[PATCH bpf-next 2/2] bpf/flow_dissector: Introduce BPF_FLOW_DISSECTOR_CONTINUE retcode for flow-dissector bpf progs

[Shmulik Ladkani <shmulik@metanetworks.com>]

Currently, attaching BPF_PROG_TYPE_FLOW_DISSECTOR programs completely
replaces the flow-dissector logic with custom dissection logic.
This forces implementors to write programs that handle dissection for
any flows expected in the namespace.

It makes sense for flow-dissector bpf programs to just augment the
dissector with custom logic (e.g. dissecting certain flows or custom
protocols), while enjoying the broad capabilities of the standard
dissector for any other traffic.

Introduce BPF_FLOW_DISSECTOR_CONTINUE retcode. Flow-dissector bpf
programs may return this to indicate no dissection was made, and
fallback to the standard dissector is requested.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
---
 include/uapi/linux/bpf.h  | 5 +++++
 net/core/flow_dissector.c | 3 +++
 2 files changed, 8 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 7bf9ba1329be..6d6654da7cef 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -5836,6 +5836,11 @@ enum bpf_ret_code {
 	 *    represented by BPF_REDIRECT above).
 	 */
 	BPF_LWT_REROUTE = 128,
+	/* BPF_FLOW_DISSECTOR_CONTINUE: used by BPF_PROG_TYPE_FLOW_DISSECTOR
+	 *   to indicate that no custom dissection was performed, and
+	 *   fallback to standard dissector is requested.
+	 */
+	BPF_FLOW_DISSECTOR_CONTINUE = 129,
 };
 
 struct bpf_sock {
diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index a01817fb4ef4..990429c69ccd 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -1022,11 +1022,14 @@ bool __skb_flow_dissect(const struct net *net,
 			prog = READ_ONCE(run_array->items[0].prog);
 			result = bpf_flow_dissect(prog, &ctx, n_proto, nhoff,
 						  hlen, flags);
+			if (result == BPF_FLOW_DISSECTOR_CONTINUE)
+				goto dissect_continue;
 			__skb_flow_bpf_to_target(&flow_keys, flow_dissector,
 						 target_container);
 			rcu_read_unlock();
 			return result == BPF_OK;
 		}
+dissect_continue:
 		rcu_read_unlock();
 	}
 
-- 
2.37.1