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

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Date:	Thu, 19 May 2016 18:56:00 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net] udp: prevent skbs lingering in tunnel socket queues

On Thu, May 19, 2016 at 3:50 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> 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).

Ah, yeah, __udp4_lib_rcv().

BTW, this part (the second half) of your patch is an optimization,
the rest (the first half) is a real bug fix. It makes sense to split
them if you plan to backport it to -stable.

Thanks.