[K42-discussion] crash in autoconf's mmap test

[Bryan S Rosenburg]

Livio is correct.  I wish I'd remembered his bug report from a few months 
back.  Andrew's test program mmap's a 1-page file on top of a page 
obtained from malloc.  We should be splitting the region backing the 
malloc pool into three regions, but instead we're simply deleting the tail 
of the original pool from the address space.  A subsequent malloc 
segfaults when it touches that part of the pool.

Livio's "fix" (which he published only to prove the nature of the bug) is 
to fill the tail with a new region.  That solution effectively zeros part 
of the malloc pool and is clearly bogus.

The suggested correct solution involves a "splitRegion" service.  I find 
that we have a method with that name implemented in ProcessReplicated, but 
it hasn't been exported or used.  I suspect it hasn't been tested.  I'll 
ask Jonathan for more of the story when he gets back from vacation (on 
Monday, I think).

- Bryan




Livio Soares <livio at eecg.toronto.edu> 
Sent by: k42-discussion-bounces at ozlabs.org
09/09/2005 11:40 AM

To
Andrew Baumann <andrewb at cse.unsw.edu.au>
cc
K42-discussion at ozlabs.org
Subject
Re: [K42-discussion] crash in autoconf's mmap test






  Hey Andrew,

  I filed  a bug report,  and a "fix"  (well, not really)  to get past the 
mmap
crash in autoconf a few months ago. Check it out here:

http://ozlabs.org/bugzilla/show_bug.cgi?id=14

  I eventually  talked to Jonathan about it  and we had some  ideas about 
fixing
the  issue  permanently  (it   included  changing  how Regions/sub-Regions 
 are
created/managed). Alas,  life has  been crazy, so  none of the  things 
discussed
went forward.

  Hope this helps,

                                 Livio
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