[PATCH v0 42/42] notifier: Return an error when callback is already registered

[Borislav Petkov]

From: Borislav Petkov <bp@xxxxxxx>

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

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

Which is unnecessary.

Return -EEXIST to signal that case so that callers can act accordingly.
Enforce callers to check the return value, leading to loud screaming
during build:

  arch/x86/kernel/cpu/mce/core.c: In function ‘mce_register_decode_chain’:
  arch/x86/kernel/cpu/mce/core.c:167:2: error: ignoring return value of \
   ‘blocking_notifier_chain_register’, declared with attribute warn_unused_result [-Werror=unused-result]
    blocking_notifier_chain_register(&x86_mce_decoder_chain, nb);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Drop the WARN too, while at it.

Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
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---
 include/linux/notifier.h |  8 ++++----
 kernel/notifier.c        | 36 +++++++++++++++++++-----------------
 2 files changed, 23 insertions(+), 21 deletions(-)

diff --git a/include/linux/notifier.h b/include/linux/notifier.h
index 87069b8459af..45cc5a8d0fd8 100644
--- a/include/linux/notifier.h
+++ b/include/linux/notifier.h
@@ -141,13 +141,13 @@ extern void srcu_init_notifier_head(struct srcu_notifier_head *nh);
 
 #ifdef __KERNEL__
 
-extern int atomic_notifier_chain_register(struct atomic_notifier_head *nh,
+extern int __must_check atomic_notifier_chain_register(struct atomic_notifier_head *nh,
 		struct notifier_block *nb);
-extern int blocking_notifier_chain_register(struct blocking_notifier_head *nh,
+extern int __must_check blocking_notifier_chain_register(struct blocking_notifier_head *nh,
 		struct notifier_block *nb);
-extern int raw_notifier_chain_register(struct raw_notifier_head *nh,
+extern int __must_check raw_notifier_chain_register(struct raw_notifier_head *nh,
 		struct notifier_block *nb);
-extern int srcu_notifier_chain_register(struct srcu_notifier_head *nh,
+extern int __must_check srcu_notifier_chain_register(struct srcu_notifier_head *nh,
 		struct notifier_block *nb);
 
 extern int atomic_notifier_chain_unregister(struct atomic_notifier_head *nh,
diff --git a/kernel/notifier.c b/kernel/notifier.c
index b8251dc0bc0f..451ef3f73ad2 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -20,13 +20,11 @@ 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;
-		}
+		if (unlikely((*nl) == n))
+			return -EEXIST;
 		if (n->priority > (*nl)->priority)
 			break;
 		nl = &((*nl)->next);
@@ -134,10 +132,11 @@ 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)
+int __must_check
+atomic_notifier_chain_register(struct atomic_notifier_head *nh,
+			       struct notifier_block *n)
 {
 	unsigned long flags;
 	int ret;
@@ -216,10 +215,11 @@ 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)
+int __must_check
+blocking_notifier_chain_register(struct blocking_notifier_head *nh,
+				 struct notifier_block *n)
 {
 	int ret;
 
@@ -335,10 +335,11 @@ 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)
+int __must_check
+raw_notifier_chain_register(struct raw_notifier_head *nh,
+			    struct notifier_block *n)
 {
 	return notifier_chain_register(&nh->head, n);
 }
@@ -406,10 +407,11 @@ 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)
+int __must_check
+srcu_notifier_chain_register(struct srcu_notifier_head *nh,
+			     struct notifier_block *n)
 {
 	int ret;
 
-- 
2.29.2