bpf: Add access to snd_cwnd and others in sock_ops

Adds read access to snd_cwnd and srtt_us fields of tcp_sock. Since these
fields are only valid if the socket associated with the sock_ops program
call is a full socket, the field is_fullsock is also added to the
bpf_sock_ops struct. If the socket is not a full socket, reading these
fields returns 0.

Note that in most cases it will not be necessary to check is_fullsock to
know if there is a full socket. The context of the call, as specified by
the 'op' field, can sometimes determine whether there is a full socket.

The struct bpf_sock_ops has the following fields added:

  __u32 is_fullsock;      /* Some TCP fields are only valid if
                           * there is a full socket. If not, the
                           * fields read as zero.
			   */
  __u32 snd_cwnd;
  __u32 srtt_us;          /* Averaged RTT << 3 in usecs */

There is a new macro, SOCK_OPS_GET_TCP32(NAME), to make it easier to add
read access to more 32 bit tcp_sock fields.
Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/filter.h

@@ -985,6 +985,7 @@ struct bpf_sock_ops_kern {
 		u32 reply;
 		u32 replylong[4];
 	};
+	u32	is_fullsock;
 };
 
 #endif /* __LINUX_FILTER_H__ */

include/net/tcp.h

@@ -2012,10 +2012,12 @@ static inline int tcp_call_bpf(struct sock *sk, int op)
 	struct bpf_sock_ops_kern sock_ops;
 	int ret;
 
-	if (sk_fullsock(sk))
+	memset(&sock_ops, 0, sizeof(sock_ops));
+	if (sk_fullsock(sk)) {
+		sock_ops.is_fullsock = 1;
 		sock_owned_by_me(sk);
+	}
 
-	memset(&sock_ops, 0, sizeof(sock_ops));
 	sock_ops.sk = sk;
 	sock_ops.op = op;
 

include/uapi/linux/bpf.h

@@ -941,6 +941,12 @@ struct bpf_sock_ops {
 	__u32 local_ip6[4];	/* Stored in network byte order */
 	__u32 remote_port;	/* Stored in network byte order */
 	__u32 local_port;	/* stored in host byte order */
+	__u32 is_fullsock;	/* Some TCP fields are only valid if
+				 * there is a full socket. If not, the
+				 * fields read as zero.
+				 */
+	__u32 snd_cwnd;
+	__u32 srtt_us;		/* Averaged RTT << 3 in usecs */
 };
 
 /* List of known BPF sock_ops operators.

net/core/filter.c

@@ -4437,6 +4437,42 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
 		*insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->dst_reg,
 				      offsetof(struct sock_common, skc_num));
 		break;
+
+	case offsetof(struct bpf_sock_ops, is_fullsock):
+		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(
+						struct bpf_sock_ops_kern,
+						is_fullsock),
+				      si->dst_reg, si->src_reg,
+				      offsetof(struct bpf_sock_ops_kern,
+					       is_fullsock));
+		break;
+
+/* Helper macro for adding read access to tcp_sock fields. */
+#define SOCK_OPS_GET_TCP32(FIELD_NAME)					      \
+	do {								      \
+		BUILD_BUG_ON(FIELD_SIZEOF(struct tcp_sock, FIELD_NAME) != 4); \
+		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(			      \
+						struct bpf_sock_ops_kern,     \
+						is_fullsock),		      \
+				      si->dst_reg, si->src_reg,		      \
+				      offsetof(struct bpf_sock_ops_kern,      \
+					       is_fullsock));		      \
+		*insn++ = BPF_JMP_IMM(BPF_JEQ, si->dst_reg, 0, 2);	      \
+		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(			      \
+						struct bpf_sock_ops_kern, sk),\
+				      si->dst_reg, si->src_reg,		      \
+				      offsetof(struct bpf_sock_ops_kern, sk));\
+		*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->dst_reg,        \
+				      offsetof(struct tcp_sock, FIELD_NAME)); \
+	} while (0)
+
+	case offsetof(struct bpf_sock_ops, snd_cwnd):
+		SOCK_OPS_GET_TCP32(snd_cwnd);
+		break;
+
+	case offsetof(struct bpf_sock_ops, srtt_us):
+		SOCK_OPS_GET_TCP32(srtt_us);
+		break;
 	}
 	return insn - insn_buf;
 }