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

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Message-ID: <20090228165624.GC26292@one.firstfloor.org>
Date:	Sat, 28 Feb 2009 17:56:24 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Pavel Machek <pavel@....cz>
Cc:	Andi Kleen <andi@...stfloor.org>,
	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 Fri, Feb 27, 2009 at 10:08:56PM +0100, Pavel Machek wrote:
> 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...

If you mean PAT caching attributes: correct it is not legal in x86 and
causes problems including data corruption.
If you mean other attributes like large page vs small page: it's normally legal,
with a few exceptions.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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