[PATCH] Adding limitation while checking for cardTable

[Neo Jia]

On 5/17/07, Y.S.Ramakrishna at sun.com <Y.S.Ramakrishna at sun.com> wrote:
> Hi Neo -- thanks for finding this problem and
> initiating a fix. I will help shepherd this bug fix via:
>
> 6559052 [OpenJDK] CardTableRS::do_MemRegion() could attempt
> out-of-bounds translation in addr_for()

Ramki,

Just saw the bug from SDN. Thanks.

Neo

>
> -- ramki.
>
> Neo Jia wrote:
>
> > On 5/10/07, Y Srinivas Ramakrishna <Y.S.Ramakrishna at sun.com> wrote:
> >
> >> Hi Neo --
> >>
> >> Thanks for the heads-up. This seems worth fixing, at least
> >> for hygiene, How about the following instead, where we avoid the
> >> addr_for() translation until we are sure it will be safe,
> >> provided that the mr argument to the method is backed by
> >> card table entries. This avoids the need for the
> >> extra compare-and-branch inside the loop in your patch.
> >>
> >
> > Ramki,
> >
> > Thanks. Your fix makes more sense than mine.
> >
> > Neo
> >
> >>   void do_MemRegion(MemRegion mr) {
> >>     HeapWord* end_of_non_clean = mr.end();
> >>     HeapWord* start_of_non_clean = end_of_non_clean;
> >>     jbyte*       entry = _ct->byte_for(mr.last());
> >>     jbyte* first_entry = _ct->byte_for(mr.start());
> >>     while (entry >= first_entry) {
> >>       jbyte entry_val = *entry;
> >>       HeapWord* cur = _ct->addr_for(entry);
> >>       if (!clear_card(entry)) {
> >>         if (start_of_non_clean < end_of_non_clean) {
> >>           MemRegion mr2(start_of_non_clean, end_of_non_clean);
> >>           _dirty_card_closure->do_MemRegion(mr2);
> >>         }
> >>         end_of_non_clean = cur;
> >>         start_of_non_clean = end_of_non_clean;
> >>       }
> >>       start_of_non_clean = cur;
> >>       entry--;
> >>     }
> >>     if (start_of_non_clean < end_of_non_clean) {
> >>       MemRegion mr2(start_of_non_clean, end_of_non_clean);
> >>       _dirty_card_closure->do_MemRegion(mr2);
> >>     }
> >>   }
> >>
> >> -- Ramki Ramakrishna
> >>
> >> Neo Jia wrote On 05/09/07 03:31 PM,:
> >>
> >> > hi,
> >> >
> >> > Acturally, the existing code works well although it misses the
> >> > limitation checking, because the current heap layout will make sure
> >> > the memory region checking failed before it goes beyond the limitation
> >> > of byte map.
> >> >
> >> > The reason is that the nursery space is on the lower address, which
> >> > will make its committed card table also on the lower address of inside
> >> > the byte maps. While walking through the card table of the mature
> >> > space and keeping reduce the entry, we will finally hit the boundary
> >> > of its byte map before doing the check of memory region, which will
> >> > fire the assert of the addr_for, if it is on the lower address.
> >> >
> >> > So, just adding more safety for this function.
> >> >
> >> > Thanks,
> >> > Neo
> >> > Index: memory/cardTableRS.cpp
> >> > ===================================================================
> >> > --- memory/cardTableRS.cpp  (revision 86)
> >> > +++ memory/cardTableRS.cpp  (working copy)
> >> > @@ -163,6 +163,9 @@
> >> >     start_of_non_clean = end_of_non_clean;
> >> >        }
> >> >        entry--;
> >> > +      // Adding a limit checking for safety.
> >> > +      if (entry < _ct->base_map())
> >> > +        break;
> >> >        start_of_non_clean = cur;
> >> >        cur = _ct->addr_for(entry);
> >> >      }
> >> > Index: memory/cardTableRS.hpp
> >> > ===================================================================
> >> > --- memory/cardTableRS.hpp  (revision 86)
> >> > +++ memory/cardTableRS.hpp  (working copy)
> >> > @@ -101,6 +101,8 @@
> >> >
> >> >    CardTableModRefBS* ct_bs() { return &_ct_bs; }
> >> >
> >> > +  jbyte * base_map() { return &(_ct_bs._byte_map[0]); }
> >> > +
> >> >    // Override.
> >> >    void prepare_for_younger_refs_iterate(bool parallel);
> >> >
> >>
> >> --
> >>
> >> ----------------------------------------------------------------------------
> >>
> >> Y. Srinivas Ramakrishna               HotSpot JVM
> >> Sun Microsystems, Inc., USCA 14-102   JWS / Software Group
> >> 4140 Network Circle                   408 276 7250 (x17250)
> >> Santa Clara, CA  95054, U.S.A.        Y dot S dot Ramakrishna at Sun
> >> dot COM
> >> ----------------------------------------------------------------------------
> >>
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