netdev - Re: [PATCH net] udp: prevent skbs lingering in tunnel socket queues

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Date:	Fri, 20 May 2016 00:50:34 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net] udp: prevent skbs lingering in tunnel socket queues

Hi Cong,

On Fri, May 20, 2016, at 00:33, Cong Wang wrote:
> On Thu, May 19, 2016 at 6:58 AM, Hannes Frederic Sowa
> <hannes@...essinduktion.org> wrote:
> > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > index 2e3ebfe5549ef5..d56c0559b477cb 100644
> > --- a/net/ipv4/udp.c
> > +++ b/net/ipv4/udp.c
> > @@ -1565,7 +1565,7 @@ int udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
> >
> >                 /* if we're overly short, let UDP handle it */
> >                 encap_rcv = ACCESS_ONCE(up->encap_rcv);
> > -               if (skb->len > sizeof(struct udphdr) && encap_rcv) {
> > +               if (encap_rcv) {
> 
> 
> I don't think you can just remove it here, l2tp_udp_recv_core() still
> relies on it:
> 
>         /* UDP has verifed checksum */
> 
>         /* UDP always verifies the packet length. */
>         __skb_pull(skb, sizeof(struct udphdr));

I think this is fine, we check on every entrance to udp that we may pull
(pskb_may_pull) an udphdr but we really never pull the header. At this
point we are guaranteed to have skb->len of at least sizeof(struct
udphdr).

Bye,
Hannes