linux-kernel - Re: [PATCH v5 1/3] livepatch: add (un)patch callbacks

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Message-ID: <85d3cd2e-f51f-621d-70c7-df602fb5004b@redhat.com>
Date:   Tue, 26 Sep 2017 15:01:52 -0400
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jessica Yu <jeyu@...nel.org>, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Chris J Arges <chris.j.arges@...onical.com>
Subject: Re: [PATCH v5 1/3] livepatch: add (un)patch callbacks

On 09/26/2017 10:49 AM, Petr Mladek wrote:
> On Thu 2017-08-31 10:53:51, Joe Lawrence wrote:
>> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
>> index b9628e43c78f..aca62c4b8616 100644
>> --- a/kernel/livepatch/core.c
>> +++ b/kernel/livepatch/core.c
>> @@ -54,11 +54,6 @@ static bool klp_is_module(struct klp_object *obj)
>>  	return obj->name;
>>  }
>>  
>> -static bool klp_is_object_loaded(struct klp_object *obj)
>> -{
>> -	return !obj->name || obj->mod;
>> -}
>> -
>>  /* sets obj->mod if object is not vmlinux and module is found */
>>  static void klp_find_object_module(struct klp_object *obj)
>>  {
>> @@ -285,6 +280,8 @@ static int klp_write_object_relocations(struct module *pmod,
>>  
>>  static int __klp_disable_patch(struct klp_patch *patch)
>>  {
>> +	struct klp_object *obj;
>> +
>>  	if (klp_transition_patch)
>>  		return -EBUSY;
>>  
>> @@ -295,6 +292,10 @@ static int __klp_disable_patch(struct klp_patch *patch)
>>  
>>  	klp_init_transition(patch, KLP_UNPATCHED);
>>  
>> +	klp_for_each_object(patch, obj)
>> +		if (patch->enabled && obj->patched)
>> +			klp_pre_unpatch_callback(obj);
>> +
>>  	/*
>>  	 * Enforce the order of the func->transition writes in
>>  	 * klp_init_transition() and the TIF_PATCH_PENDING writes in
>> @@ -388,13 +389,18 @@ static int __klp_enable_patch(struct klp_patch *patch)
>>  		if (!klp_is_object_loaded(obj))
>>  			continue;
>>  
>> -		ret = klp_patch_object(obj);
>> +		ret = klp_pre_patch_callback(obj);
>>  		if (ret) {
>> -			pr_warn("failed to enable patch '%s'\n",
>> -				patch->mod->name);
>> +			pr_warn("pre-patch callback failed for object '%s'\n",
>> +				klp_is_module(obj) ? obj->name : "vmlinux");
>> +			goto err;
>> +		}
>>  
>> -			klp_cancel_transition();
>> -			return ret;
>> +		ret = klp_patch_object(obj);
>> +		if (ret) {
>> +			pr_warn("failed to patch object '%s'\n",
>> +				klp_is_module(obj) ? obj->name : "vmlinux");
> 
> We should call klp_post_unpatch_callback(obj) here to make it
> synchronous.

Are you talking about the error path?  As its coded here,
klp_cancel_transition() will call klp_complete_transition() with
klp_target_state = KLP_UNPATCHED and then klp_complete_transition()'s
done: code will call klp_post_unpatch_callback() on all the necessary
kobj's.  Is there something asynchronous about that?

> Well, what about calling:
> 
>       klp_pre_patch_callback() inside klp_patch_object() and
>       klp_post_unpatch_callback() inside klp_unpatch_object()

v1 started out that way, but we migrated to placing these around the
callers of klp_(un)patch_object() to try and better line up the
locations of the pre- hooks  with the post- hook locations.

I can take a second look at reversing this decision, but that may take a
little time while I page all the testing corner cases back into my brain :)

> By other words, we would do the two operations. It would have
> two advantages:
> 
>    + error handling for free
>    + no need for the strange callbacks_enabled flag

Indeed, it would be nice to ditch that callbacks_enabled wart.

> It would require the more strict consistency model if there
> is a dependency between the callbacks and patches from various
> modules. But we would probably need the consistency model
> in this case anyway.
> 
>> +			goto err;
> 
>>  		}
>>  	}
>>  
> 
> Otherwise I think that we are getting close.
> 
> Best Regards,
> Petr
> 
> PS: I hope that the above problem and solution has not been mentioned
> yet. I am sorry if it was. I am a bit lost in many mails after
> vacation, sickness, and conference.

I think the only other outstanding issue before rolling a v6 is the one
that Miroslav raised about the error path in klp_module_coming():

  https://marc.info/?l=linux-kernel&m=150590635602784&w=2
  https://marc.info/?l=linux-kernel&m=150592065007463&w=2

Thanks,

-- Joe