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/bpf_helpers.h>
12#include <bpf/bpf_tracing.h>
13#include <bpf/bpf_core_read.h>
14#include "trace_common.h"
15
16struct {
17	__uint(type, BPF_MAP_TYPE_HASH);
18	__type(key, struct sockaddr_in);
19	__type(value, struct sockaddr_in);
20	__uint(max_entries, 256);
21} dnat_map SEC(".maps");
22
23/* kprobe is NOT a stable ABI
24 * kernel functions can be removed, renamed or completely change semantics.
25 * Number of arguments and their positions can change, etc.
26 * In such case this bpf+kprobe example will no longer be meaningful
27 *
28 * This example sits on a syscall, and the syscall ABI is relatively stable
29 * of course, across platforms, and over time, the ABI may change.
30 */
31SEC("kprobe/" SYSCALL(sys_connect))
32int bpf_prog1(struct pt_regs *ctx)
33{
34	struct pt_regs *real_regs = (struct pt_regs *)PT_REGS_PARM1_CORE(ctx);
35	void *sockaddr_arg = (void *)PT_REGS_PARM2_CORE(real_regs);
36	int sockaddr_len = (int)PT_REGS_PARM3_CORE(real_regs);
37	struct sockaddr_in new_addr, orig_addr = {};
38	struct sockaddr_in *mapped_addr;
39
40	if (sockaddr_len > sizeof(orig_addr))
41		return 0;
42
43	if (bpf_probe_read_user(&orig_addr, sizeof(orig_addr), sockaddr_arg) != 0)
44		return 0;
45
46	mapped_addr = bpf_map_lookup_elem(&dnat_map, &orig_addr);
47	if (mapped_addr != NULL) {
48		memcpy(&new_addr, mapped_addr, sizeof(new_addr));
49		bpf_probe_write_user(sockaddr_arg, &new_addr,
50				     sizeof(new_addr));
51	}
52	return 0;
53}
54
55char _license[] SEC("license") = "GPL";
56u32 _version SEC("version") = LINUX_VERSION_CODE;