[PATCH] kernel/notifier.c: remove notifier_chain_register

[Xiaoming Ni]

Registering the same notifier to a hook repeatedly can cause the hook
list to form a ring or lose other members of the list.

case1: An infinite loop in notifier_chain_register can cause soft lockup
        atomic_notifier_chain_register(&test_notifier_list, &test_notifier1);
        atomic_notifier_chain_register(&test_notifier_list, &test_notifier1);
        atomic_notifier_chain_register(&test_notifier_list, &test_notifier2);

case2: An infinite loop in notifier_chain_register can cause soft lockup
        atomic_notifier_chain_register(&test_notifier_list, &test_notifier1);
        atomic_notifier_chain_register(&test_notifier_list, &test_notifier1);
        atomic_notifier_call_chain(&test_notifier_list, 0, NULL);

case3: lose other hook "test_notifier2"
        atomic_notifier_chain_register(&test_notifier_list, &test_notifier1);
        atomic_notifier_chain_register(&test_notifier_list, &test_notifier2);
        atomic_notifier_chain_register(&test_notifier_list, &test_notifier1);

case4: Unregister returns 0, but the hook is still in the linked list,
        and it is not really registered. If you call notifier_call_chain
        after ko is unloaded, it will trigger oops.

If the system is configured with softlockup_panic and the same
hook is repeatedly registered on the panic_notifier_list, it
will cause a loop panic.

The only difference between notifier_chain_cond_register and
notifier_chain_register is that a check is added in order to
avoid registering the same notifier multiple times to the same hook.
So consider removing notifier_chain_register and replacing it
with notifier_chain_cond_register.

Signed-off-by: Xiaoming Ni <nixiaom...@huawei.com>
---
 kernel/notifier.c | 26 ++++++--------------------
 1 file changed, 6 insertions(+), 20 deletions(-)

diff --git a/kernel/notifier.c b/kernel/notifier.c
index d9f5081..56efd54 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -19,20 +19,6 @@
  *     are layered on top of these, with appropriate locking added.
  */
 
-static int notifier_chain_register(struct notifier_block **nl,
-               struct notifier_block *n)
-{
-       while ((*nl) != NULL) {
-               WARN_ONCE(((*nl) == n), "double register detected");
-               if (n->priority > (*nl)->priority)
-                       break;
-               nl = &((*nl)->next);
-       }
-       n->next = *nl;
-       rcu_assign_pointer(*nl, n);
-       return 0;
-}
-
 static int notifier_chain_cond_register(struct notifier_block **nl,
                struct notifier_block *n)
 {
@@ -127,7 +113,7 @@ int atomic_notifier_chain_register(struct 
atomic_notifier_head *nh,
        int ret;
 
        spin_lock_irqsave(&nh->lock, flags);
-       ret = notifier_chain_register(&nh->head, n);
+       ret = notifier_chain_cond_register(&nh->head, n);
        spin_unlock_irqrestore(&nh->lock, flags);
        return ret;
 }
@@ -223,10 +209,10 @@ int blocking_notifier_chain_register(struct 
blocking_notifier_head *nh,
         * such times we must not call down_write().
         */
        if (unlikely(system_state == SYSTEM_BOOTING))
-               return notifier_chain_register(&nh->head, n);
+               return notifier_chain_cond_register(&nh->head, n);
 
        down_write(&nh->rwsem);
-       ret = notifier_chain_register(&nh->head, n);
+       ret = notifier_chain_cond_register(&nh->head, n);
        up_write(&nh->rwsem);
        return ret;
 }
@@ -349,7 +335,7 @@ int blocking_notifier_call_chain(struct 
blocking_notifier_head *nh,
 int raw_notifier_chain_register(struct raw_notifier_head *nh,
                struct notifier_block *n)
 {
-       return notifier_chain_register(&nh->head, n);
+       return notifier_chain_cond_register(&nh->head, n);
 }
 EXPORT_SYMBOL_GPL(raw_notifier_chain_register);
 
@@ -431,10 +417,10 @@ int srcu_notifier_chain_register(struct 
srcu_notifier_head *nh,
         * such times we must not call mutex_lock().
         */
        if (unlikely(system_state == SYSTEM_BOOTING))
-               return notifier_chain_register(&nh->head, n);
+               return notifier_chain_cond_register(&nh->head, n);
 
        mutex_lock(&nh->mutex);
-       ret = notifier_chain_register(&nh->head, n);
+       ret = notifier_chain_cond_register(&nh->head, n);
        mutex_unlock(&nh->mutex);
        return ret;
 }
-- 
1.8.5.6