Re: [PATCH 5/8] xdl_change_compact(): fix compaction heuristic to adjust io

> On Thu, Aug 04, 2016 at 12:00:33AM +0200, Michael Haggerty wrote:
> 
>> The code branch used for the compaction heuristic incorrectly forgot to
>> keep io in sync while the group was shifted. I think that could have
>> led to reading past the end of the rchgo array.
>>
>> Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
>> ---
>> I didn't actually try to verify the presence of a bug, because it
>> seems like more work than worthwhile. But here is my reasoning:
>>
>> If io is not decremented correctly during one iteration of the outer
>> `while` loop, then it will loose sync with the `end` counter. In
>> particular it will be too large.
>>
>> Suppose that the next iterations of the outer `while` loop (i.e.,
>> processing the next block of add/delete lines) don't have any sliders.
>> Then the `io` counter would be incremented by the number of
>> non-changed lines in xdf, which is the same as the number of
>> non-changed lines in xdfo that *should have* followed the group that
>> experienced the malfunction. But since `io` was too large at the end
>> of that iteration, it will be incremented past the end of the
>> xdfo->rchg array, and will try to read that memory illegally.
> 
> Hmm. In the loop:
> 
>   while (rchgo[io])
> 	io++;
> 
> that implies that rchgo has a zero-marker that we can rely on hitting.