linux-kernel - Re: [PATCH v3 5/6] x86/ftrace: Use text

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Date:   Tue, 15 Oct 2019 09:28:02 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jpoimboe@...hat.com, jeyu@...nel.org
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

On Fri, 11 Oct 2019 14:59:03 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Oct 10, 2019 at 07:28:19PM +0200, Peter Zijlstra wrote:
> 
> > Really the best solution is to move all the poking into
> > ftrace_module_init(), before we mark it RO+X. That's what I'm going to
> > do for jump_label and static_call as well, I just need to add that extra
> > notifier callback.  
> 
> OK, so I started writing that patch... or rather, I wrote the patch and
> started on the Changelog when I ran into trouble describing why we need
> it.
> 
> That is, I'm struggling to explain why we cannot flip
> prepare_coming_module() and complete_formation().
> 
> Yes, it breaks ftrace, but I'm thinking that is all it breaks. So let me
> see if we can cure that.

You are mainly worried about making text that is executable into
read-write again. What if we kept my one patch that just changed the
module in ftrace_module_enable() from read-only to read-write, but
before we ever set it executable.

Jessica, would this patch break anything?

It moves the setting of the module execution to after calling both
ftrace_module_enable() and klp_module_coming().

This would make it possible to do the module code and still keep with
the no executable code becoming writable.

-- Steve

diff --git a/kernel/module.c b/kernel/module.c
index ff2d7359a418..6e2fd40a6ed9 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3728,7 +3728,6 @@ static int complete_formation(struct module *mod, struct load_info *info)
 
 	module_enable_ro(mod, false);
 	module_enable_nx(mod);
-	module_enable_x(mod);
 
 	/* Mark state as coming so strong_try_module_get() ignores us,
 	 * but kallsyms etc. can see us. */
@@ -3751,6 +3750,11 @@ static int prepare_coming_module(struct module *mod)
 	if (err)
 		return err;
 
+	/* Make module executable after ftrace is enabled */
+	mutex_lock(&module_mutex);
+	module_enable_x(mod);
+	mutex_unlock(&module_mutex);
+
 	blocking_notifier_call_chain(&module_notify_list,
 				     MODULE_STATE_COMING, mod);
 	return 0;