netdev - Re: [PATCH net-next v3 3/4] bpf: BPF for lightweight tunnel infrastructure

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Date:   Tue, 29 Nov 2016 23:01:52 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Thomas Graf <tgraf@...g.ch>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, daniel@...earbox.net,
        tom@...bertland.com, roopa@...ulusnetworks.com,
        hannes@...essinduktion.org
Subject: Re: [PATCH net-next v3 3/4] bpf: BPF for lightweight tunnel
 infrastructure

On Wed, Nov 30, 2016 at 07:48:51AM +0100, Thomas Graf wrote:
> On 11/29/16 at 04:15pm, Alexei Starovoitov wrote:
> > On Tue, Nov 29, 2016 at 02:21:22PM +0100, Thomas Graf wrote:
> > ...
> > > +#define LWT_BPF_MAX_HEADROOM 128
> > 
> > why 128?
> > btw I'm thinking for XDP to use 256, so metadata can be stored in there.
> 
> It's an arbitrary limit to catch obvious misconfiguration. I'm absolutely
> fine with bumping it to 256.
> 
> > > +static int run_lwt_bpf(struct sk_buff *skb, struct bpf_lwt_prog *lwt,
> > > +		       struct dst_entry *dst, bool can_redirect)
> > > +{
> > > +	int ret;
> > > +
> > > +	/* Preempt disable is needed to protect per-cpu redirect_info between
> > > +	 * BPF prog and skb_do_redirect(). The call_rcu in bpf_prog_put() and
> > > +	 * access to maps strictly require a rcu_read_lock() for protection,
> > > +	 * mixing with BH RCU lock doesn't work.
> > > +	 */
> > > +	preempt_disable();
> > > +	rcu_read_lock();
> > > +	bpf_compute_data_end(skb);
> > > +	ret = BPF_PROG_RUN(lwt->prog, skb);
> > > +	rcu_read_unlock();
> > > +
> > > +	switch (ret) {
> > > +	case BPF_OK:
> > > +		break;
> > > +
> > > +	case BPF_REDIRECT:
> > > +		if (!can_redirect) {
> > > +			WARN_ONCE(1, "Illegal redirect return code in prog %s\n",
> > > +				  lwt->name ? : "<unknown>");
> > > +			ret = BPF_OK;
> > > +		} else {
> > > +			ret = skb_do_redirect(skb);
> > 
> > I think this assumes that program did bpf_skb_push and L2 header is present.
> > Would it make sense to check that mac_header < network_header here to make
> > sure that it actually happened? I think the cost of single 'if' isn't much.
> > Also skb_do_redirect() can redirect to l3 tunnels like ipip ;)
> > so program shouldn't be doing bpf_skb_push in such case...
> 
> We are currently guaranteeing mac_header <= network_header given that
> bpf_skb_push() is calling skb_reset_mac_header() unconditionally.
> 
> Even if a program were to push an L2 header and then redirect to an l3
> tunnel, __bpf_redirect_no_mac will pull it off again and correct the
> mac_header offset.

yes. that part is fine.

> Should we check in __bpf_redirect_common() whether mac_header <
> nework_header then or add it to lwt-bpf conditional on
> dev_is_mac_header_xmit()?

may be only extra 'if' in lwt-bpf is all we need?
I'm still missing what will happen if we 'forget' to do
bpf_skb_push() inside the lwt-bpf program, but still do redirect
in lwt_xmit stage to l2 netdev...