text: /* Copyright (c) 2016 Sargun Dhillon <sargun@sargun.me>
   *
   * 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.
   */
  #include <linux/skbuff.h>
  #include <linux/netdevice.h>
  #include <uapi/linux/bpf.h>
  #include <linux/version.h>
  #include "bpf_helpers.h"
  #include "bpf_tracing.h"
  
  struct bpf_map_def SEC("maps") dnat_map = {
  	.type = BPF_MAP_TYPE_HASH,
  	.key_size = sizeof(struct sockaddr_in),
  	.value_size = sizeof(struct sockaddr_in),
  	.max_entries = 256,
  };
  
  /* kprobe is NOT a stable ABI
   * kernel functions can be removed, renamed or completely change semantics.
   * Number of arguments and their positions can change, etc.
   * In such case this bpf+kprobe example will no longer be meaningful
   *
   * This example sits on a syscall, and the syscall ABI is relatively stable
   * of course, across platforms, and over time, the ABI may change.
   */
  SEC("kprobe/sys_connect")
  int bpf_prog1(struct pt_regs *ctx)
  {
  	struct sockaddr_in new_addr, orig_addr = {};
  	struct sockaddr_in *mapped_addr;
  	void *sockaddr_arg = (void *)PT_REGS_PARM2(ctx);
  	int sockaddr_len = (int)PT_REGS_PARM3(ctx);
  
  	if (sockaddr_len > sizeof(orig_addr))
  		return 0;
  
  	if (bpf_probe_read_user(&orig_addr, sizeof(orig_addr), sockaddr_arg) != 0)
  		return 0;
  
  	mapped_addr = bpf_map_lookup_elem(&dnat_map, &orig_addr);
  	if (mapped_addr != NULL) {
  		memcpy(&new_addr, mapped_addr, sizeof(new_addr));
  		bpf_probe_write_user(sockaddr_arg, &new_addr,
  				     sizeof(new_addr));
  	}
  	return 0;
  }
  
  char _license[] SEC("license") = "GPL";
  u32 _version SEC("version") = LINUX_VERSION_CODE;
  