linux-kernel - Re: [RFC PATCH 5/5] livepatch: Do not manually track kobject initialization

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Date:   Thu, 2 May 2019 09:30:44 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Tobin C. Harding" <tobin@...nel.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 5/5] livepatch: Do not manually track kobject
 initialization

On Thu 2019-05-02 09:12:32, Greg Kroah-Hartman wrote:
> On Thu, May 02, 2019 at 12:31:42PM +1000, Tobin C. Harding wrote:
> > Currently we use custom logic to track kobject initialization.  Recently
> > a predicate function was added to the kobject API so we now no longer
> > need to do this.
> > 
> > Use kobject API to check for initialized state of kobjects instead of
> > using custom logic to track state.
> > 
> > Signed-off-by: Tobin C. Harding <tobin@...nel.org>
> > ---
> >  include/linux/livepatch.h |  6 ------
> >  kernel/livepatch/core.c   | 18 +++++-------------
> >  2 files changed, 5 insertions(+), 19 deletions(-)
> > 
> > @@ -626,7 +626,7 @@ static void __klp_free_objects(struct klp_patch *patch, bool nops_only)
> >  		list_del(&obj->node);
> >  
> >  		/* Might be called from klp_init_patch() error path. */
> > -		if (obj->kobj_added) {
> > +		if (kobject_is_initialized(&obj->kobj)) {
> >  			kobject_put(&obj->kobj);
> >  		} else if (obj->dynamic) {
> >  			klp_free_object_dynamic(obj);
> 
> Same here, let's not be lazy.
> 
> The code should "know" if the kobject has been initialized or not
> because it is the entity that asked for it to be initialized.  Don't add
> extra logic to the kobject core (like the patch before this did) just
> because this one subsystem wanted to only write 1 "cleanup" function.

We use kobject for a mix of statically and dynamically defined
structures[*]. And we misunderstood the behavior of kobject_init().

Anyway, the right solution is to call kobject_init()
already in klp_init_patch_early() for the statically
defined structures and in klp_alloc*() for the dynamically
allocated ones. Then we could simply call kobject_put()
every time.

Tobin, this goes deeper into the livepatching code that
you probably expected. Do you want to do the above
suggested change or should I prepare the patch?

Anyway, thanks for working on this.


[*] Yes, we know that kobject was not designed for
    static structures. We even tried to use them but
    there was a lot of extra code with not a big benefit.

Best Regards,
Petr