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Date: Sun, 7 Feb 2016 07:46:48 -0800
From: Jeremiah Mahler <jmmahler@gmail.com>
To: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Matthew Wilcox <matthew.r.wilcox@intel.com>,
        Hugh Dickins <hughd@google.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Stephen Rothwell <sfr@canb.auug.org.au>
Subject: Re: [REGRESSION] mm: filemap_map_pages NULL pointer dereference
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Konstantin,

On Sun, Feb 07, 2016 at 11:27:53AM +0300, Konstantin Khlebnikov wrote:
> On Sat, Feb 6, 2016 at 9:18 PM, Jeremiah Mahler <jmmahler@gmail.com> wrote:
[...]
> >> -static __always_inline unsigned
> >> +static __always_inline long
> >>  radix_tree_chunk_size(struct radix_tree_iter *iter)
> >>  {
> >>       return iter->next_index - iter->index;
> >> @@ -434,9 +434,9 @@ radix_tree_next_slot(void **slot, struct
> >>                       return slot + offset + 1;
> >>               }
> >>       } else {
> >> -             unsigned size = radix_tree_chunk_size(iter) - 1;
> >> +             long size = radix_tree_chunk_size(iter);
> >>
> >> -             while (size--) {
> >> +             while (--size > 0) {
> >>                       slot++;
> >>                       iter->index++;
> >>                       if (likely(*slot))
> >> _
> >>
> >
> > I have applied this patch to my kernel and so far the bug has not
> > come back.  Thanks for the quick fix.
> >
> > Although I don't quite understand how this fixes the slot==NULL problem.
> > Unless I am missing something, it looks like the while loop will be
> > executed the same number of times but the size variable will no
> > longer go negative as it did before.
> 
> That's simple. Slot is dereferenced after checking remaining size.
> Old version checked only for != 0. After iter-retry size is zero and
> afrer "- 1" it overlaps into positive range. In new version it's signed and
> checked for > 0.
> 

OK, I get it now.  Thanks for the explanation.

-- 
- Jeremiah Mahler