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Message-ID: <2cd57c900511120632y1e7993ber@mail.gmail.com>
Date: Sat, 12 Nov 2005 22:32:34 +0800
From: Coywolf Qi Hunt <coywolf@gmail.com>
To: Andi Kleen <ak@suse.de>
Subject: Re: [patch] mark text section read-only
Cc: coywolf@sosdg.org, linux-kernel@vger.kernel.org,
       Arjan van de Ven <arjan@infradead.org>, Josh Boyer <jdub@us.ibm.com>,
       akpm@osdl.org
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2005/11/12, Andi Kleen <ak@suse.de>:
> On Friday 11 November 2005 20:04, Coywolf Qi Hunt wrote:
> > On Sat, Nov 12, 2005 at 02:57:02AM +0800, Coywolf Qi Hunt wrote:
> > > And we could also mark text section read-only and data/stack section
> > > noexec if NX is supported. But I doubt the whole thing would really
> > > help much. Kill the kernel thread? We can't. We only run into a panic.
> > > Anyway I'd attach a quick patch to mark text section read only in the
> > > next mail.
>
>
> I think this whole thing is only usable as a debugging option. It shouldn't
> be used by default on production systems because it will increase TLB
> pressure by splitting up the large pages used by kernel. And TLB pressure
> is critical in many workloads.
>
> It definitely shouldn't be on by default.
>
> Then the text section will likely not be page aligned, so it would be
> surprising if it even worked.

It works. I have tested it with { c=_stext[0]; _stext[0]=c;}. No
effect when it's disabled; panic when it's enabled.

The symbol `_text' is always page aligned. `_etext' is not, but we don't care.

(Bugs: It would conflict with kprobes.)

>
> At least on x86-64 it is pretty useless too because the .text section can
> be accessed over its alias in the direct mapping.

OK, for x86 only then.

>
> Overall I doubt it is worth it even as a debugging option. I so far cannot
> remember a single bug that was caused by overwriting kernel text.

I had the same concern basically. But I am convinced after seeing the
bug Nikita Danilov points out.
--
Coywolf Qi Hunt
http://sosdg.org/~coywolf/
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