linux-kernel - Re: [PATCH v13 06/12] livepatch: Simplify API by removing registration step

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Date:   Wed, 17 Oct 2018 14:06:57 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Jiri Kosina <jikos@...nel.org>, Miroslav Benes <mbenes@...e.cz>,
        Jason Baron <jbaron@...mai.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v13 06/12] livepatch: Simplify API by removing
 registration step

On Mon, Oct 15, 2018 at 02:37:07PM +0200, Petr Mladek wrote:
> @@ -319,96 +316,66 @@ forced it is guaranteed that no task sleeps or runs in the old code.
>  5. Livepatch life-cycle
>  =======================
>  
> -Livepatching defines four basic operations that define the life cycle of each
> -live patch: registration, enabling, disabling and unregistration.  There are
> -several reasons why it is done this way.
> +Livepatches get automatically enabled when the respective module is loaded.

(only true if the module enables the patch in its init function)

> @@ -502,6 +483,9 @@ static void klp_free_objects(struct klp_patch *patch)
>  }
>  
>  /*
> + * The synchronous variant is needed when the patch is freed in
> + * the klp_enable_patch() error paths.
> + *

Hm?  This comment seems confusingly out of context.

> @@ -528,6 +512,23 @@ static void klp_free_patch_finish(struct klp_patch *patch)
>  		kobject_put(&patch->kobj);
>  		wait_for_completion(&patch->finish);
>  	}
> +
> +	/* Put the module after the last access to struct klp_patch. */
> +	if (patch->module_put)
> +		module_put(patch->mod);
> +}
> +
> +/*
> + * The livepatch might be freed from sysfs interface created by the patch.
> + * This work allows to wait until the interface is destroyed in a separate
> + * context.
> + */
> +static void klp_free_patch_fn(struct work_struct *work)

To clarify that it's a work function, how about calling it
"klp_free_patch_work_fn"?

>  static int klp_init_func(struct klp_object *obj, struct klp_func *func)
> @@ -642,116 +643,38 @@ static int klp_init_patch(struct klp_patch *patch)
>  	struct klp_object *obj;
>  	int ret;
>  
> -	if (!patch->objs)
> -		return -EINVAL;
> -
> -	mutex_lock(&klp_mutex);
> -
>  	patch->enabled = false;
> -	patch->forced = false;
> +	patch->module_put = false;
>  	INIT_LIST_HEAD(&patch->list);
> +	INIT_WORK(&patch->free_work, klp_free_patch_fn);
>  	init_completion(&patch->finish);
>  
> +	if (!patch->objs)
> +		return -EINVAL;
> +
> +	/*
> +	 * A reference is taken on the patch module to prevent it from being
> +	 * unloaded.
> +	 */
> +	if (!try_module_get(patch->mod))
> +		return -ENODEV;

This comment isn't needed.  It describes what try_module_get() does,
which is common kernel knowledge.

> +	patch->module_put = true;

The naming and semantics of the 'module_put' field are a little
confusing.  It's false in two cases:

1) try_module_get() failure
2) forced patch

Maybe we can get rid of the need for the first case by moving the
try_module_get() call to klp_enable_patch(), before calling
klp_init_lists().  Then klp_free_patch_finish() will always be called
with a module reference, so it doesn't have to check the 'module_put'
field.

We'd still need it for the force case, but then it can just be called
'forced' again.

> --- a/samples/livepatch/livepatch-callbacks-demo.c
> +++ b/samples/livepatch/livepatch-callbacks-demo.c
> @@ -184,22 +184,11 @@ static struct klp_patch patch = {
>  
>  static int livepatch_callbacks_demo_init(void)
>  {
> -	int ret;
> -
> -	ret = klp_register_patch(&patch);
> -	if (ret)
> -		return ret;
> -	ret = klp_enable_patch(&patch);
> -	if (ret) {
> -		WARN_ON(klp_unregister_patch(&patch));
> -		return ret;
> -	}
> -	return 0;
> +	return klp_enable_patch(&patch);
>  }
>  
>  static void livepatch_callbacks_demo_exit(void)
>  {
> -	WARN_ON(klp_unregister_patch(&patch));
>  }

This module exit function is no longer needed.

>  
>  module_init(livepatch_callbacks_demo_init);
> diff --git a/samples/livepatch/livepatch-sample.c b/samples/livepatch/livepatch-sample.c
> index de30d1ba4791..88afb708a48d 100644
> --- a/samples/livepatch/livepatch-sample.c
> +++ b/samples/livepatch/livepatch-sample.c
> @@ -66,22 +66,11 @@ static struct klp_patch patch = {
>  
>  static int livepatch_init(void)
>  {
> -	int ret;
> -
> -	ret = klp_register_patch(&patch);
> -	if (ret)
> -		return ret;
> -	ret = klp_enable_patch(&patch);
> -	if (ret) {
> -		WARN_ON(klp_unregister_patch(&patch));
> -		return ret;
> -	}
> -	return 0;
> +	return klp_enable_patch(&patch);
>  }
>  
>  static void livepatch_exit(void)
>  {
> -	WARN_ON(klp_unregister_patch(&patch));
>  }

Ditto.

-- 
Josh