netdev - Re: [bpf-next V2 PATCH 06/15] tun: convert to use generic xdp_frame and xdp_return

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Date:   Thu, 8 Mar 2018 16:16:25 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     BjörnTöpel <bjorn.topel@...el.com>,
        magnus.karlsson@...el.com, Jason Wang <jasowang@...hat.com>
Cc:     netdev@...r.kernel.org, eugenia@...lanox.com,
        John Fastabend <john.fastabend@...il.com>,
        Eran Ben Elisha <eranbe@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>, galp@...lanox.com,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Tariq Toukan <tariqt@...lanox.com>, brouer@...hat.com
Subject: Re: [bpf-next V2 PATCH 06/15] tun: convert to use generic xdp_frame
 and xdp_return_frame API

Hi Jason,

Please see below FIXME, which is actually a question to you.

On Thu, 08 Mar 2018 14:08:11 +0100 Jesper Dangaard Brouer <brouer@...hat.com> wrote:

> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 475088f947bb..cd046cf31b77 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
[...]

> @@ -1290,17 +1290,18 @@ static const struct net_device_ops tun_netdev_ops = {
>  static int tun_xdp_xmit(struct net_device *dev, struct xdp_buff *xdp)
>  {
>  	struct tun_struct *tun = netdev_priv(dev);
> -	struct xdp_buff *buff = xdp->data_hard_start;
> -	int headroom = xdp->data - xdp->data_hard_start;
> +	struct xdp_frame *frame;
>  	struct tun_file *tfile;
>  	u32 numqueues;
>  	int ret = 0;
>  
> -	/* Assure headroom is available and buff is properly aligned */
> -	if (unlikely(headroom < sizeof(*xdp) || tun_is_xdp_buff(xdp)))
> -		return -ENOSPC;
> +	/* FIXME: Explain why this check is the needed! */
> +	if (unlikely(tun_is_xdp_frame(xdp)))
> +		return -EBADRQC;
>  
> -	*buff = *xdp;
> +	frame = convert_to_xdp_frame(xdp);
> +	if (unlikely(!frame))
> +		return -EOVERFLOW;

To Jason, in the FIXME, I'm inheriting a check you put in, but I don't
understand why this check was needed?

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