netdev - Re: [PATCH 2/2] xdp: adjust xdp redirect tracepoint to include return error code

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Date:   Thu, 17 Aug 2017 21:28:57 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     netdev@...r.kernel.org, brouer@...hat.com
Subject: Re: [PATCH 2/2] xdp: adjust xdp redirect tracepoint to include
 return error code


On Thu, 17 Aug 2017 11:46:10 -0700 John Fastabend <john.fastabend@...il.com> wrote:

> On 08/17/2017 09:22 AM, Jesper Dangaard Brouer wrote:
> > The return error code need to be included in the tracepoint
> > xdp:xdp_redirect, else its not possible to distinguish successful or
> > failed XDP_REDIRECT transmits.
> > 
> > XDP have no queuing mechanism. Thus, it is fairly easily to overrun a
> > NIC transmit queue.  The eBPF program invoking helpers (bpf_redirect
> > or bpf_redirect_map) to redirect a packet doesn't get any feedback
> > whether the packet was actually transmitted.
> > 
> > Info on failed transmits in the tracepoint xdp:xdp_redirect, is
> > interesting as this opens for providing a feedback-loop to the
> > receiving XDP program.
> > 
> > Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> > ---  
> 
> [...]
> 
> > @@ -2532,12 +2535,14 @@ int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
> >  	ri->map = NULL;
> >  	if (unlikely(!fwd)) {
> >  		bpf_warn_invalid_xdp_redirect(index);
> > -		return -EINVAL;
> > +		err = -EINVAL;
> > +		goto out;  
> 
> It doesn't look like there is a check in trace_xdp_redirect to
> avoid dereferencing a NULL fwd pointer here (*to in trace code
> path). Did I miss something?

Nice that you spotted this in your review, but the __string() macro
used in trace code already takes case of this, see output:

 xdp:xdp_redirect: prog=39cf08f65683838a from=ixgbe2 to=(null) action=REDIRECT err=-22
 
> >  	}
> >  
> > -	trace_xdp_redirect(dev, fwd, xdp_prog, XDP_REDIRECT);
> > -
> > -	return __bpf_tx_xdp(fwd, NULL, xdp, 0);
> > +	err = __bpf_tx_xdp(fwd, NULL, xdp, 0);
> > +out:
> > +	trace_xdp_redirect(dev, fwd, xdp_prog, XDP_REDIRECT, err);
> > +	return err;
> >  }
> >  EXPORT_SYMBOL_GPL(xdp_do_redirect);

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer