Re: [PATCH bpf-next 07/11] bpf: Add helper to retrieve socket in BPF

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Thu, Sep 13, 2018 at 01:55:01PM -0700, Joe Stringer wrote:
> On Thu, 13 Sep 2018 at 12:06, Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Wed, Sep 12, 2018 at 5:06 PM, Alexei Starovoitov
> > <alexei.starovoitov@gmail.com> wrote:
> > > On Tue, Sep 11, 2018 at 05:36:36PM -0700, Joe Stringer wrote:
> > >> This patch adds new BPF helper functions, bpf_sk_lookup_tcp() and
> > >> bpf_sk_lookup_udp() which allows BPF programs to find out if there is a
> > >> socket listening on this host, and returns a socket pointer which the
> > >> BPF program can then access to determine, for instance, whether to
> > >> forward or drop traffic. bpf_sk_lookup_xxx() may take a reference on the
> > >> socket, so when a BPF program makes use of this function, it must
> > >> subsequently pass the returned pointer into the newly added sk_release()
> > >> to return the reference.
> > >>
> > >> By way of example, the following pseudocode would filter inbound
> > >> connections at XDP if there is no corresponding service listening for
> > >> the traffic:
> > >>
> > >>   struct bpf_sock_tuple tuple;
> > >>   struct bpf_sock_ops *sk;
> > >>
> > >>   populate_tuple(ctx, &tuple); // Extract the 5tuple from the packet
> > >>   sk = bpf_sk_lookup_tcp(ctx, &tuple, sizeof tuple, netns, 0);
> > > ...
> > >> +struct bpf_sock_tuple {
> > >> +     union {
> > >> +             __be32 ipv6[4];
> > >> +             __be32 ipv4;
> > >> +     } saddr;
> > >> +     union {
> > >> +             __be32 ipv6[4];
> > >> +             __be32 ipv4;
> > >> +     } daddr;
> > >> +     __be16 sport;
> > >> +     __be16 dport;
> > >> +     __u8 family;
> > >> +};
> > >
> > > since we can pass ptr_to_packet into map lookup and other helpers now,
> > > can you move 'family' out of bpf_sock_tuple and combine with netns_id arg?
> > > then progs wouldn't need to copy bytes from the packet into tuple
> > > to do a lookup.
> 
> If I follow, you're proposing that users should be able to pass a
> pointer to the source address field of the L3 header, and assuming
> that the L3 header ends with saddr+daddr (no options/extheaders), and
> is immediately followed by the sport/dport then a packet pointer
> should work for performing socket lookup. Then it is up to the BPF
> program writer to ensure that this is the case, or otherwise fall back
> to populating a copy of the sock tuple on the stack.

yep.

> > have been thinking more about it.
> > since only ipv4 and ipv6 supported may be use size of bpf_sock_tuple
> > to infer family inside the helper, so it doesn't need to be passed explicitly?
> 
> Let me make sure I understand the proposal here.
> 
> The current structure and function prototypes are:
> 
> struct bpf_sock_tuple {
>       union {
>               __be32 ipv6[4];
>               __be32 ipv4;
>       } saddr;
>       union {
>               __be32 ipv6[4];
>               __be32 ipv4;
>       } daddr;
>       __be16 sport;
>       __be16 dport;
>       __u8 family;
> };
...
> You're proposing something like:
> 
> struct bpf_sock_tuple4 {
>       __be32 saddr;
>       __be32 daddr;
>       __be16 sport;
>       __be16 dport;
>       __u8 family;
> };
> 
> struct bpf_sock_tuple6 {
>       __be32 saddr[4];
>       __be32 daddr[4];
>       __be16 sport;
>       __be16 dport;
>       __u8 family;
> };

I think the split is unnecessary.
I'm proposing:
struct bpf_sock_tuple {
      union {
              __be32 ipv6[4];
              __be32 ipv4;
      } saddr;
      union {
              __be32 ipv6[4];
              __be32 ipv4;
      } daddr;
      __be16 sport;
      __be16 dport;
};

that points directly into the packet (when ipv4 options are not there)
and bpf_sk_lookup_tcp() uses 'size' argument to figure out ipv4/ipv6 family.