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

Petr Mladek
2018-10-18T12:33:33Z

On Wed 2018-10-17 14:06:57, Josh Poimboeuf wrote:
> 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)

Great catch! Will fix it.

> > @@ -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.

Ah, the comment is just a left over from the previous version. It does
not longer make sense. I'll remove it.

> > @@ -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"?

OK

> > 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.

Yup. I'll remove it. Note that it was there even before. I have just
moved it with the code.

> > + 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.

Great idea! I'll do it in v14.

> > --- 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.

I have been there ;-) It is required. Otherewise the module
can't get removed. See the following code in kernel/module.c:

SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
unsigned int, flags)
{
[...]
/* If it has an init func, it must have an exit func to unload */
if (mod->init && !mod->exit) {
forced = try_force_unload(flags);
if (!forced) {
/* This module can't be removed */
ret = -EBUSY;
goto out;
}
}

Best Regards,
Petr