Re: [PATCH v0 00/42] notifiers: Return an error when callback is already registered

[Borislav Petkov <bp@xxxxxxxxx>]

On Mon, Nov 08, 2021 at 03:24:39PM +0100, Borislav Petkov wrote:
> I guess I can add another indirection to notifier_chain_register() and
> avoid touching all the call sites.

IOW, something like this below.

This way I won't have to touch all the callsites and the registration
routines would still return a proper value instead of returning 0
unconditionally.

---
diff --git a/kernel/notifier.c b/kernel/notifier.c
index b8251dc0bc0f..04f08b2ef17f 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -19,14 +19,12 @@ BLOCKING_NOTIFIER_HEAD(reboot_notifier_list);
  *     are layered on top of these, with appropriate locking added.
  */
 
-static int notifier_chain_register(struct notifier_block **nl,
-               struct notifier_block *n)
+static int __notifier_chain_register(struct notifier_block **nl,
+                                    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);
@@ -36,6 +34,18 @@ static int notifier_chain_register(struct notifier_block 
**nl,
        return 0;
 }
 
+static int notifier_chain_register(struct notifier_block **nl,
+                                  struct notifier_block *n)
+{
+       int ret = __notifier_chain_register(nl, n);
+
+       if (ret == -EEXIST)
+               WARN(1, "double register of notifier callback %ps detected",
+                       n->notifier_call);
+
+       return ret;
+}
+
 static int notifier_chain_unregister(struct notifier_block **nl,
                struct notifier_block *n)
 {


-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette