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

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