linux-kernel - Re: livepatch/module: remove livepatch module notifier

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Message-ID: <20160205041157.GB24068@packer-debian-8-amd64.digitalocean.com>
Date:	Thu, 4 Feb 2016 23:11:57 -0500
From:	Jessica Yu <jeyu@...hat.com>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	Miroslav Benes <mbenes@...e.cz>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: livepatch/module: remove livepatch module notifier

+++ Petr Mladek [04/02/16 15:39 +0100]:
>On Mon 2016-02-01 20:17:36, Jessica Yu wrote:
[ snipped since email is getting long ]
>> diff --git a/kernel/module.c b/kernel/module.c
>> index b05d466..71c77ed 100644
>> --- a/kernel/module.c
>> +++ b/kernel/module.c
>> @@ -53,6 +53,7 @@
>>  #include <asm/sections.h>
>>  #include <linux/tracepoint.h>
>>  #include <linux/ftrace.h>
>> +#include <linux/livepatch.h>
>>  #include <linux/async.h>
>>  #include <linux/percpu.h>
>>  #include <linux/kmemleak.h>
>> @@ -981,6 +982,7 @@ SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
>>  		mod->exit();
>>  	blocking_notifier_call_chain(&module_notify_list,
>>  				     MODULE_STATE_GOING, mod);
>> +	klp_module_disable(mod);
>>  	ftrace_release_mod(mod);
>>
>>  	async_synchronize_full();
>> @@ -3297,6 +3299,7 @@ fail:
>>  	module_put(mod);
>>  	blocking_notifier_call_chain(&module_notify_list,
>>  				     MODULE_STATE_GOING, mod);
>> +	klp_module_disable(mod);
>>  	ftrace_release_mod(mod);
>>  	free_module(mod);
>>  	wake_up_all(&module_wq);
>> @@ -3375,6 +3378,10 @@ static int complete_formation(struct module *mod, struct load_info *info)
>>  	mutex_unlock(&module_mutex);
>>
>>  	ftrace_module_enable(mod);
>> +	err = klp_module_enable(mod);
>> +	if (err)
>> +		goto out;
>
>If you go out here, you need to revert some some operations
>that are normally done in the bug_cleanup: goto target
>in load_module(). In particular, you need to do:
>
>	/* module_bug_cleanup needs module_mutex protection */
>	mutex_lock(&module_mutex);
>	module_bug_cleanup(mod);
>	mutex_unlock(&module_mutex);
>
>	ftrace_release_mod(mod);
>
>	/* we can't deallocate the module until we clear memory protection */
>	module_disable_ro(mod);
>	module_disable_nx(mod);
>
>
>IMHO, it would make sense to somehow split the complete_formation() function
>and avoid a code duplication in the error paths.

Argh, thank you for catching that. I think we could split up complete_formation()
into two functions in order to make the error handling work. 

We could probably take out the coming notifier calls, ftrace_module_enable(),
and klp_module_enable() out of complete_formation(), and put them in another
function, maybe called prepare_coming_module(), that would be called right
after complete_formation(). It might look something like this:

@@ -3614,6 +3621,9 @@ static int load_module(struct load_info *info, const char __user *uargs,
        err = complete_formation(mod, info); 
        if (err)
                goto ddebug_cleanup;
+       err = prepare_coming_module(mod); // calls ftrace_module_enable(), klp_module_enable(), then coming notifiers
+       if (err) // means that klp_module_enable failed
+               goto bug_cleanup;
 
        /* Module is ready to execute: parsing args may do that. */
        after_dashes = parse_args(mod->name, mod->args, mod->kp, mod->num_kp,
@@ -3621,7 +3631,7 @@ static int load_module(struct load_info *info, const char __user *uargs,
                                  unknown_module_param_cb);
        if (IS_ERR(after_dashes)) {
                err = PTR_ERR(after_dashes);
-               goto bug_cleanup;
+               goto coming_cleanup;
        } else if (after_dashes) {
                pr_warn("%s: parameters '%s' after `--' ignored\n",
                       mod->name, after_dashes);

Now for the error conditions. If complete_formation() fails, goto
ddebug_cleanup. If prepare_coming_module() fails (at that point,
module_enable_{ro,nx} and module_bug_finalize() have already finished), goto
bug_cleanup. Everything else that fails afterwards (meaning klp_module_enable,
ftrace_module_enable, and the coming notifiers have finished) goto
coming_cleanup. ftrace_release_mod() gets called in the goto free_module label
so we don't have to call it in coming_module.

@@ -3649,16 +3659,16 @@ static int load_module(struct load_info *info, const char __user *uargs,
 
+ coming_cleanup:
+       blocking_notifier_call_chain(&module_notify_list,
+                                    MODULE_STATE_GOING, mod);
+       klp_module_disable(mod);
  bug_cleanup:
        /* module_bug_cleanup needs module_mutex protection */
        mutex_lock(&module_mutex);
        module_bug_cleanup(mod);
        mutex_unlock(&module_mutex);
 
-       blocking_notifier_call_chain(&module_notify_list,
-                                    MODULE_STATE_GOING, mod);
-       klp_module_disable(mod);
-
        /* we can't deallocate the module until we clear memory protection */
        module_disable_ro(mod);
        module_disable_nx(mod);

Does all this look ok? Also, one last thing, I noticed that module->state isn't
set to MODULE_STATE_GOING anywhere before the going notifier chain is called in
the bug_cleanup label (I think it is still COMING at that point), so the
klp_module_disable call right afterwards would have bailed out because of that.
To be consistent, shouldn't it be set before the going notifiers are called?

Thanks,
Jessica