bpf: Add gso_size to __sk_buff

BPF programs may want to know whether an skb is gso. The canonical
answer is skb_is_gso(skb), which tests that gso_size != 0.

Expose this field in the same manner as gso_segs. That field itself
is not a sufficient signal, as the comment in skb_shared_info makes
clear: gso_segs may be zero, e.g., from dodgy sources.

Also prepare net/bpf/test_run for upcoming BPF_PROG_TEST_RUN tests
of the feature.
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200303200503.226217-2-willemdebruijn.kernel@gmail.com

include/uapi/linux/bpf.h

@@ -3176,6 +3176,7 @@ struct __sk_buff {
 	__u32 wire_len;
 	__u32 gso_segs;
 	__bpf_md_ptr(struct bpf_sock *, sk);
+	__u32 gso_size;
 };
 
 struct bpf_tunnel_key {

net/bpf/test_run.c

@@ -277,6 +277,12 @@ static int convert___skb_to_skb(struct sk_buff *skb, struct __sk_buff *__skb)
 	/* gso_segs is allowed */
 
 	if (!range_is_zero(__skb, offsetofend(struct __sk_buff, gso_segs),
+			   offsetof(struct __sk_buff, gso_size)))
+		return -EINVAL;
+
+	/* gso_size is allowed */
+
+	if (!range_is_zero(__skb, offsetofend(struct __sk_buff, gso_size),
 			   sizeof(struct __sk_buff)))
 		return -EINVAL;
 
@@ -297,6 +303,7 @@ static int convert___skb_to_skb(struct sk_buff *skb, struct __sk_buff *__skb)
 	if (__skb->gso_segs > GSO_MAX_SEGS)
 		return -EINVAL;
 	skb_shinfo(skb)->gso_segs = __skb->gso_segs;
+	skb_shinfo(skb)->gso_size = __skb->gso_size;
 
 	return 0;
 }

net/core/filter.c

@@ -7139,6 +7139,27 @@ static u32 flow_dissector_convert_ctx_access(enum bpf_access_type type,
 	return insn - insn_buf;
 }
 
+static struct bpf_insn *bpf_convert_shinfo_access(const struct bpf_insn *si,
+						  struct bpf_insn *insn)
+{
+	/* si->dst_reg = skb_shinfo(SKB); */
+#ifdef NET_SKBUFF_DATA_USES_OFFSET
+	*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, end),
+			      BPF_REG_AX, si->src_reg,
+			      offsetof(struct sk_buff, end));
+	*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, head),
+			      si->dst_reg, si->src_reg,
+			      offsetof(struct sk_buff, head));
+	*insn++ = BPF_ALU64_REG(BPF_ADD, si->dst_reg, BPF_REG_AX);
+#else
+	*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, end),
+			      si->dst_reg, si->src_reg,
+			      offsetof(struct sk_buff, end));
+#endif
+
+	return insn;
+}
+
 static u32 bpf_convert_ctx_access(enum bpf_access_type type,
 				  const struct bpf_insn *si,
 				  struct bpf_insn *insn_buf,
@@ -7461,26 +7482,21 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
 		break;
 
 	case offsetof(struct __sk_buff, gso_segs):
-		/* si->dst_reg = skb_shinfo(SKB); */
-#ifdef NET_SKBUFF_DATA_USES_OFFSET
-		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, end),
-				      BPF_REG_AX, si->src_reg,
-				      offsetof(struct sk_buff, end));
-		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, head),
-				      si->dst_reg, si->src_reg,
-				      offsetof(struct sk_buff, head));
-		*insn++ = BPF_ALU64_REG(BPF_ADD, si->dst_reg, BPF_REG_AX);
-#else
-		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, end),
-				      si->dst_reg, si->src_reg,
-				      offsetof(struct sk_buff, end));
-#endif
+		insn = bpf_convert_shinfo_access(si, insn);
 		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct skb_shared_info, gso_segs),
 				      si->dst_reg, si->dst_reg,
 				      bpf_target_off(struct skb_shared_info,
 						     gso_segs, 2,
 						     target_size));
 		break;
+	case offsetof(struct __sk_buff, gso_size):
+		insn = bpf_convert_shinfo_access(si, insn);
+		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct skb_shared_info, gso_size),
+				      si->dst_reg, si->dst_reg,
+				      bpf_target_off(struct skb_shared_info,
+						     gso_size, 2,
+						     target_size));
+		break;
 	case offsetof(struct __sk_buff, wire_len):
 		BUILD_BUG_ON(sizeof_field(struct qdisc_skb_cb, pkt_len) != 4);