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

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Date:	Sun, 22 Feb 2009 18:50:00 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	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

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?

-Andi

>
> Signed-off-by: Steven Rostedt <srostedt@...hat.com>
> ---
>  arch/x86/include/asm/ftrace.h |   10 ++++++++++
>  arch/x86/kernel/ftrace.c      |   20 ++++++++++++++++++++
>  arch/x86/mm/init_32.c         |   27 ++++++++++++++++++++++++---
>  arch/x86/mm/init_64.c         |   29 ++++++++++++++++++++++++-----
>  4 files changed, 78 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
> index b55b4a7..5564cf3 100644
> --- a/arch/x86/include/asm/ftrace.h
> +++ b/arch/x86/include/asm/ftrace.h
> @@ -80,4 +80,14 @@ extern void return_to_handler(void);
>  #endif /* __ASSEMBLY__ */
>  #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
>  
> +#ifndef __ASSEMBLY__
> +#ifdef CONFIG_DEBUG_RODATA
> +void set_kernel_text_rw(void);
> +void set_kernel_text_ro(void);
> +#else
> +static inline void set_kernel_text_rw(void) { }
> +static inline void set_kernel_text_ro(void) { }
> +#endif
> +#endif /* __ASSEMBLY__ */
> +

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