > > +{
> > > +       char __user *start = uaddr, *end;
> > > +
> > > +       if (unlikely(size == 0))
> > > +               return 0;
> > > +       if (!PAGE_ALIGNED(uaddr)) {
> > > +               if (unlikely(__put_user(0, uaddr) != 0))
> > > +                       return size;
> > > +               uaddr = (char __user *)PAGE_ALIGN((unsigned long)uaddr);
> > > +       }
> > > +       end = (char __user *)PAGE_ALIGN((unsigned long)start + size);
> > > +       if (unlikely(end < start))
> > > +               end = NULL;
> > > +       while (uaddr != end) {
> > > +               if (unlikely(__put_user(0, uaddr) != 0))
> > > +                       goto out;
> > > +               uaddr += PAGE_SIZE;
> >
> > Won't we loop endlessly or corrupt some unwanted page when 'end' was
> > set to NULL?
> 
> What do you mean? We set 'end' to NULL when start + size < start
> exactly so that the loop will stop when uaddr wraps around.

But think about x86-64.  The virtual address space (unless you have 5
level PTs) looks like:

[0, 2^47)		userspace
[2^47, 2^64 - 2^47)	hole
[2^64 - 2^47, 2^64)	kernel space

If we try to copy from the hole we'll get some kind of fault (I forget
the details).  We have to stop at the top of userspace.