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

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Message-ID: <20180913205740.a3zqadwevavshopv@ast-mbp>
Date:   Thu, 13 Sep 2018 13:57:42 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...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 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.

> 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.