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

[Lars Ellenberg]

On Tue, Sep 23, 2014 at 07:08:24PM +0200, Marc Schiffbauer wrote:
> * 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

Yeah.

Why would sending a signal to the current thread prevent a counter from
overflowing. How could that possibly improve the overall situation,
even assuming that the overflow would have bad side effects?

Never mind.  This won't be a fruitful discussion...

-- 
: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

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