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

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Message-ID: <20090223023332.GA5430@Krystal>
Date:	Sun, 22 Feb 2009 21:33:33 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Andi Kleen <andi@...stfloor.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>,
	"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

* Steven Rostedt (rostedt@...dmis.org) wrote:
> 
> 
> 
> On Sun, 22 Feb 2009, 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?
> > 
> 
> We are changing over 19000 locations in the kernel. This touches almost 
> all kernel text pages anyway. You want to map a page in and out for over 
> 19000 locations?
> 
> -- Steve
> 

Hi Steve,

Can you provide numbers to indicate why it's required to be so intrusive
in the kernel mappings while doing these modifications ? I think opening
such time window where standard code mapping is writeable globally in
config RO_DATA kernels could open the door to unexpected side-effects,
so ideally going through the "backdoor" page mapped by text_poke seems
safer. Given similar performance, I would tend to use a text_poke-like
approach.

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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