linux-kernel - Re: [PATCH] sys_getppid oopses on debug kernel

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Date:	Tue, 08 Aug 2006 19:54:19 +0400
From:	Kirill Korotaev <dev@...ru>
To:	Dave Hansen <haveblue@...ibm.com>
CC:	Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH] sys_getppid oopses on debug kernel

>>>Accessing freed memory is a bug, always, not just *only* when slab
>>>debugging is on, right?  Doesn't this mean we could get junk, or that
>>>the reader could potentially run off a bad pointer?
>>
>>no, read the comment in sys_getppid.
>>It is a valid optimization. _safe_ and alowing to bypass taking the lock.
>>BUT! This optimization relies on the fact that kernel memory (DMA + normal zone)
>>is always mapped into virtual address space.
>>Which is invalid for debug kernels only.
> 
> 
> Actually, it might also be invalid in hypervisor environments.  s390 and
> Xen use ballooning drivers to tell the hypervisor which pages are not
> currently in use by the OS so that they may be used in virtual machines
> elsewhere.
> 
> I'm cc'ing the s390 guys.  Will the s390 kernel oops if it accesses a
> page which was ballooned back to the hypervisor?

Yeah, a minute after my reply I got your idea and sent a new patch
which always takes the lock :)))

Thanks,
Kirill

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