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/[email protected] for example. This version is an alternative of https://lore.kernel.org/r/[email protected] which needed to touch every caller not checking the registration routine's return value. Signed-off-by: Borislav Petkov <[email protected]> Reviewed-by: Sebastian Andrzej Siewior <[email protected]> Acked-by: Alan Stern <[email protected]> Link: https://lore.kernel.org/r/[email protected]
torvalds · Dec 29, 2021 · 5abb065 · 5abb065
1 parent fc74e0a
commit 5abb065
Showing 1 changed file with 8 additions and 7 deletions.
diff --git a/kernel/notifier.c b/kernel/notifier.c
@@ -20,12 +20,13 @@ BLOCKING_NOTIFIER_HEAD(reboot_notifier_list);
  */
 
 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");
- return 0;
+ WARN(1, "notifier callback %ps already registered",
+ n->notifier_call);
+ return -EEXIST;
  }
  if (n->priority > (*nl)->priority)
  break;
@@ -134,7 +135,7 @@ static int notifier_call_chain_robust(struct notifier_block **nl,
  *
  * 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)
@@ -216,7 +217,7 @@ NOKPROBE_SYMBOL(atomic_notifier_call_chain);
  * 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)
@@ -335,7 +336,7 @@ EXPORT_SYMBOL_GPL(blocking_notifier_call_chain);
  * 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)
@@ -406,7 +407,7 @@ EXPORT_SYMBOL_GPL(raw_notifier_call_chain);
  * 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)