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

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Message-ID: <20180309094630.062a9362@redhat.com>
Date:   Fri, 9 Mar 2018 09:46:30 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     BjörnTöpel <bjorn.topel@...el.com>,
        magnus.karlsson@...el.com, 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

On Fri, 9 Mar 2018 15:16:35 +0800
Jason Wang <jasowang@...hat.com> wrote:

> On 2018年03月08日 23:16, Jesper Dangaard Brouer wrote:
> > 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?
> >  
> 
> Sorry for the late reply.
> 
> I think it was used to make sure to not use misaligned or invalid 
> pointer that caller passed to us.

Okay, but I don't think this can happen, thus I'm going to remove this
check in V3.

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