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 15:23:46 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	"rostedt@...dmis.org" <rostedt@...dmis.org>
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 14:35 -0700, Steven Rostedt wrote: 
> On Tue, 2009-10-27 at 14:26 -0800, Suresh Siddha wrote:
> >  
> > +	/*
> > +	 * 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?

Yes. it will be ip = ip for 32bit.

> > +	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.

I just saw one more place calling do_ftrace_mod_code(). So moved this
check inside the do_ftrace_mod_code(). Does this cover all the cases?
Thanks.

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 5a1b975..e239fd7 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -189,9 +189,23 @@ static void wait_for_nmi(void)
 	nmi_wait_count++;
 }
 
+static inline int
+within(unsigned long addr, unsigned long start, unsigned long end)
+{
+	return addr >= start && addr < end;
+}
+
 static int
 do_ftrace_mod_code(unsigned long ip, void *new_code)
 {
+	/*
+	 * On x86_64, we use the kernel identity mapping instead of the
+	 * kernel text mapping to modify the kernel text. For 32bit kernels,
+	 * these mappings are same.
+	 */
+	if (within(ip, (unsigned long)_text, (unsigned long)_etext))
+		ip = (unsigned long)__va(__pa(ip));
+
 	mod_code_ip = (void *)ip;
 	mod_code_newcode = new_code;
 




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