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

On 08/17/2017 12:28 PM, Jesper Dangaard Brouer wrote:

On Thu, 17 Aug 2017 11:46:10 -0700 John Fastabend [off-list ref] wrote:

quoted

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);

I think we should drop the warn_invalid now that we have a tracepoint.
The tracepoint is much nicer for debugging vs a warning for what might
be a valid case depending on xdp program.

quoted

quoted

-		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

Great thanks.