bpf: Introduce SK_LOOKUP program type with a dedicated attach point

Add a new program type BPF_PROG_TYPE_SK_LOOKUP with a dedicated attach type
BPF_SK_LOOKUP. The new program kind is to be invoked by the transport layer
when looking up a listening socket for a new connection request for
connection oriented protocols, or when looking up an unconnected socket for
a packet for connection-less protocols.

When called, SK_LOOKUP BPF program can select a socket that will receive
the packet. This serves as a mechanism to overcome the limits of what
bind() API allows to express. Two use-cases driving this work are:

 (1) steer packets destined to an IP range, on fixed port to a socket

     192.0.2.0/24, port 80 -> NGINX socket

 (2) steer packets destined to an IP address, on any port to a socket

     198.51.100.1, any port -> L7 proxy socket

In its run-time context program receives information about the packet that
triggered the socket lookup. Namely IP version, L4 protocol identifier, and
address 4-tuple. Context can be further extended to include ingress
interface identifier.

To select a socket BPF program fetches it from a map holding socket
references, like SOCKMAP or SOCKHASH, and calls bpf_sk_assign(ctx, sk, ...)
helper to record the selection. Transport layer then uses the selected
socket as a result of socket lookup.

In its basic form, SK_LOOKUP acts as a filter and hence must return either
SK_PASS or SK_DROP. If the program returns with SK_PASS, transport should
look for a socket to receive the packet, or use the one selected by the
program if available, while SK_DROP informs the transport layer that the
lookup should fail.

This patch only enables the user to attach an SK_LOOKUP program to a
network namespace. Subsequent patches hook it up to run on local delivery
path in ipv4 and ipv6 stacks.

Suggested-by: Marek Majkowski <marek@cloudflare.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200717103536.397595-3-jakub@cloudflare.com

1 files changed, 5 insertions, 0 deletions

diff --git a/kernel/bpf/net_namespace.c b/kernel/bpf/net_namespace.c
index e9c8e26ac8f2..38b368bccda2 100644
--- a/kernel/bpf/net_namespace.c
+++ b/kernel/bpf/net_namespace.c
@@ -373,6 +373,8 @@ static int netns_bpf_max_progs(enum netns_bpf_attach_type type)
 	switch (type) {
 	case NETNS_BPF_FLOW_DISSECTOR:
 		return 1;
+	case NETNS_BPF_SK_LOOKUP:
+		return 64;
 	default:
 		return 0;
 	}
@@ -403,6 +405,9 @@ static int netns_bpf_link_attach(struct net *net, struct bpf_link *link,
 	case NETNS_BPF_FLOW_DISSECTOR:
 		err = flow_dissector_bpf_prog_attach_check(net, link->prog);
 		break;
+	case NETNS_BPF_SK_LOOKUP:
+		err = 0; /* nothing to check */
+		break;
 	default:
 		err = -EINVAL;
 		break;