linux-kernel - Re: [PATCH] ftrace/module: Allow ftrace to make only loaded module text read-write

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Date:   Fri, 11 Oct 2019 13:09:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Jessica Yu <jeyu@...nel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] ftrace/module: Allow ftrace to make only loaded module
 text read-write

On Thu, Oct 10, 2019 at 01:20:13PM -0400, Steven Rostedt wrote:
> Hmm, I'm lost at what the below is trying to do with respect to the
> above.

The below is an alternative approach for the module load issue. It
accepts we patch 'late' and then uses text_poke_bp().

It works. We can then look at moving all that patching to
ftrace_module_init() later when we figure out how to do it across
architectures.

> > --- a/arch/x86/kernel/ftrace.c
> > +++ b/arch/x86/kernel/ftrace.c
> > @@ -34,6 +34,8 @@
> >  
> >  #ifdef CONFIG_DYNAMIC_FTRACE
> >  
> > +static int ftrace_poke_late = 0;
> > +
> >  int ftrace_arch_code_modify_prepare(void)
> >      __acquires(&text_mutex)
> >  {
> > @@ -43,12 +45,15 @@ int ftrace_arch_code_modify_prepare(void
> >  	 * ftrace has it set to "read/write".
> >  	 */
> >  	mutex_lock(&text_mutex);
> > +	ftrace_poke_late = 1;
> >  	return 0;
> >  }
> >  
> >  int ftrace_arch_code_modify_post_process(void)
> >      __releases(&text_mutex)
> >  {
> > +	text_poke_finish();
> > +	ftrace_poke_late = 0;
> >  	mutex_unlock(&text_mutex);
> >  	return 0;
> >  }
> > @@ -116,7 +121,10 @@ ftrace_modify_code_direct(unsigned long
> >  		return ret;
> >  
> >  	/* replace the text with the new text */
> > -	text_poke_early((void *)ip, new_code, MCOUNT_INSN_SIZE);
> > +	if (ftrace_poke_late)
> > +		text_poke_queue((void *)ip, new_code, MCOUNT_INSN_SIZE, NULL);
> > +	else
> > +		text_poke_early((void *)ip, new_code, MCOUNT_INSN_SIZE);
> >  	return 0;
> >  }
> >  
>