Subject: Re: [BUG bpf-next] bpf/net: Hitting gpf when running selftests

From: John Fastabend <[email-masked]>
Date: Fri, 01 Sep 2023 13:22:24 -0700

Jiri Olsa wrote:
> On Fri, Sep 01, 2023 at 05:10:43PM +0800, Xu Kuohai wrote:
> 
> SNIP
> 
> > > > Trying to come up with some nice fix now.
> > > 
> > > Something like this it fixes the splat, but need to think if it
> > > introduces anything or some better way to do this. Basic idea
> > > is to bump user->refcnt because we have two references to the
> > > skb and want to ensure we really only kfree_skb() the skb
> > > after both references are dropped.
> > > 
> > > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> > > index a0659fc29bcc..6c31eefbd777 100644
> > > --- a/net/core/skmsg.c
> > > +++ b/net/core/skmsg.c
> > > @@ -612,12 +612,18 @@ static int sk_psock_skb_ingress_self(struct sk_psock *psock, struct sk_buff *skb
> > >   static int sk_psock_handle_skb(struct sk_psock *psock, struct sk_buff *skb,
> > >                                 u32 off, u32 len, bool ingress)
> > >   {
> > > +       int err = 0;
> > > +
> > >          if (!ingress) {
> > >                  if (!sock_writeable(psock->sk))
> > >                          return -EAGAIN;
> > >                  return skb_send_sock(psock->sk, skb, off, len);
> > >          }
> > > -       return sk_psock_skb_ingress(psock, skb, off, len);
> > > +       skb_get(skb);
> > > +       err = sk_psock_skb_ingress(psock, skb, off, len);
> > > +       if (err < 0)
> > > +               kfree_skb(skb);
> > > +       return err;
> > >   }
> > >   static void sk_psock_skb_state(struct sk_psock *psock,
> > > @@ -685,9 +691,7 @@ static void sk_psock_backlog(struct work_struct *work)
> > >                  } while (len);
> > >                  skb = skb_dequeue(&psock->ingress_skb);
> > > -               if (!ingress) {
> > > -                       kfree_skb(skb);
> > > -               }
> > > +               kfree_skb(skb);
> > >          }
> > >   end:
> > >          mutex_unlock(&psock->work_mutex);
> > > .
> > 
> > With this fix, the crash is gone.
> 
> +1, same on my setup
> 
> jirka

Sent a patch. Add tested-by and acks if you have time. Thanks!