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

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Date:	09 Aug 2006 05:09:11 +0200
From:	Andi Kleen <ak@...e.de>
To:	Kirill Korotaev <dev@...ru>
Cc:	Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	haveblue@...ibm.com, linux-arch@...r.kernel.org
Subject: Re: [PATCH] sys_getppid oopses on debug kernel

Kirill Korotaev <dev@...ru> writes:

[adding linux-arch]

> > 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.

In x86 arch code we would use __get_user for this (and we do in a couple 
of places). But it wouldn't be portable because sometimes _user is 
in a different address space.

Maybe it would be time to make a similar facility (read/write_kernel_safe() or similar)
with error return available to generic code? 

It should be easy to implement - iirc near all architectures already
use the exception handling frame work and it is a simple extension 
of that. x86 could just define it to __put/get_user

-Andi
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