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

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Message-ID: <CANn89iK_HyO_9Ek0VRcvjsM-mY-_kUvDsVPkh2iUvi-qcAhDCA@mail.gmail.com>
Date: Wed, 17 Dec 2025 16:44:04 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: ALOK TIWARI <alok.a.tiwari@...cle.com>
Cc: Aditya Gupta <adityag@...ux.ibm.com>, linux-kernel@...r.kernel.org, 
	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [External] : Re: [REPORT] Null pointer deref in net/core/dev.c on PowerPC

On Wed, Dec 17, 2025 at 4:38 PM ALOK TIWARI <alok.a.tiwari@...cle.com> wrote:
>
>
>
> 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?

I do not think this is unlikely(), otherwise my recent optimizations should not
have been needed in the first place.

Just leave that to cpu branch prediction.