Subject: Re: [net PATCH 1/4] virtio_net: disable XDP_REDIRECT in receive_mergeable() case

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 27 Feb 2018 02:40:44 +0200

On Tue, Feb 20, 2018 at 02:32:04PM +0100, Jesper Dangaard Brouer wrote:
> The virtio_net code have three different RX code-paths in receive_buf().
> Two of these code paths can handle XDP, but one of them is broken for
> at least XDP_REDIRECT.
> 
> Function(1): receive_big() does not support XDP.
> Function(2): receive_small() support XDP fully and uses build_skb().
> Function(3): receive_mergeable() broken XDP_REDIRECT uses napi_alloc_skb().
> 
> The simple explanation is that receive_mergeable() is broken because
> it uses napi_alloc_skb(), which violates XDP given XDP assumes packet
> header+data in single page and enough tail room for skb_shared_info.
> 
> The longer explaination is that receive_mergeable() tries to
> work-around and satisfy these XDP requiresments e.g. by having a
> function xdp_linearize_page() that allocates and memcpy RX buffers
> around (in case packet is scattered across multiple rx buffers).  This
> does currently satisfy XDP_PASS, XDP_DROP and XDP_TX (but only because
> we have not implemented bpf_xdp_adjust_tail yet).
> 
> The XDP_REDIRECT action combined with cpumap is broken, and cause hard
> to debug crashes.  The main issue is that the RX packet does not have
> the needed tail-room (SKB_DATA_ALIGN(skb_shared_info)), causing
> skb_shared_info to overlap the next packets head-room (in which cpumap
> stores info).
> 
> Reproducing depend on the packet payload length and if RX-buffer size
> happened to have tail-room for skb_shared_info or not.  But to make
> this even harder to troubleshoot, the RX-buffer size is runtime
> dynamically change based on an Exponentially Weighted Moving Average
> (EWMA) over the packet length, when refilling RX rings.
> 
> This patch only disable XDP_REDIRECT support in receive_mergeable()
> case, because it can cause a real crash.
> 
> IMHO we should consider NOT supporting XDP in receive_mergeable() at
> all, because the principles behind XDP are to gain speed by (1) code
> simplicity, (2) sacrificing memory and (3) where possible moving
> runtime checks to setup time.  These principles are clearly being
> violated in receive_mergeable(), that e.g. runtime track average
> buffer size to save memory consumption.
> 
> In the longer run, we should consider introducing a separate receive
> function when attaching an XDP program, and also change the memory
> model to be compatible with XDP when attaching an XDP prog.

I agree with a separate function approach.

So each buffer is tagged as xdp/non xdp, we check that
and handle appropriately - where non xdp could be handled
by the generic path.

> Fixes: 186b3c998c50 ("virtio-net: support XDP_REDIRECT")
> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> ---
>  drivers/net/virtio_net.c |    7 -------
>  1 file changed, 7 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 626c27352ae2..0ca91942a884 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -677,7 +677,6 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  	struct bpf_prog *xdp_prog;
>  	unsigned int truesize;
>  	unsigned int headroom = mergeable_ctx_to_headroom(ctx);
> -	int err;
>  
>  	head_skb = NULL;
>  
> @@ -754,12 +753,6 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  				goto err_xdp;
>  			rcu_read_unlock();
>  			goto xdp_xmit;
> -		case XDP_REDIRECT:
> -			err = xdp_do_redirect(dev, &xdp, xdp_prog);
> -			if (!err)
> -				*xdp_xmit = true;
> -			rcu_read_unlock();
> -			goto xdp_xmit;
>  		default:
>  			bpf_warn_invalid_xdp_action(act);
>  		case XDP_ABORTED: