Re: [RFC PATCH] livepatch: allow removal of a disabled patch

[Miroslav Benes <mbenes@suse.cz>]

On Mon, 2 May 2016, Josh Poimboeuf wrote:

> On Mon, May 02, 2016 at 01:57:22PM +0200, Miroslav Benes wrote:
> > Currently we do not allow patch module to unload since there is no
> > method to determine if a task is still running in the patched code.
> > 
> > The consistency model gives us the way because when the patching
> > finishes we know that all tasks were marked as safe to call a new
> > patched function. Thus every new call to the function calls the new
> > patched code and at the same time no task can be somewhere in the old
> > code, because it had to leave that code to be marked as safe.
> > 
> > We can safely let the patch module go after that.
> 
> I found this a little confusing because it talks about patching, whereas
> we really want to remove the patch module after _unpatching_ it.

You're right. I'll rephrase that.

> > Completion is used for synchronization between module removal and sysfs
> > infrastructure. Note that we still do not allow the removal for
> > immediate model, that is no consistency model.
> > 
> > With this change a call to try_module_get() is moved to
> > __klp_enable_patch from klp_register_patch to make module reference
> > counting symmetric (module_put() is in a patch disable path) and to
> > allow to take a new reference to a disabled module when being enabled.
> 
> One minor issue here: for patch->immediate, if somebody enabled and
> disabled the patch several times, the module ref count would keep
> increasing.  Probably not a big deal... maybe we don't need to worry
> about it.

That is unfortunately true. I don't have a solution in my pocket which 
would be 100% reliable. At the same time I don't see a practical problem. 
Yes, refcount could overflow, but that shouldn't be a problem, should it? 
Anyway, I'll note it in the changelog.

> > Also all kobject_put(&patch->kobj) calls are moved outside of klp_mutex
> > lock protection to prevent a deadlock situation when
> > klp_unregister_patch is called and sysfs directories are removed. There
> > is no need to do the same for other kobject_put() callsites as we
> > currently do not have their sysfs counterparts.
> > 
> > Signed-off-by: Miroslav Benes <mbenes@suse.cz>
> > ---
> > Based on Josh's v2.
> > 
> > There are still two things which need to be discussed.
> > 
> > 1. Do we really need a completion? If I am not missing something
> > kobject_del() always waits for sysfs callers to leave thanks to kernfs
> > active protection. The only problem is in kobject_release() when
> > CONFIG_DEBUG_KOBJECT_RELEASE=y. In this case kobject_delayed_cleanup()
> > is delay-scheduled, which is a problem for us. For this we definitely
> > need the completion, right? JiriS, is this correct?
> 
> Sysfs and kobjects hurt my brain; I need to take a refresher course, so
> I don't have an answer to this yet :-)  Maybe Jiri S can enlighten us.

:)

> > 2. I refuse to remove a module when patch->immediate is set or one of
> > its func->immediate is set. We can do slightly better, because we can
> > allow the module to go if patch->immediate is false and all func with
> > immediate set to true are from some object which is not loaded. This is
> > for discussion because it needs more changes in the code (I'd like to
> > call klp_is_object_loaded() which is somewhere else and so on).
> 
> I don't think we need to worry about doing that unless somebody
> complains about it.  It's kind of obscure and patch removal isn't very
> important anyway...

Agreed.

Thanks,
Miroslav