linux-kernel - Re: [PATCH] [GIT PULL] tracing: allow to change permissions for text with dynamic ftrace enabled

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Date:	Tue, 27 Oct 2009 17:35:18 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] [GIT PULL] tracing: allow to change permissions for
 text with dynamic ftrace enabled

On Tue, 2009-10-27 at 14:26 -0800, Suresh Siddha wrote:
> On Tue, 2009-10-27 at 11:33 -0700, Steven Rostedt wrote:
> > On Tue, 2009-10-27 at 11:20 -0800, Suresh Siddha wrote:
> > 
> > > Steven, Is it possible for dynamic ftrace to use the kernel identity
> > > mapping instead of the kernel text mapping for converting the tracing
> > > calls to nops etc.
> > 
> > Not sure what you mean by "kernel identity" mapping.
> 
> 64bit has the kernel text pages mapped at two locations. kernel identity
> mapping  (__PAGE_OFFSET) and kernel image/text mapping
> (__START_KERNEL_map).

I didn't realize that x86_64 mapped it two different ways.

> 
> DEBUG_RODATA patch was trying to preserve large page mapping (for perf
> reasons) for kernel text. We can use the identity mapping for modifying
> the kernel text.
> 
> This patch seems to fix.
> 
> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> index 5a1b975..7e1799b 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -215,6 +215,11 @@ do_ftrace_mod_code(unsigned long ip, void *new_code)
>  }
>  
> 
> +static inline int
> +within(unsigned long addr, unsigned long start, unsigned long end)
> +{
> +	return addr >= start && addr < end;
> +}
>  
> 
>  static unsigned char ftrace_nop[MCOUNT_INSN_SIZE];
> @@ -248,6 +253,14 @@ ftrace_modify_code(unsigned long ip, unsigned char *old_code,
>  	if (memcmp(replaced, old_code, MCOUNT_INSN_SIZE) != 0)
>  		return -EINVAL;
>  
> +	/*
> +	 * On x86_64, we use the kernel identity mapping instead of the
> +	 * kernel text mapping to modify the kernel text. This is a nop
> +	 * for 32bit kernels.
> +	 */

Is it really a nop on 32bit? Does it just turn into ip = ip?


> +	if (within(ip, (unsigned long)_text, (unsigned long)_etext))
> +		ip = (unsigned long)__va(__pa(ip));
> +
>  	/* replace the text with the new text */
>  	if (do_ftrace_mod_code(ip, new_code))
>  		return -EPERM;

I'll test it out, and if it does work, you can write up a formal patch
and remove the !define that I added.

Thanks,

-- Steve


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