Re: [PATCH net-next 1/2] bpf: enable non-root eBPF programs

[Alexei Starovoitov]

On 10/6/15 1:39 AM, Daniel Borkmann wrote:
> > > [...] Also classic BPF would then need to test for it, since a socket
> > > filter
> > > doesn't really know whether native eBPF is loaded there or a
> > > classic-to-eBPF
> > > transformed one, and classic never makes use of this. Anyway, it
> > > could be done
> > > by adding a bit flag cb_access:1 to the bpf_prog, set it during eBPF
> > > verification phase, and test it inside sk_filter() if I see it
> > > correctly.
> >
> > That could also be done in an unlikely() branch, to keep the cost to
> > the non-eBPF
> > case near zero.
>
> Yes, agreed. For the time being, the majority of users are coming from the
> classic BPF side anyway and the unlikely() could still be changed later on
> if it should not be the case anymore. The flag and bpf_func would share the
> same cacheline as well.

was also thinking that we can do it only in paths that actually
have multiple protocol layers, since today bpf is mainly used with
tcpdump(raw_socket) and new af_packet fanout both have cb cleared
on RX, because it just came out of alloc_skb and no layers were called,
and on TX we can clear 20 bytes in dev_queue_xmit_nit().
af_unix/netlink also have clean skb. Need to analyze tun and sctp...
but it feels overly fragile to save a branch in sk_filter,
so planning to go with
if(unlikely(prog->cb_access)) memset in sk_filter().

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