bpf: add TCP_SAVE_SYN/TCP_SAVED_SYN sample program

Sample program which shows TCP_SAVE_SYN/TCP_SAVED_SYN usage example:
bpf program which is doing TOS/TCLASS reflection (server would reply
with a same TOS/TCLASS as client).
Signed-off-by: Nikita V. Shirokov <tehnerd@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

samples/bpf/Makefile

@@ -153,6 +153,7 @@ always += tcp_cong_kern.o
 always += tcp_iw_kern.o
 always += tcp_clamp_kern.o
 always += tcp_basertt_kern.o
+always += tcp_tos_reflect_kern.o
 always += xdp_redirect_kern.o
 always += xdp_redirect_map_kern.o
 always += xdp_redirect_cpu_kern.o

samples/bpf/tcp_tos_reflect_kern.c

+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2018 Facebook
+ *
+ * BPF program to automatically reflect TOS option from received syn packet
+ *
+ * Use load_sock_ops to load this BPF program.
+ */
+
+#include <uapi/linux/bpf.h>
+#include <uapi/linux/tcp.h>
+#include <uapi/linux/if_ether.h>
+#include <uapi/linux/if_packet.h>
+#include <uapi/linux/ip.h>
+#include <uapi/linux/ipv6.h>
+#include <uapi/linux/in.h>
+#include <linux/socket.h>
+#include "bpf_helpers.h"
+#include "bpf_endian.h"
+
+#define DEBUG 1
+
+#define bpf_printk(fmt, ...)					\
+({								\
+	       char ____fmt[] = fmt;				\
+	       bpf_trace_printk(____fmt, sizeof(____fmt),	\
+				##__VA_ARGS__);			\
+})
+
+SEC("sockops")
+int bpf_basertt(struct bpf_sock_ops *skops)
+{
+	char header[sizeof(struct ipv6hdr)];
+	struct ipv6hdr *hdr6;
+	struct iphdr *hdr;
+	int hdr_size = 0;
+	int save_syn = 1;
+	int tos = 0;
+	int rv = 0;
+	int op;
+
+	op = (int) skops->op;
+
+#ifdef DEBUG
+	bpf_printk("BPF command: %d\n", op);
+#endif
+	switch (op) {
+	case BPF_SOCK_OPS_TCP_LISTEN_CB:
+		rv = bpf_setsockopt(skops, SOL_TCP, TCP_SAVE_SYN,
+				   &save_syn, sizeof(save_syn));
+		break;
+	case BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB:
+		if (skops->family == AF_INET)
+			hdr_size = sizeof(struct iphdr);
+		else
+			hdr_size = sizeof(struct ipv6hdr);
+		rv = bpf_getsockopt(skops, SOL_TCP, TCP_SAVED_SYN,
+				    header, hdr_size);
+		if (!rv) {
+			if (skops->family == AF_INET) {
+				hdr = (struct iphdr *) header;
+				tos = hdr->tos;
+				if (tos != 0)
+					bpf_setsockopt(skops, SOL_IP, IP_TOS,
+						       &tos, sizeof(tos));
+			} else {
+				hdr6 = (struct ipv6hdr *) header;
+				tos = ((hdr6->priority) << 4 |
+				       (hdr6->flow_lbl[0]) >>  4);
+				if (tos)
+					bpf_setsockopt(skops, SOL_IPV6,
+						       IPV6_TCLASS,
+						       &tos, sizeof(tos));
+			}
+			rv = 0;
+		}
+		break;
+	default:
+		rv = -1;
+	}
+#ifdef DEBUG
+	bpf_printk("Returning %d\n", rv);
+#endif
+	skops->reply = rv;
+	return 1;
+}
+char _license[] SEC("license") = "GPL";