[tip: core/core] notifier: Return an error when a callback has already been registered

[tip-bot2 for Borislav Petkov]

The following commit has been merged into the core/core branch of tip:

Commit-ID:     5abb065dca7301de90b7c44bbcdc378e49e4d362
Gitweb:        https://git.kernel.org/tip/5abb065dca7301de90b7c44bbcdc378e49e4d362
Author:        Borislav Petkov <bp@xxxxxxx>
AuthorDate:    Wed, 01 Dec 2021 22:28:14 +01:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Wed, 29 Dec 2021 10:37:33 +01:00

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@xxxxxxx

for example.

This version is an alternative of

  https://lore.kernel.org/r/20211108101157.15189-1-bp@xxxxxxxxx

which needed to touch every caller not checking the registration
routine's return value.

Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/YcSWNdUBS8A2ZB3s@xxxxxxx
---
 kernel/notifier.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

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