On Fri, 29 Jan 2016, Josh Poimboeuf wrote:
> On Fri, Jan 29, 2016 at 12:40:14PM -0500, Steven Rostedt wrote:
> > [ Added Rusty, as he's still maintainer of the module code ]
> >
> > On Fri, 29 Jan 2016 11:30:10 -0600
> > Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >
> > > On Fri, Jan 29, 2016 at 05:30:46PM +0100, Miroslav Benes wrote:
> > > > Otherwise than that it looks good. I agree there are advantages to split
> > > > the notifiers. For example we can replace the coming one with the function
> > > > call somewhere in load_module() to improve error handling if the patching
> > > > fails while loading a module. This would be handy with a consistency model
> > > > in the future.
> > >
> > > Yeah, we'll need something like that eventually. Though we'll need to
> > > make sure that ftrace_module_enable() is still called beforehand, after
> > > setting MODULE_STATE_COMING state, due to the race described in 5156dca.
> > >
> > > Something like:
> > >
> > > [note: klp_module_notify_coming() is replaced with klp_module_enable()]
> > >
> > > diff --git a/kernel/module.c b/kernel/module.c
> > > index 8358f46..aeabd81 100644
> > > --- a/kernel/module.c
> > > +++ b/kernel/module.c
> > > @@ -3371,6 +3371,13 @@ static int complete_formation(struct module *mod, struct load_info *info)
> > > mod->state = MODULE_STATE_COMING;
> > > mutex_unlock(&module_mutex);
> > >
> > > + ftrace_module_enable(mod);
> > > + err = klp_module_enable(mod);
> > > + if (err) {
> > > + ftrace_release_mod(mod);
> > > + return err;
> > > + }
> > > +
> > > blocking_notifier_call_chain(&module_notify_list,
> > > MODULE_STATE_COMING, mod);
> > > return 0;
> > > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > > index eca592f..c42cf37 100644
> > > --- a/kernel/trace/ftrace.c
> > > +++ b/kernel/trace/ftrace.c
> > > @@ -5045,9 +5045,6 @@ static int ftrace_module_notify(struct notifier_block *self,
> > > struct module *mod = data;
> > >
> > > switch (val) {
> > > - case MODULE_STATE_COMING:
> > > - ftrace_module_enable(mod);
> > > - break;
> > > case MODULE_STATE_GOING:
> > > ftrace_release_mod(mod);
> > > break;
> >
> > If we end up doing something like this, I would just say punt and have
> > the ftrace code be hardcoded into the module code and remove the
> > notifiers completely. ftrace (and live kernel patching for that matter)
> > are rather special. They are not a filesystem or driver. They are core
> > utilities and having them called directly from the module code may be
> > prudent and better to understand and control.
>
> Agreed, and we might as well make this change now to avoid more churn
> later.
It is possible to achieve the same goal even with the notifiers. They are
processed synchronously in complete_formation(). So we can put our klp
hook after that, right? Or better, put it to load_module() after
complete_formation() call. There is an error handling code even today
(that is, parse_args() or mod_sysfs_setup() can fail). Moreover, we'll
have a hook there with Jessica's relocation rework patch set.
But Steven's reasoning is convincing, so I'm all up for it.
Regards,
Miroslav
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