linux-kernel - Re: [PATCH 4/6] ftrace, x86: make kernel text writable only for conversions

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Date:	Fri, 27 Feb 2009 22:08:56 +0100
From:	Pavel Machek <pavel@....cz>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 4/6] ftrace, x86: make kernel text writable only for
	conversions

On Sun 2009-02-22 18:50:00, Andi Kleen wrote:
> Steven Rostedt <rostedt@...dmis.org> writes:
> 
> > From: Steven Rostedt <srostedt@...hat.com>
> >
> > Impact: keep kernel text read only
> >
> > Because dynamic ftrace converts the calls to mcount into and out of
> > nops at run time, we needed to always keep the kernel text writable.
> >
> > But this defeats the point of CONFIG_DEBUG_RODATA. This patch converts
> > the kernel code to writable before ftrace modifies the text, and converts
> > it back to read only afterward.
> >
> > The conversion is done via stop_machine and no IPIs may be executed
> > at that time. The kernel text is set to write just before calling
> > stop_machine and set to read only again right afterward.
> 
> The very old text poke code I had for this just used a dynamic
> mapping elsewhere instead to modify the code. That's much less
> intrusive than changing the complete mappings. Any reason you can't use 
> that too?

Is it legal to have two mappings of same page with different
attributes? IIRC some processors did not like that...

-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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