Re: [External] : Re: [REPORT] Null pointer deref in net/core/dev.c on PowerPC

[ALOK TIWARI <alok.a.tiwari@oracle.com>]

On 12/17/2025 8:52 PM, Eric Dumazet wrote:
>>> I will send the following fix, thanks.
>>>
>>> diff --git a/net/core/dev.c b/net/core/dev.c
>>> index 9094c0fb8c68..36dc5199037e 100644
>>> --- a/net/core/dev.c
>>> +++ b/net/core/dev.c
>>> @@ -4241,9 +4241,11 @@ static inline int __dev_xmit_skb(struct sk_buff
>>> *skb, struct Qdisc *q,
>>>                   int count = 0;
>>>
>>>                   llist_for_each_entry_safe(skb, next, ll_list, ll_node) {
>>> -                       prefetch(next);
>>> -                       prefetch(&next->priority);
>>> -                       skb_mark_not_on_list(skb);
>>> +                       if (next) {
>>> +                               prefetch(next);
>>> +                               prefetch(&next->priority);
>>> +                               skb_mark_not_on_list(skb);
>>> +                       }
>>>                           rc = dev_qdisc_enqueue(skb, q, &to_free, txq);
>>>                           count++;
>>>                   }
>>>
>> why not only ?
>> if (likely(next)) {
>>       prefetch(next);
>>       prefetch(&next->priority);
>> }
> Because we also can avoid clearing skb->next, we know it is already NULL.
> 
> Since we pay the price of a conditional, let's amortize its cost :/

Thanks a lot for the explanation, I understand the goal of amortizing 
the cost and avoiding unnecessary writes to skb->next.
Would it make sense to add if (likely(next)) around the prefetch?

Thanks,
Alok