netdev - Re: [PATCH bpf-next V7 4/8] bpf: add BPF-helper for MTU checking

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Message-ID: <20201214125242.7cea3ecb@carbon>
Date:   Mon, 14 Dec 2020 12:52:42 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     sdf@...gle.com
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        maze@...gle.com, lmb@...udflare.com, shaun@...era.io,
        Lorenzo Bianconi <lorenzo@...nel.org>, marek@...udflare.com,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, eyal.birger@...il.com,
        colrack@...il.com, brouer@...hat.com
Subject: Re: [PATCH bpf-next V7 4/8] bpf: add BPF-helper for MTU checking

On Thu, 3 Dec 2020 10:25:41 -0800
sdf@...gle.com wrote:

> > +BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
> > +	   u32, ifindex, u32 *, mtu_len, s32, len_diff, u64, flags)
> > +{
> > +	int ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
> > +	struct net_device *dev = skb->dev;
> > +	int len;
> > +	int mtu;
> > +
> > +	if (flags & ~(BPF_MTU_CHK_SEGS))
> > +		return -EINVAL;
> > +
> > +	dev = __dev_via_ifindex(dev, ifindex);
> > +	if (!dev)
> > +		return -ENODEV;
> > +
> > +	mtu = READ_ONCE(dev->mtu);
> > +
> > +	/* TC len is L2, remove L2-header as dev MTU is L3 size */  
> 
> [..]
> > +	len = skb->len - ETH_HLEN;  
> Any reason not to do s/ETH_HLEN/dev->hard_header_len/ (or min_header_len?)
> thought this patch?

Will fix in V9.

There is a very small (performance) overhead, but mostly because
net_device struct layout have placed mtu and hard_header_len on
different cache-lines. (This is something that should be fixed
separately).

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