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

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Date:	Tue, 08 Aug 2006 19:43:40 +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>
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.

> It seems that this patch only papers over the problem in the case when
> it is observed, but doesn't really even fix the normal case.
> 
> Could we use a seqlock to determine when real_parent is in flux, and
> re-read real_parent until we get a consistent one?  We could use in in
> lieu of the existing for() loop.

Kirill

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