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...
