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

Borislav Petkov
2021-11-08T10:13:43Z

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 registration has happened at the call site:

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

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@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ayush Sawal <ayush.sawal@chelsio.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Rohit Maheshwari <rohitm@chelsio.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Vinay Kumar Yadav <vinay.yadav@chelsio.com>
Cc: alsa-devel@alsa-project.org
Cc: bcm-kernel-feedback-list@broadcom.com
Cc: intel-gfx@lists.freedesktop.org
Cc: intel-gvt-dev@lists.freedesktop.org
Cc: linux-alpha@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-clk@vger.kernel.org
Cc: linux-crypto@vger.kernel.org
Cc: linux-edac@vger.kernel.org
Cc: linux-fbdev@vger.kernel.org
Cc: linux-hyperv@vger.kernel.org
Cc: linux-iio@vger.kernel.org
Cc: linux-leds@vger.kernel.org
Cc: linux-mips@vger.kernel.org
Cc: linux-parisc@vger.kernel.org
Cc: linux-pm@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
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Cc: x86@kernel.org
Cc: xen-devel@lists.xenproject.org
---
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