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

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Date:   Fri, 18 Aug 2017 10:19:54 -0400
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Petr Mladek <pmladek@...e.com>, Nicolai Stange <nstange@...e.de>
Cc:     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

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().

> 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.

-- Joe