bpf: allow BPF programs access skb_shared_info->gso_segs field

This adds the ability to read gso_segs from a BPF program.

v3: Use BPF_REG_AX instead of BPF_REG_TMP for the temporary register,
    as suggested by Martin.

v2: refined Eddie Hao patch to address Alexei feedback.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Eddie Hao <eddieh@google.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/uapi/linux/bpf.h

@@ -2540,6 +2540,7 @@ struct __sk_buff {
 	__bpf_md_ptr(struct bpf_flow_keys *, flow_keys);
 	__u64 tstamp;
 	__u32 wire_len;
+	__u32 gso_segs;
 };
 
 struct bpf_tunnel_key {

net/core/filter.c

@@ -6700,6 +6700,27 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
 							     target_size));
 		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, head),
+				      si->dst_reg, si->src_reg,
+				      offsetof(struct sk_buff, head));
+		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, end),
+				      BPF_REG_AX, si->src_reg,
+				      offsetof(struct sk_buff, end));
+		*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_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, wire_len):
 		BUILD_BUG_ON(FIELD_SIZEOF(struct qdisc_skb_cb, pkt_len) != 4);
 

tools/include/uapi/linux/bpf.h

@@ -2540,6 +2540,7 @@ struct __sk_buff {
 	__bpf_md_ptr(struct bpf_flow_keys *, flow_keys);
 	__u64 tstamp;
 	__u32 wire_len;
+	__u32 gso_segs;
 };
 
 struct bpf_tunnel_key {

tools/testing/selftests/bpf/test_verifier.c

@@ -5663,6 +5663,42 @@ static struct bpf_test tests[] = {
 		.result = ACCEPT,
 		.prog_type = BPF_PROG_TYPE_CGROUP_SKB,
 	},
+	{
+		"read gso_segs from CGROUP_SKB",
+		.insns = {
+			BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
+				    offsetof(struct __sk_buff, gso_segs)),
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+		},
+		.result = ACCEPT,
+		.prog_type = BPF_PROG_TYPE_CGROUP_SKB,
+	},
+	{
+		"write gso_segs from CGROUP_SKB",
+		.insns = {
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_STX_MEM(BPF_W, BPF_REG_1, BPF_REG_0,
+				    offsetof(struct __sk_buff, gso_segs)),
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+		},
+		.result = REJECT,
+		.result_unpriv = REJECT,
+		.errstr = "invalid bpf_context access off=164 size=4",
+		.prog_type = BPF_PROG_TYPE_CGROUP_SKB,
+	},
+	{
+		"read gso_segs from CLS",
+		.insns = {
+			BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
+				    offsetof(struct __sk_buff, gso_segs)),
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+		},
+		.result = ACCEPT,
+		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
+	},
 	{
 		"multiple registers share map_lookup_elem result",
 		.insns = {