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

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Message-ID: <20191015160912.GB21110@linux-8ccs>
Date:   Tue, 15 Oct 2019 18:09:13 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, 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
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

+++ Steven Rostedt [15/10/19 09:28 -0400]:
>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;

As long as we enable x before parse_args(), which this patch does, then
I don't think this change would break anything.