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.

Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/YcSWNdUBS8A2ZB3s@zn.tnic

diff --git a/kernel/notifier.c b/kernel/notifier.c
index b8251dc..ba005eb 100644 (file)
--- 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)