bpf: add napi_id read access to __sk_buff

Add napi_id access to __sk_buff for socket filter program types, tc
program types and other bpf_convert_ctx_access() users. Having access
to skb->napi_id is useful for per RX queue listener siloing, f.e.
in combination with SO_ATTACH_REUSEPORT_EBPF and when busy polling is
used, meaning SO_REUSEPORT enabled listeners can then select the
corresponding socket at SYN time already [1]. The skb is marked via
skb_mark_napi_id() early in the receive path (e.g., napi_gro_receive()).

Currently, sockets can only use SO_INCOMING_NAPI_ID from 6d4339028b35
("net: Introduce SO_INCOMING_NAPI_ID") as a socket option to look up
the NAPI ID associated with the queue for steering, which requires a
prior sk_mark_napi_id() after the socket was looked up.

Semantics for the __sk_buff napi_id access are similar, meaning if
skb->napi_id is < MIN_NAPI_ID (e.g. outgoing packets using sender_cpu),
then an invalid napi_id of 0 is returned to the program, otherwise a
valid non-zero napi_id.

  [1] http://netdevconf.org/2.1/slides/apr6/dumazet-BUSY-POLLING-Netdev-2.1.pdf

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/bpf.h

@@ -603,6 +603,7 @@ struct __sk_buff {
 	__u32 tc_classid;
 	__u32 data;
 	__u32 data_end;
+	__u32 napi_id;
 };
 
 struct bpf_tunnel_key {

net/core/filter.c

@@ -53,6 +53,7 @@
 #include <net/dst_metadata.h>
 #include <net/dst.h>
 #include <net/sock_reuseport.h>
+#include <net/busy_poll.h>
 
 /**
  *	sk_filter_trim_cap - run a packet through a socket filter
@@ -3201,6 +3202,19 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
 			*insn++ = BPF_MOV64_REG(si->dst_reg, si->dst_reg);
 		else
 			*insn++ = BPF_MOV64_IMM(si->dst_reg, 0);
+#endif
+		break;
+
+	case offsetof(struct __sk_buff, napi_id):
+#if defined(CONFIG_NET_RX_BUSY_POLL)
+		BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, napi_id) != 4);
+
+		*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->src_reg,
+				      offsetof(struct sk_buff, napi_id));
+		*insn++ = BPF_JMP_IMM(BPF_JGE, si->dst_reg, MIN_NAPI_ID, 1);
+		*insn++ = BPF_MOV64_IMM(si->dst_reg, 0);
+#else
+		*insn++ = BPF_MOV64_IMM(si->dst_reg, 0);
 #endif
 		break;
 	}

tools/include/uapi/linux/bpf.h

@@ -603,6 +603,7 @@ struct __sk_buff {
 	__u32 tc_classid;
 	__u32 data;
 	__u32 data_end;
+	__u32 napi_id;
 };
 
 struct bpf_tunnel_key {

tools/testing/selftests/bpf/test_verifier.c

@@ -772,6 +772,9 @@ static struct bpf_test tests[] = {
 			BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
 				    offsetof(struct __sk_buff, vlan_tci)),
 			BPF_JMP_IMM(BPF_JGE, BPF_REG_0, 0, 0),
+			BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
+				    offsetof(struct __sk_buff, napi_id)),
+			BPF_JMP_IMM(BPF_JGE, BPF_REG_0, 0, 0),
 			BPF_EXIT_INSN(),
 		},
 		.result = ACCEPT,