linux-kernel - Re: [PATCH V4 1/3] livepatch: remove 'struct completion finish' from klp

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Date:   Tue, 2 Nov 2021 16:56:10 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>
Subject: Re: [PATCH V4 1/3] livepatch: remove 'struct completion finish' from
 klp_patch

On Tue 2021-11-02 22:59:30, Ming Lei wrote:
> The completion finish is just for waiting release of the klp_patch
> object, then releases module refcnt. We can simply drop the module
> refcnt in the kobject release handler of klp_patch.
> 
> This way also helps to support allocating klp_patch from heap.

IMHO, this is wrong assumption. kobject_put() might do everyting
asynchronously, see:

   kobject_put()
     kobject_release()
       INIT_DELAYED_WORK(&kobj->release, kobject_delayed_cleanup);
       schedule_delayed_work(&kobj->release, delay);

   asynchronously:

     kobject_delayed_cleanup()
      kobject_cleanup()
	__kobject_del()


> Signed-off-by: Ming Lei <ming.lei@...hat.com>
> ---
>  include/linux/livepatch.h |  1 -
>  kernel/livepatch/core.c   | 12 +++---------
>  2 files changed, 3 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> index 2614247a9781..9712818997c5 100644
> --- a/include/linux/livepatch.h
> +++ b/include/linux/livepatch.h
> @@ -170,7 +170,6 @@ struct klp_patch {
>  	bool enabled;
>  	bool forced;
>  	struct work_struct free_work;
> -	struct completion finish;
>  };
>  
>  #define klp_for_each_object_static(patch, obj) \
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index 335d988bd811..b967b4b0071b 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -551,10 +551,10 @@ static int klp_add_nops(struct klp_patch *patch)
>  
>  static void klp_kobj_release_patch(struct kobject *kobj)
>  {
> -	struct klp_patch *patch;
> +	struct klp_patch *patch = container_of(kobj, struct klp_patch, kobj);
>  
> -	patch = container_of(kobj, struct klp_patch, kobj);
> -	complete(&patch->finish);
> +	if (!patch->forced)
> +		module_put(patch->mod);
>  }
>  
>  static struct kobj_type klp_ktype_patch = {
> @@ -678,11 +678,6 @@ static void klp_free_patch_finish(struct klp_patch *patch)
>  	 * cannot get enabled again.
>  	 */
>  	kobject_put(&patch->kobj);
> -	wait_for_completion(&patch->finish);
> -
> -	/* Put the module after the last access to struct klp_patch. */
> -	if (!patch->forced)
> -		module_put(patch->mod);

klp_free_patch_finish() does not longer wait until the release
callbacks are called.

klp_free_patch_finish() is called also in klp_enable_patch() error
path.

klp_enable_patch() is called in module_init(). For example, see
samples/livepatch/livepatch-sample.c

The module must not get removed until the release callbacks are called.
Does the module loader check the module reference counter when
module_init() fails?

Best Regards,
Petr