Re: [PATCH 11/15] static_call: Add inline static call infrastructure

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 06, 2019 at 10:24:17PM +0000, Nadav Amit wrote:

> > +static void static_call_del_module(struct module *mod)
> > +{
> > +	struct static_call_site *start = mod->static_call_sites;
> > +	struct static_call_site *stop = mod->static_call_sites +
> > +					mod->num_static_call_sites;
> > +	struct static_call_site *site;
> > +	struct static_call_key *key, *prev_key = NULL;
> > +	struct static_call_mod *site_mod;
> > +
> > +	for (site = start; site < stop; site++) {
> > +		key = static_call_key(site);
> > +		if (key == prev_key)
> > +			continue;
> > +		prev_key = key;
> > +
> > +		list_for_each_entry(site_mod, &key->site_mods, list) {
> > +			if (site_mod->mod == mod) {
> > +				list_del(&site_mod->list);
> > +				kfree(site_mod);
> > +				break;
> > +			}
> > +		}
> > +	}
> 
> I think that for safety, when a module is removed, all the static-calls
> should be traversed to check that none of them calls any function in the
> removed module. If that happens, perhaps it should be poisoned.

We don't do that for normal indirect calls either.. I suppose we could
here, but meh.

> > +}
> > +
> > +static int static_call_module_notify(struct notifier_block *nb,
> > +				     unsigned long val, void *data)
> > +{
> > +	struct module *mod = data;
> > +	int ret = 0;
> > +
> > +	cpus_read_lock();
> > +	static_call_lock();
> > +
> > +	switch (val) {
> > +	case MODULE_STATE_COMING:
> > +		module_disable_ro(mod);
> > +		ret = static_call_add_module(mod);
> > +		module_enable_ro(mod, false);
> 
> Doesn’t it cause some pages to be W+X ? Can it be avoided?

I don't know why it does this, jump_labels doesn't seem to need this,
and I'm not seeing what static_call needs differently.

> > +		if (ret) {
> > +			WARN(1, "Failed to allocate memory for static calls");
> > +			static_call_del_module(mod);
> 
> If static_call_add_module() succeeded in changing some of the calls, but not
> all, I don’t think that static_call_del_module() will correctly undo
> static_call_add_module(). The code transformations, I think, will remain.

Hurm, jump_labels has the same problem.

I wonder why kernel/module.c:prepare_coming_module() doesn't propagate
the error from the notifier call. If it were to do that, I think we'll
abort the module load and any modifications get lost anyway.