notifier: Return an error when a callback has already been registered

Return -EEXIST when a notifier callback has already been registered on a notifier chain. This should avoid any homegrown registration tracking at the callsite like https://lore.kernel.org/amd-gfx/20210512013058.6827-1-mukul.joshi@amd.com for example. This version is an alternative of https://lore.kernel.org/r/20211108101157.15189-1-bp@alien8.de which needed to touch every caller not checking the registration routine's return value.

Borislav Petkov committed 4 years ago

Commit 5abb0651 parent fc74e0a1 file · +8 −7Browse files

Unified · Split

kernel/notifier.c+8−7

  */
 static int notifier_chain_register(struct notifier_block **nl,
-		struct notifier_block *n)
+				   struct notifier_block *n)
 {
 	while ((*nl) != NULL) {
 		if (unlikely((*nl) == n)) {
-			WARN(1, "double register detected");
+			WARN(1, "notifier callback %ps already registered",
-			return 0;
+			     n->notifier_call);
+			return -EEXIST;
 		}
 		if (n->priority > (*nl)->priority)
 			break;
  *
  *	Adds a notifier to an atomic notifier chain.
  *
- *	Currently always returns zero.
+ *	Returns 0 on success, %-EEXIST on error.
  */
 int atomic_notifier_chain_register(struct atomic_notifier_head *nh,
 		struct notifier_block *n)
  *	Adds a notifier to a blocking notifier chain.
  *	Must be called in process context.
  *
- *	Currently always returns zero.
+ *	Returns 0 on success, %-EEXIST on error.
  */
 int blocking_notifier_chain_register(struct blocking_notifier_head *nh,
 		struct notifier_block *n)
  *	Adds a notifier to a raw notifier chain.
  *	All locking must be provided by the caller.
  *
- *	Currently always returns zero.
+ *	Returns 0 on success, %-EEXIST on error.
  */
 int raw_notifier_chain_register(struct raw_notifier_head *nh,
 		struct notifier_block *n)
  *	Adds a notifier to an SRCU notifier chain.
  *	Must be called in process context.
  *
- *	Currently always returns zero.
+ *	Returns 0 on success, %-EEXIST on error.
  */
 int srcu_notifier_chain_register(struct srcu_notifier_head *nh,
 		struct notifier_block *n)