linux-kernel - Re: [PATCH 0/7] livepatch,module: Remove .klp.arch and module_disable

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Date:   Wed, 15 Apr 2020 16:24:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jessica Yu <jeyu@...nel.org>
Subject: Re: [PATCH 0/7] livepatch,module: Remove .klp.arch and
 module_disable_ro()

On Tue, Apr 14, 2020 at 02:08:14PM -0500, Josh Poimboeuf wrote:
> On Tue, Apr 14, 2020 at 08:27:26PM +0200, Peter Zijlstra wrote:
> > On Tue, Apr 14, 2020 at 11:28:36AM -0500, Josh Poimboeuf wrote:
> > > Better late than never, these patches add simplifications and
> > > improvements for some issues Peter found six months ago, as part of his
> > > non-writable text code (W^X) cleanups.
> > 
> > Excellent stuff, thanks!!
> >
> > I'll go brush up these two patches then:
> > 
> >   https://lkml.kernel.org/r/20191018074634.801435443@infradead.org
> >   https://lkml.kernel.org/r/20191018074634.858645375@infradead.org
> 
> Ah right, I meant to bring that up.  I actually played around with those
> patches.  While it would be nice to figure out a way to converge the
> ftrace module init, I didn't really like the first patch.

ftrace only needs it done after ftrace_module_enable(), which is before
the notifier chain happens, so we can simply do something like so
instead:

diff --git a/kernel/module.c b/kernel/module.c
index a3a8f6d0e144..89f8d02c3c3e 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3700,6 +3700,10 @@ static int prepare_coming_module(struct module *mod)
 	if (err)
 		return err;
 
+	module_enable_ro(mod, false);
+	module_enable_nx(mod);
+	module_enable_x(mod);
+
 	err = blocking_notifier_call_chain_robust(&module_notify_list,
 			MODULE_STATE_COMING, MODULE_STATE_GOING, mod);
 	err = notifier_to_errno(err);
@@ -3845,10 +3849,6 @@ static int load_module(struct load_info *info, const char __user *uargs,
 	if (err)
 		goto bug_cleanup;
 
-	module_enable_ro(mod, false);
-	module_enable_nx(mod);
-	module_enable_x(mod);
-
 	/* Module is ready to execute: parsing args may do that. */
 	after_dashes = parse_args(mod->name, mod->args, mod->kp, mod->num_kp,
 				  -32768, 32767, mod,

> It bothers me that both the notifiers and the module init() both see the
> same MODULE_STATE_COMING state, but only in the former case is the text
> writable.
> 
> I think it's cognitively simpler if MODULE_STATE_COMING always means the
> same thing, like the comments imply, "fully formed" and thus
> not-writable:
> 
> enum module_state {
> 	MODULE_STATE_LIVE,	/* Normal state. */
> 	MODULE_STATE_COMING,	/* Full formed, running module_init. */
> 	MODULE_STATE_GOING,	/* Going away. */
> 	MODULE_STATE_UNFORMED,	/* Still setting it up. */
> };
> 
> And, it keeps tighter constraints on what a notifier can do, which is a
> good thing if we can get away with it.

Moo! -- but jump_label and static_call are on the notifier chain and I
was hoping to make it cheaper for them. Should we perhaps weane them off the
notifier and, like ftrace/klp put in explicit calls?

It'd make the error handling in prepare_coming_module() a bigger mess,
but it should work.