Re: [RFC PATCH] livepatch/module: Do not patch modules that are not ready

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Tue, Mar 03, 2015 at 06:34:57PM +0100, Petr Mladek wrote:
> Ah, I have missed the comments in the code in the first reply.
> > Why do we need two flags for this?  The notifer already
> > sets/clears obj->mod, so can we rely on the value obj->mod to determine
> > if the notifier already ran?
> 
> We need this for new patches. There we do not know if the module
> notifier has already been called or not, see also below.
> 
> Of course, we might use more optimal storage for the two flags
> if requested. For example, two bits in an unsigned char.
> 
> 
> > For example:
> > 
> > @@ -89,7 +89,7 @@ static bool klp_is_object_loaded(struct klp_object *obj)
> >  /* sets obj->mod if object is not vmlinux and module is found */
> >  static void klp_find_object_module(struct klp_object *obj)
> >  {
> > -	if (!klp_is_module(obj))
> > +	if (!klp_is_module(obj) || obj->mod)
> >  		return;
> >  
> >  	mutex_lock(&module_mutex);
> > @@ -98,7 +98,9 @@ static void klp_find_object_module(struct klp_object *obj)
> >  	 * the klp_mutex, which is also taken by the module notifier.  This
> >  	 * prevents any module from unloading until we release the klp_mutex.
> >  	 */
> > -	obj->mod = find_module(obj->name);
> > +	mod = find_module(obj->name);
> > +	if (mod->state == MODULE_STATE_LIVE)
> 
> The check of the MODULE_STATE_LIVE is not enough. We need to init
> modules when a new patch is registered, the module is still in
> MODULE_STATE_COMING and the module notify handler has already
> been called.

Ok, thanks for explaining.  I think I got it now.  But the two module
flags aren't ideal.

MODULE_STATE_COMING means that ftrace is already initialized, so I think
it doesn't _have_ to be the notifier which does the work.  Instead, can
we just let it be done by whoever gets there first?  Like this:


diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 01ca088..05390ab 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -89,16 +89,29 @@ static bool klp_is_object_loaded(struct klp_object *obj)
 /* sets obj->mod if object is not vmlinux and module is found */
 static void klp_find_object_module(struct klp_object *obj)
 {
-	if (!klp_is_module(obj))
+	struct module *mod;
+
+	if (!klp_is_module(obj) || obj->mod)
 		return;
 
 	mutex_lock(&module_mutex);
+
 	/*
 	 * We don't need to take a reference on the module here because we have
 	 * the klp_mutex, which is also taken by the module notifier.  This
 	 * prevents any module from unloading until we release the klp_mutex.
 	 */
-	obj->mod = find_module(obj->name);
+	mod = find_module(obj->name);
+
+	/*
+	 * MODULE_STATE_COMING means we got to the module first before the
+	 * notifier did.  ftrace is already initialized, so it's fine to go
+	 * ahead and start using it.
+	 */
+	if (mod->state == MODULE_STATE_COMING ||
+	    mod->state == MODULE_STATE_LIVE)
+		obj->mod = mod;
+
 	mutex_unlock(&module_mutex);
 }
 
@@ -697,8 +710,6 @@ static void klp_free_object_loaded(struct klp_object *obj)
 {
 	struct klp_func *func;
 
-	obj->mod = NULL;
-
 	for (func = obj->funcs; func->old_name; func++)
 		func->old_addr = 0;
 }
@@ -967,10 +978,31 @@ static int klp_module_notify(struct notifier_block *nb, unsigned long action,
 				continue;
 
 			if (action == MODULE_STATE_COMING) {
+
+				/*
+				 * There's a small window where a register or
+				 * enable call may have already done this
+				 * initialization.  First one there wins.
+				 */
+				if (obj->mod)
+					continue;
+
 				obj->mod = mod;
 				klp_module_notify_coming(patch, obj);
-			} else /* MODULE_STATE_GOING */
+			} else {
+				/* MODULE_STATE_GOING */
+
+				/*
+				 * There's a small window where a register or
+				 * enable call may have already done this
+				 * teardown.  First one there wins.
+				 */
+				if (!obj->mod)
+					continue;
+
 				klp_module_notify_going(patch, obj);
+				obj->mod = NULL;
+			}
 
 			break;
 		}