bpf: Sample bpf program to set initial window

The sample bpf program, tcp_rwnd_kern.c, sets the initial
advertized window to 40 packets in an environment where
distinct IPv6 prefixes indicate that both hosts are not
in the same data center.

Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 70 insertions, 0 deletions

diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index e29370a19957..ca955280cbeb 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -114,6 +114,7 @@ always += xdp_tx_iptunnel_kern.o
 always += test_map_in_map_kern.o
 always += cookie_uid_helper_example.o
 always += tcp_synrto_kern.o
+always += tcp_rwnd_kern.o
 
 HOSTCFLAGS += -I$(objtree)/usr/include
 HOSTCFLAGS += -I$(srctree)/tools/lib/
diff --git a/samples/bpf/tcp_rwnd_kern.c b/samples/bpf/tcp_rwnd_kern.c
new file mode 100644
index 000000000000..3f2a228f81ce
--- /dev/null
+++ b/samples/bpf/tcp_rwnd_kern.c
@@ -0,0 +1,69 @@
+/* Copyright (c) 2017 Facebook
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of version 2 of the GNU General Public
+ * License as published by the Free Software Foundation.
+ *
+ * BPF program to set initial receive window to 40 packets when using IPv6
+ * and the first 5.5 bytes of the IPv6 addresses are not the same (in this
+ * example that means both hosts are not the same datacenter).
+ *
+ * Use load_sock_ops to load this BPF program.
+ */
+
+#include <uapi/linux/bpf.h>
+#include <uapi/linux/if_ether.h>
+#include <uapi/linux/if_packet.h>
+#include <uapi/linux/ip.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_rwnd(struct bpf_sock_ops *skops)
+{
+        int rv = -1;
+        int op;
+
+        /* For testing purposes, only execute rest of BPF program
+         * if neither port numberis 55601
+         */
+        if (bpf_ntohl(skops->remote_port) !=
+            55601 && skops->local_port != 55601)
+                return -1;
+
+        op = (int) skops->op;
+
+#ifdef DEBUG
+        bpf_printk("BPF command: %d\n", op);
+#endif
+
+        /* Check for RWND_INIT operation and IPv6 addresses */
+        if (op == BPF_SOCK_OPS_RWND_INIT &&
+                skops->family == AF_INET6) {
+
+                /* If the first 5.5 bytes of the IPv6 address are not the same
+                 * then both hosts are not in the same datacenter
+                 * so use a larger initial advertized window (40 packets)
+                 */
+                if (skops->local_ip6[0] != skops->remote_ip6[0] ||
+                    (bpf_ntohl(skops->local_ip6[1]) & 0xfffff000) !=
+                    (bpf_ntohl(skops->remote_ip6[1]) & 0xfffff000))
+                        rv = 40;
+        }
+#ifdef DEBUG
+        bpf_printk("Returning %d\n", rv);
+#endif
+        skops->reply = rv;
+        return 1;
+}
+char _license[] SEC("license") = "GPL";