[PATCH v3 bpf-next] bpf: allow BPF programs access skb_shared_info->gso_segs field

[Eric Dumazet <edumazet@google.com>]

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>
---
 include/uapi/linux/bpf.h                    |  1 +
 net/core/filter.c                           | 21 ++++++++++++
 tools/include/uapi/linux/bpf.h              |  1 +
 tools/testing/selftests/bpf/test_verifier.c | 36 +++++++++++++++++++++
 4 files changed, 59 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 91c43884f295f60a85268ddf0020bf8aa47f8329..2940a9854f6d8e493518ca894e0c9c630ae4ab7a 100644
--- a/include/uapi/linux/bpf.h
+++ b/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 {
diff --git a/net/core/filter.c b/net/core/filter.c
index 2b3b436ef5457bf44c99780d6dec0b5f403f005c..8e587dd1da200494b900bd14eec7f875f5453773 100644
--- a/net/core/filter.c
+++ b/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);
 
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 91c43884f295f60a85268ddf0020bf8aa47f8329..2940a9854f6d8e493518ca894e0c9c630ae4ab7a 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/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 {
diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
index 2fd90d4568926d13542783c870507d43a6d6bb64..2c46531044bdf9ec1e4fa47e2c94c9edb0ac3d08 100644
--- a/tools/testing/selftests/bpf/test_verifier.c
+++ b/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 = {
-- 
2.20.1.321.g9e740568ce-goog