1/* Copyright (c) 2016 Sargun Dhillon <sargun@sargun.me>
 2 *
 3 * This program is free software; you can redistribute it and/or
 4 * modify it under the terms of version 2 of the GNU General Public
 5 * License as published by the Free Software Foundation.
 6 */
 7#include <linux/skbuff.h>
 8#include <linux/netdevice.h>
 9#include <uapi/linux/bpf.h>
10#include <linux/version.h>
11#include "bpf_helpers.h"
12
13struct bpf_map_def SEC("maps") dnat_map = {
14	.type = BPF_MAP_TYPE_HASH,
15	.key_size = sizeof(struct sockaddr_in),
16	.value_size = sizeof(struct sockaddr_in),
17	.max_entries = 256,
18};
19
20/* kprobe is NOT a stable ABI
21 * kernel functions can be removed, renamed or completely change semantics.
22 * Number of arguments and their positions can change, etc.
23 * In such case this bpf+kprobe example will no longer be meaningful
24 *
25 * This example sits on a syscall, and the syscall ABI is relatively stable
26 * of course, across platforms, and over time, the ABI may change.
27 */
28SEC("kprobe/sys_connect")
29int bpf_prog1(struct pt_regs *ctx)
30{
31	struct sockaddr_in new_addr, orig_addr = {};
32	struct sockaddr_in *mapped_addr;
33	void *sockaddr_arg = (void *)PT_REGS_PARM2(ctx);
34	int sockaddr_len = (int)PT_REGS_PARM3(ctx);
35
36	if (sockaddr_len > sizeof(orig_addr))
37		return 0;
38
39	if (bpf_probe_read(&orig_addr, sizeof(orig_addr), sockaddr_arg) != 0)
40		return 0;
41
42	mapped_addr = bpf_map_lookup_elem(&dnat_map, &orig_addr);
43	if (mapped_addr != NULL) {
44		memcpy(&new_addr, mapped_addr, sizeof(new_addr));
45		bpf_probe_write_user(sockaddr_arg, &new_addr,
46				     sizeof(new_addr));
47	}
48	return 0;
49}
50
51char _license[] SEC("license") = "GPL";
52u32 _version SEC("version") = LINUX_VERSION_CODE;