Re: livepatch/module: remove livepatch module notifier

[Jessica Yu]

+++ Petr Mladek [04/02/16 15:39 +0100]:
> On Mon 2016-02-01 20:17:36, Jessica Yu wrote:
[ snipped since email is getting long ]
> > diff --git a/kernel/module.c b/kernel/module.c
> > index b05d466..71c77ed 100644
> > --- a/kernel/module.c
> > +++ b/kernel/module.c
> > @@ -53,6 +53,7 @@
> >  #include <asm/sections.h>
> >  #include <linux/tracepoint.h>
> >  #include <linux/ftrace.h>
> > +#include <linux/livepatch.h>
> >  #include <linux/async.h>
> >  #include <linux/percpu.h>
> >  #include <linux/kmemleak.h>
> > @@ -981,6 +982,7 @@ SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
> >  		mod->exit();
> >  	blocking_notifier_call_chain(&module_notify_list,
> >  				     MODULE_STATE_GOING, mod);
> > +	klp_module_disable(mod);
> >  	ftrace_release_mod(mod);
> >
> >  	async_synchronize_full();
> > @@ -3297,6 +3299,7 @@ fail:
> >  	module_put(mod);
> >  	blocking_notifier_call_chain(&module_notify_list,
> >  				     MODULE_STATE_GOING, mod);
> > +	klp_module_disable(mod);
> >  	ftrace_release_mod(mod);
> >  	free_module(mod);
> >  	wake_up_all(&module_wq);
> > @@ -3375,6 +3378,10 @@ static int complete_formation(struct module *mod, struct load_info *info)
> >  	mutex_unlock(&module_mutex);
> >
> >  	ftrace_module_enable(mod);
> > +	err = klp_module_enable(mod);
> > +	if (err)
> > +		goto out;
>
> If you go out here, you need to revert some some operations
> that are normally done in the bug_cleanup: goto target
> in load_module(). In particular, you need to do:
>
> 	/* module_bug_cleanup needs module_mutex protection */
> 	mutex_lock(&module_mutex);
> 	module_bug_cleanup(mod);
> 	mutex_unlock(&module_mutex);
>
> 	ftrace_release_mod(mod);
>
> 	/* we can't deallocate the module until we clear memory protection */
> 	module_disable_ro(mod);
> 	module_disable_nx(mod);
>
>
> IMHO, it would make sense to somehow split the complete_formation() function
> and avoid a code duplication in the error paths.

Argh, thank you for catching that. I think we could split up complete_formation()
into two functions in order to make the error handling work. 

We could probably take out the coming notifier calls, ftrace_module_enable(),
and klp_module_enable() out of complete_formation(), and put them in another
function, maybe called prepare_coming_module(), that would be called right
after complete_formation(). It might look something like this:

@@ -3614,6 +3621,9 @@ static int load_module(struct load_info *info, const char __user *uargs,
       err = complete_formation(mod, info); 
       if (err)
               goto ddebug_cleanup;
+       err = prepare_coming_module(mod); // calls ftrace_module_enable(), klp_module_enable(), then coming notifiers
+       if (err) // means that klp_module_enable failed
+               goto bug_cleanup;

       /* Module is ready to execute: parsing args may do that. */
       after_dashes = parse_args(mod->name, mod->args, mod->kp, mod->num_kp,
@@ -3621,7 +3631,7 @@ static int load_module(struct load_info *info, const char __user *uargs,
                                 unknown_module_param_cb);
       if (IS_ERR(after_dashes)) {
               err = PTR_ERR(after_dashes);
-               goto bug_cleanup;
+               goto coming_cleanup;
       } else if (after_dashes) {
               pr_warn("%s: parameters '%s' after `--' ignored\n",
                      mod->name, after_dashes);

Now for the error conditions. If complete_formation() fails, goto
ddebug_cleanup. If prepare_coming_module() fails (at that point,
module_enable_{ro,nx} and module_bug_finalize() have already finished), goto
bug_cleanup. Everything else that fails afterwards (meaning klp_module_enable,
ftrace_module_enable, and the coming notifiers have finished) goto
coming_cleanup. ftrace_release_mod() gets called in the goto free_module label
so we don't have to call it in coming_module.

@@ -3649,16 +3659,16 @@ static int load_module(struct load_info *info, const char __user *uargs,

+ coming_cleanup:
+       blocking_notifier_call_chain(&module_notify_list,
+                                    MODULE_STATE_GOING, mod);
+       klp_module_disable(mod);
 bug_cleanup:
       /* module_bug_cleanup needs module_mutex protection */
       mutex_lock(&module_mutex);
       module_bug_cleanup(mod);
       mutex_unlock(&module_mutex);

-       blocking_notifier_call_chain(&module_notify_list,
-                                    MODULE_STATE_GOING, mod);
-       klp_module_disable(mod);
-
       /* we can't deallocate the module until we clear memory protection */
       module_disable_ro(mod);
       module_disable_nx(mod);

Does all this look ok? Also, one last thing, I noticed that module->state isn't
set to MODULE_STATE_GOING anywhere before the going notifier chain is called in
the bug_cleanup label (I think it is still COMING at that point), so the
klp_module_disable call right afterwards would have bailed out because of that.
To be consistent, shouldn't it be set before the going notifiers are called?

Thanks,
Jessica