Re: [PATCH -v2] notifier: Return an error when a callback has already been registered

[Alan Stern]

On Thu, Dec 23, 2021 at 04:31:01PM +0100, Borislav Petkov wrote:
> From: Borislav Petkov <bp@xxxxxxx>
> 
> 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>
> ---

Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

>  kernel/notifier.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/notifier.c b/kernel/notifier.c
> index b8251dc0bc0f..ba005ebf4730 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)
> -- 
> 2.29.2
> 
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette