debug: add notifier chain debugging

during some development we suspected a case where we left something
in a notifier chain that was from a module that was unloaded already...
and that sort of thing is rather hard to track down.

This patch adds a very simple sanity check (which isn't all that
expensive) to make sure the notifier we're about to call is
actually from either the kernel itself of from a still-loaded
module, avoiding a hard-to-chase-down crash.
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/notifier.c

@@ -21,6 +21,10 @@ BLOCKING_NOTIFIER_HEAD(reboot_notifier_list);
 static int notifier_chain_register(struct notifier_block **nl,
 		struct notifier_block *n)
 {
+	if (!kernel_text_address((unsigned long)n->notifier_call)) {
+		WARN(1, "Invalid notifier registered!");
+		return 0;
+	}
 	while ((*nl) != NULL) {
 		if (n->priority > (*nl)->priority)
 			break;
@@ -34,6 +38,10 @@ static int notifier_chain_register(struct notifier_block **nl,
 static int notifier_chain_cond_register(struct notifier_block **nl,
 		struct notifier_block *n)
 {
+	if (!kernel_text_address((unsigned long)n->notifier_call)) {
+		WARN(1, "Invalid notifier registered!");
+		return 0;
+	}
 	while ((*nl) != NULL) {
 		if ((*nl) == n)
 			return 0;
@@ -82,6 +90,14 @@ static int __kprobes notifier_call_chain(struct notifier_block **nl,
 
 	while (nb && nr_to_call) {
 		next_nb = rcu_dereference(nb->next);
+
+#ifdef CONFIG_DEBUG_NOTIFIERS
+		if (!kernel_text_address((unsigned long)nb->notifier_call)) {
+			WARN(1, "Invalid notifier called!");
+			nb = next_nb;
+			continue;
+		}
+#endif
 		ret = nb->notifier_call(nb, val, v);
 
 		if (nr_calls)

lib/Kconfig.debug

@@ -536,6 +536,16 @@ config DEBUG_SG
 
 	  If unsure, say N.
 
+config DEBUG_NOTIFIERS
+	bool "Debug notifier call chains"
+	depends on DEBUG_KERNEL
+	help
+	  Enable this to turn on sanity checking for notifier call chains.
+	  This is most useful for kernel developers to make sure that
+	  modules properly unregister themselves from notifier chains.
+	  This is a relatively cheap check but if you care about maximum
+	  performance, say N.
+
 config FRAME_POINTER
 	bool "Compile the kernel with frame pointers"
 	depends on DEBUG_KERNEL && \