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

[Andi Kleen <andi@firstfloor.org>]

Steven Rostedt <rostedt@goodmis.org> writes:

> From: Steven Rostedt <srostedt@redhat.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@redhat.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@linux.intel.com -- Speaking for myself only.