netdev - Re: [bpf-next, v3 1/5] flow_dissector: implements flow dissector BPF hook

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Date:   Thu, 13 Sep 2018 17:00:59 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Petar Penkov <peterpenkov96@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        simon.horman@...ronome.com, ecree@...arflare.com,
        songliubraving@...com, Tom Herbert <tom@...bertland.com>,
        Petar Penkov <ppenkov@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [bpf-next, v3 1/5] flow_dissector: implements flow dissector BPF hook

On Thu, Sep 13, 2018 at 4:57 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Thu, Sep 13, 2018 at 04:51:49PM -0400, Willem de Bruijn wrote:
> > On Thu, Sep 13, 2018 at 3:45 PM Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> > >
> > > On Thu, Sep 13, 2018 at 10:45:53AM -0700, Petar Penkov wrote:
> > > > From: Petar Penkov <ppenkov@...gle.com>
> > > >
> > > > Adds a hook for programs of type BPF_PROG_TYPE_FLOW_DISSECTOR and
> > > > attach type BPF_FLOW_DISSECTOR that is executed in the flow dissector
> > > > path. The BPF program is per-network namespace.
> > > >
> > > > Signed-off-by: Petar Penkov <ppenkov@...gle.com>
> > > > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> > > ...
> > > > @@ -2333,6 +2335,7 @@ struct __sk_buff {
> > > >       /* ... here. */
> > > >
> > > >       __u32 data_meta;
> > > > +     struct bpf_flow_keys *flow_keys;
> > >
> > > the bpf prog form patch 4 looks much better now. Thanks!
> > >
> > > >  };
> > > >
> > > >  struct bpf_tunnel_key {
> > > > @@ -2778,4 +2781,27 @@ enum bpf_task_fd_type {
> > > >       BPF_FD_TYPE_URETPROBE,          /* filename + offset */
> > > >  };
> > > >
> > > > +struct bpf_flow_keys {
> > > > +     __u16   nhoff;
> > > > +     __u16   thoff;
> > > > +     __u16   addr_proto;                     /* ETH_P_* of valid addrs */
> > > > +     __u8    is_frag;
> > > > +     __u8    is_first_frag;
> > > > +     __u8    is_encap;
> > > > +     __be16  n_proto;
> > > > +     __u8    ip_proto;
> > > > +     union {
> > > > +             struct {
> > > > +                     __be32  ipv4_src;
> > > > +                     __be32  ipv4_dst;
> > > > +             };
> > > > +             struct {
> > > > +                     __u32   ipv6_src[4];    /* in6_addr; network order */
> > > > +                     __u32   ipv6_dst[4];    /* in6_addr; network order */
> > > > +             };
> > > > +     };
> > > > +     __be16  sport;
> > > > +     __be16  dport;
> > > > +};
> > >
> > > can you please pack it?
> > > struct bpf_flow_keys {
> > >         __u16                      nhoff;                /*     0     2 */
> > >         __u16                      thoff;                /*     2     2 */
> > >         __u16                      addr_proto;           /*     4     2 */
> > >         __u8                       is_frag;              /*     6     1 */
> > >         __u8                       is_first_frag;        /*     7     1 */
> > >         __u8                       is_encap;             /*     8     1 */
> > >
> > >         /* XXX 1 byte hole, try to pack */
> > >
> > >         __be16                     n_proto;              /*    10     2 */
> > >         __u8                       ip_proto;             /*    12     1 */
> > >
> > >         /* XXX 3 bytes hole, try to pack */
> > >
> > >         union {
> > >
> > > also is_frag and other fields are not used by the kernel and
> > > only used by the prog to pass data between tail_calls ?
> >
> > No, these are mapped directly onto fields in struct flow_keys
> > on return from the BPF program in __skb_flow_bpf_to_target.
> > For is_frag, for instance:
> >
> >        if (flow_keys->is_frag)
> >                key_control->flags |= FLOW_DIS_IS_FRAGMENT;
>
> right. my search-fu failed me. only packing is needed then.
>
> > This is true for all fields in the struct except nhoff.
>
> > > In such case reserve some space in bpf_flow_keys similar to skb->cb
> > > so it can contain any fields and accommodate for inevitable changes
> > > to bpf flow dissector prog in the future.
> >
> > Do you mean a second scratch space akin to cb[], just a few
> > reserved padding bytes?
>
> it looks to me it's possible to rearrange the fields to avoid all holes,
> so no extra padding bytes necessary.

Absolutely. We forgot to run pahole earlier. Updating now.

> > We have given some thought to forward compatibility. The existing
> > fields cannot be changed, but it should be fine if we need to expand
> > the struct later.
>
> let's keep cb-like idea for later. It seems to me we can add it to
> the end of bpf_flow_keys any time later.