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

[Eddie Hao <eddieh@google.com>]

This adds the ability to read gso_segs from a BPF program. This is useful
for user space monitoring systems to get gso_segs statistics for each
arbitrary group of flows defined by the BPF program.

Signed-off-by: Eddie Hao <eddieh@google.com>
---
 include/uapi/linux/bpf.h | 1 +
 net/core/filter.c        | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index b7db3261c62d..282085445616 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2266,6 +2266,7 @@ struct __sk_buff {
 	/* ... here. */
 
 	__u32 data_meta;
+	unsigned short gso_segs;
 };
 
 struct bpf_tunnel_key {
diff --git a/net/core/filter.c b/net/core/filter.c
index b9ec916f4e3a..f17e249f3fbb 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5820,6 +5820,15 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
 				      bpf_target_off(struct sock_common,
 						     skc_num, 2, target_size));
 		break;
+
+	case offsetof(struct __sk_buff, gso_segs):
+		BUILD_BUG_ON(FIELD_SIZEOF(struct skb_shared_info, gso_segs) !=
+					  sizeof(unsigned short));
+		off = offsetof(struct sk_buff, end);
+		off += offsetof(struct skb_shared_info, gso_segs);
+		*insn++ = BPF_LDX_MEM(BPF_SIZEOF(unsigned short), si->dst_reg,
+						 si->src_reg, off);
+		break;
 	}
 
 	return insn - insn_buf;
-- 
2.18.0.203.gfac676dfb9-goog