linux-kernel - Re: [PATCH v4] livepatch: introduce shadow variable API

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Date:   Fri, 18 Aug 2017 16:46:41 +0200
From:   Nicolai Stange <nstange@...e.de>
To:     Joe Lawrence <joe.lawrence@...hat.com>
Cc:     Petr Mladek <pmladek@...e.com>, Nicolai Stange <nstange@...e.de>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>
Subject: Re: [PATCH v4] livepatch: introduce shadow variable API

Joe Lawrence <joe.lawrence@...hat.com> writes:

> On 08/18/2017 10:04 AM, Petr Mladek wrote:
>> On Fri 2017-08-18 15:44:29, Nicolai Stange wrote:
>>> Joe Lawrence <joe.lawrence@...hat.com> writes:
>>>
>>> <snip>
>>>> +
>>>> +/**
>>>> + * klp_shadow_get() - retrieve a shadow variable data pointer
>>>> + * @obj:	pointer to parent object
>>>> + * @id:		data identifier
>>>> + *
>>>> + * Return: the shadow variable data element, NULL on failure.
>>>> + */
>>>> +void *klp_shadow_get(void *obj, unsigned long id)
>>>> +{
>>>> +	struct klp_shadow *shadow;
>>>> +
>>>> +	rcu_read_lock();
>>>> +
>>>> +	hash_for_each_possible_rcu(klp_shadow_hash, shadow, node,
>>>> +				   (unsigned long)obj) {
>>>> +
>>>> +		if (klp_shadow_match(shadow, obj, id)) {
>>>> +			rcu_read_unlock();
>>>> +			return shadow->data;
>>>
>>> I had to think a moment about what protects shadow from getting freed by
>>> a concurrent detach after that rcu_read_unlock(). Then I noticed that if
>>> obj and the livepatch are alive, then so is shadow, because there
>>> obviously hasn't been any reason to detach it.
>>>
>>> So maybe it would be nice to have an additional comment at
>>> klp_shadow_detach() that it's the API user's responsibility not to use a
>>> shadow instance after detaching it...
>
> Nicolai, I can add something like "This function releases the memory for
> this shadow variable instance, callers should stop referencing it
> accordingly."  Similar text for klp_shadow_detach_all().

Perfect, thanks!


>> Good point. In fact, it might make sense to rename the functions:
>> 
>>      attach -> create
>>      detach -> destroy
>> 
>> The name detach suggests that the variable is just not connected to
>> the parent object but that it is still accessible/usable.
>
> FWIW, kpatch calls them "kpatch_shadow_alloc" and "kpatch_shadow_free".
> Now that it's clear that we're not going separate shadow variable
> allocation from hash table insertion, going back to alloc/create and
> destroy/free is fine w/me.

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