[RFC PATCH] notifier: Return non-null when callback already registered

[Borislav Petkov <bp@alien8.de>]

From: Borislav Petkov <bp@suse.de>

The notifier registration routine doesn't return a proper error value
when a callback has already been registered, leading people to track
whether that regisration has happened at the call site:

  https://lore.kernel.org/amd-gfx/20210512013058.6827-1-mukul.joshi@amd.com/

Which is unnecessary.

Return a non-null to signal that case so that callers can act
accordingly.

Signed-off-by: Borislav Petkov <bp@suse.de>
---
 kernel/notifier.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/notifier.c b/kernel/notifier.c
index 1b019cbca594..ff7a3198c5fc 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -25,7 +25,7 @@ static int notifier_chain_register(struct notifier_block **nl,
 	while ((*nl) != NULL) {
 		if (unlikely((*nl) == n)) {
 			WARN(1, "double register detected");
-			return 0;
+			return 1;
 		}
 		if (n->priority > (*nl)->priority)
 			break;
@@ -134,7 +134,7 @@ 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, !0 on error.
  */
 int atomic_notifier_chain_register(struct atomic_notifier_head *nh,
 		struct notifier_block *n)
@@ -235,7 +235,7 @@ 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, !0 on error.
  */
 int blocking_notifier_chain_register(struct blocking_notifier_head *nh,
 		struct notifier_block *n)
@@ -354,7 +354,7 @@ 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, !0 on error.
  */
 int raw_notifier_chain_register(struct raw_notifier_head *nh,
 		struct notifier_block *n)
@@ -425,7 +425,7 @@ 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, !0 on error.
  */
 int srcu_notifier_chain_register(struct srcu_notifier_head *nh,
 		struct notifier_block *n)
-- 
2.29.2