Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

From: Jessica Yu <jeyu@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	mhiramat@kernel.org, bristot@redhat.com, jbaron@akamai.com,
	torvalds@linux-foundation.org, tglx@linutronix.de,
	mingo@kernel.org, namit@vmware.com, hpa@zytor.com,
	luto@kernel.org, ard.biesheuvel@linaro.org, jpoimboe@redhat.com
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()
Date: Tue, 15 Oct 2019 18:09:13 +0200	[thread overview]
Message-ID: <20191015160912.GB21110@linux-8ccs> (raw)
In-Reply-To: <20191015092802.5c9aea5e@gandalf.local.home>

+++ Steven Rostedt [15/10/19 09:28 -0400]:
>On Fri, 11 Oct 2019 14:59:03 +0200
>Peter Zijlstra <peterz@infradead.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.