Re: [regression] UDP recv data corruption

[Paolo Abeni <pabeni@redhat.com>]

On Fri, 2021-07-02 at 17:23 +0200, Matthias Treydte wrote:
> Quoting Paolo Abeni <pabeni@redhat.com>:
> 
> > ---
> > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> > index 54e06b88af69..458c888337a5 100644
> > --- a/net/ipv4/udp_offload.c
> > +++ b/net/ipv4/udp_offload.c
> > @@ -526,6 +526,8 @@ struct sk_buff *udp_gro_receive(struct list_head  
> > *head, struct sk_buff *skb,
> >                 if ((!sk && (skb->dev->features & NETIF_F_GRO_UDP_FWD)) ||
> >                     (sk && udp_sk(sk)->gro_enabled) ||  
> > NAPI_GRO_CB(skb)->is_flist)
> >                         pp =  
> > call_gro_receive(udp_gro_receive_segment, head, skb);
> > +               else
> > +                       goto out;
> >                 return pp;
> >         }
> 
> Impressive! This patch, applied to 5.13, fixes the problem. What I  
> like even more is that it again confirms my suspicion that an "if"  
> without an "else" is always a code smell. :-)

Thank you for the quick feedback! I'll submit formally soon, after more
tests. I'll probably change the code to be something hopefully more
readable, as follow:

---
diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index 7a670757f37a..b3aabc886763 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -551,8 +551,10 @@ struct sk_buff *udp_gro_receive(struct list_head *head, struct sk_buff *skb,
 
                if ((!sk && (skb->dev->features & NETIF_F_GRO_UDP_FWD)) ||
                    (sk && udp_sk(sk)->gro_enabled) || NAPI_GRO_CB(skb)->is_flist)
-                       pp = call_gro_receive(udp_gro_receive_segment, head, skb);
-               return pp;
+                       return call_gro_receive(udp_gro_receive_segment, head, skb);
+
+               /* no GRO, be sure flush the current packet */
+               goto out;
        }
---

> With this and the reproducer in my previous mail, is there still value  
> in doing the "perf" stuff?

Not needed, thank you!

Would be great instead if you could have a spin to the proposed variant
above - not stritly needed, I'm really asking for a few extra miles
here ;)

Cheers,

Paolo