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 [off-list ref] wrote:

On 08/17/2017 09:22 AM, Jesper Dangaard Brouer wrote:

quoted

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 <redacted>
---  

[...]

quoted

@@ -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
 

quoted

 	}
 
-	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