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

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Message-ID: <87pobtj8hu.fsf@suse.de>
Date:   Fri, 18 Aug 2017 15:44:29 +0200
From:   Nicolai Stange <nstange@...e.de>
To:     Joe Lawrence <joe.lawrence@...hat.com>
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>, Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH v4] livepatch: introduce shadow variable API

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...

Thanks,

Nicolai


> +		}
> +	}
> +
> +	rcu_read_unlock();
> +
> +	return NULL;
> +}
> +EXPORT_SYMBOL_GPL(klp_shadow_get);
<snap>


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