Re: [Drbd-dev] drbd 8.4.3: refcounter overflow on re-sync

[Marc Schiffbauer <m@sys4.de>]

* Lars Ellenberg schrieb am 23.09.14 um 13:03 Uhr:
>On Fri, Sep 19, 2014 at 05:16:53PM +0200, Marc Schiffbauer wrote:
>> * Lars Ellenberg schrieb am 19.09.14 um 16:48 Uhr:
>> >On Fri, Sep 19, 2014 at 11:49:09AM +0200, Marc Schiffbauer wrote:
>> >>Hi,
>> >>
>> >
>> >If you resolve that to a code line,
>> >I may be able to figure out what PAX is talking about.
>> >
>> >But from this stack trace alone, I have absolutely no idea what PAX
>> >is trying to say, which refcount could possibly be meant there,
>> >let alone why it could possibly overflow or.
>> >
>> >Ah, ok. Looking at [1], "PaX Team" says:
>> >.---
>> >| after having looked at the drbd code a bit i think this could be a
>> >| real bug in drbd but only upstream can tell for sure so you'll have to
>> >| contact them. you can show them the following that i figured out so far:
>> >|
>> >| the refcount overflow was detected in
>> >| drivers/block/drbd/drbd_bitmap.c:bm_page_io_async at the
>> >|
>> >| atomic_add(len >> 9, &mdev->rs_sect_ev)
>> >
>> >Well, yes, why would it not overflow.
>> >It is *not* a refcount.
>> >It is an atomic counter.
>> >It is meant to overflow.
>>
>> Ok, then I can report this back and it should be fixed in PaX as a
>> false postive. Thanks for clarifying this.
>
>I still don't get why PAX is even sending a signal there.
>How could sending a signal possibly help against an overflowing counter?


PAX is a patchset for the linux kernel that hardens the kernel in 
several ways.

The REFCOUNT feature will detect and prevent overflowing of 
refcounters.

This is a good introduction of what it does:
http://wiki.gentoo.org/wiki/Hardened/PaX_Quickstart


-Marc


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