[PATCH bpf-next 1/2] bpf: add csum/ip_summed fields to __sk_buff

[Menglong Dong]

For now, we have to call some helpers when we need to update the csum,
such as bpf_l4_csum_replace, bpf_l3_csum_replace, etc. These helpers are
not inlined, which causes poor performance.

In fact, we can define our own csum update functions in BPF program
instead of bpf_l3_csum_replace, which is totally inlined and efficient.
However, we can't do this for bpf_l4_csum_replace for now, as we can't
update skb->csum, which can make skb->csum invalid in the rx path with
CHECKSUM_COMPLETE mode.

What's more, we can't use the direct data access and have to use
skb_store_bytes() with the BPF_F_RECOMPUTE_CSUM flag in some case, such
as modifing the vni in the vxlan header and the underlay udp header has
no checksum.

With the read/write accessing to skb->csum and read accessing to
skb->ip_summed, now we can define the inlined csum update functions in
libbpf, which are much more efficient.

Signed-off-by: Menglong Dong <menglong8.dong@xxxxxxxxx>
---
 include/linux/skbuff.h         |  2 ++
 include/uapi/linux/bpf.h       |  2 ++
 net/core/filter.c              | 22 ++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h |  2 ++
 4 files changed, 28 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index ea5c8ab3ed00..a0ec404c7009 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1077,8 +1077,10 @@ struct sk_buff {
 /* if you move pkt_type around you also must adapt those constants */
 #ifdef __BIG_ENDIAN_BITFIELD
 #define PKT_TYPE_MAX	(7 << 5)
+#define IP_SUMMED_RSH	1
 #else
 #define PKT_TYPE_MAX	7
+#define IP_SUMMED_RSH	5
 #endif
 #define PKT_TYPE_OFFSET		offsetof(struct sk_buff, __pkt_type_offset)
 
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 754e68ca8744..b450e27f5a8d 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -6148,6 +6148,8 @@ struct __sk_buff {
 	__u8  tstamp_type;
 	__u32 :24;		/* Padding, future use. */
 	__u64 hwtstamp;
+	__u32 csum;
+	__u32 ip_summed;
 };
 
 struct bpf_tunnel_key {
diff --git a/net/core/filter.c b/net/core/filter.c
index 24061f29c9dd..23c22d88da1b 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -8858,6 +8858,7 @@ static bool tc_cls_act_is_valid_access(int off, int size,
 		case bpf_ctx_range_till(struct __sk_buff, cb[0], cb[4]):
 		case bpf_ctx_range(struct __sk_buff, tstamp):
 		case bpf_ctx_range(struct __sk_buff, queue_mapping):
+		case bpf_ctx_range(struct __sk_buff, csum):
 			break;
 		default:
 			return false;
@@ -8885,6 +8886,8 @@ static bool tc_cls_act_is_valid_access(int off, int size,
 		 */
 		((struct bpf_prog *)prog)->tstamp_type_access = 1;
 		return size == sizeof(__u8);
+	case bpf_ctx_range_till(struct __sk_buff, csum, ip_summed):
+		return size == sizeof(__u32);
 	}
 
 	return bpf_skb_is_valid_access(off, size, type, prog, info);
@@ -9513,6 +9516,25 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
 #endif
 		break;
 
+	case offsetof(struct __sk_buff, ip_summed):
+		*target_size = 1;
+		*insn++ = BPF_LDX_MEM(BPF_B, si->dst_reg, si->src_reg,
+				      PKT_TYPE_OFFSET);
+		*insn++ = BPF_ALU32_IMM(BPF_RSH, si->dst_reg, IP_SUMMED_RSH);
+		*insn++ = BPF_ALU32_IMM(BPF_AND, si->dst_reg, 3);
+		break;
+
+	case offsetof(struct __sk_buff, csum):
+		if (type == BPF_WRITE)
+			*insn++ = BPF_EMIT_STORE(BPF_W, si,
+						 bpf_target_off(struct sk_buff, csum, 4,
+								target_size));
+		else
+			*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->src_reg,
+					      bpf_target_off(struct sk_buff, csum, 4,
+							     target_size));
+		break;
+
 	case offsetof(struct __sk_buff, queue_mapping):
 		if (type == BPF_WRITE) {
 			u32 off = bpf_target_off(struct sk_buff, queue_mapping, 2, target_size);
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 7f24d898efbb..31fd5ee40864 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -6148,6 +6148,8 @@ struct __sk_buff {
 	__u8  tstamp_type;
 	__u32 :24;		/* Padding, future use. */
 	__u64 hwtstamp;
+	__u32 csum;
+	__u32 ip_summed;
 };
 
 struct bpf_tunnel_key {
-- 
2.39.2