CTF-All-In-One/doc/5.11_retdec.md
2017-12-15 18:35:06 +08:00

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# 5.11 RetDec 反编译器
- [RetDec 简介](#retdec-简介)
- [安装](#安装)
- [使用方法](#使用方法)
- [r2pipe decompiler](#r2pipe-decompiler)
- [参考资料](#参考资料)
前面介绍过 IDA Pro其 F5 已经具有巨强大的反编译能力了,但这本书一直到现在,由于本人的某种执念,都是在硬怼汇编代码,没有用到 IDA虽说这样能锻炼到我们的汇编能力但也可以说是无故加大了逆向的难度。但现在事情出现了转机安全公司 Avast 开源了它的反编译器 RetDec能力虽不及 IDA目前也只支持 32 位,但好歹有了第一步,未来会好起来的。
## RetDec 简介
[RetDec](https://retdec.com/) 是一个可重定向的机器码反编译器,它基于 LLVM支持各种体系结构、操作系统和文件格式
- 支持的文件格式ELFPEMach-OCOFFAR存档Intel HEX 和原始机器码。
- 支持的体系结构(仅限 32 位Intel x86ARMMIPSPIC32 和 PowerPC。
## 安装
在 Linux 上,你需要自己构建和安装。
安装依赖:
```
$ sudo apt-get install build-essential cmake coreutils wget bc graphviz upx flex bison zlib1g-dev libtinfo-dev autoconf pkg-config m4 libtool
```
把项目连同子模块一起拉下来:
```
$ git clone --recursive https://github.com/avast-tl/retdec
```
接下来要注意了,由于项目自己的问题,在运行 cmake 的时候一定指定一个干净的目录,不要在默认的 `/usr` 或者 `/usr/local` 里,可以像下面这样:
```
$ cd retdec
$ mkdir build && cd build
$ cmake .. -DCMAKE_INSTALL_PREFIX=/usr/local/retdec
$ make && sudo make install
```
## 入门
安装完成后,我们用 helloword 大法试一下,注意将其编译成 32 位:
```C
#include <stdio.h>
int main() {
printf("hello world!\n");
return 0;
}
```
运行 decompile.sh 反编译它,我们截取出部分重要的过程和输出:
```sh
$ /usr/local/retdec/bin/decompile.sh a.out
##### Checking if file is a Mach-O Universal static library...
RUN: /usr/local/retdec/bin/macho-extractor --list /home/firmy/test/a.out
##### Checking if file is an archive...
RUN: /usr/local/retdec/bin/ar-extractor --arch-magic /home/firmy/test/a.out
##### Gathering file information...
RUN: /usr/local/retdec/bin/fileinfo -c /home/firmy/test/a.out.c.json --similarity /home/firmy/test/a.out --no-hashes=all --crypto /usr/local/retdec/bin/../share/generic/yara_patterns/signsrch/signsrch.yara
##### Trying to unpack /home/firmy/test/a.out into /home/firmy/test/a.out-unpacked.tmp by using generic unpacker...
RUN: /usr/local/retdec/bin/unpacker -d /usr/local/retdec/bin/unpacker-plugins -o /home/firmy/test/a.out-unpacked.tmp /home/firmy/test/a.out
##### Trying to unpack /home/firmy/test/a.out into /home/firmy/test/a.out-unpacked.tmp by using UPX...
RUN: upx -d /home/firmy/test/a.out -o /home/firmy/test/a.out-unpacked.tmp
##### Decompiling /home/firmy/test/a.out into /home/firmy/test/a.out.c.backend.bc...
RUN: /usr/local/retdec/bin/bin2llvmir -provider-init -config-path /home/firmy/test/a.out.c.json -decoder -disable-inlining -disable-simplify-libcalls -inst-opt -verify -volatilize -instcombine -reassociate -volatilize -control-flow -cfg-fnc-detect -main-detection -register -stack -control-flow -cond-branch-opt -syscalls -idioms-libgcc -constants -param-return -local-vars -type-conversions -simple-types -generate-dsm -remove-asm-instrs -select-fncs -unreachable-funcs -type-conversions -stack-protect -verify -instcombine -tbaa -targetlibinfo -basicaa -domtree -simplifycfg -domtree -early-cse -lower-expect -targetlibinfo -tbaa -basicaa -globalopt -mem2reg -instcombine -simplifycfg -basiccg -domtree -early-cse -lazy-value-info -jump-threading -correlated-propagation -simplifycfg -instcombine -simplifycfg -reassociate -domtree -loops -loop-simplify -lcssa -loop-rotate -licm -lcssa -instcombine -scalar-evolution -loop-simplifycfg -loop-simplify -aa -loop-accesses -loop-load-elim -lcssa -indvars -loop-idiom -loop-deletion -memdep -gvn -memdep -sccp -instcombine -lazy-value-info -jump-threading -correlated-propagation -domtree -memdep -dse -dce -bdce -adce -die -simplifycfg -instcombine -strip-dead-prototypes -globaldce -constmerge -constprop -instnamer -domtree -instcombine -never-returning-funcs -adapter-methods -class-hierarchy -instcombine -tbaa -targetlibinfo -basicaa -domtree -simplifycfg -domtree -early-cse -lower-expect -targetlibinfo -tbaa -basicaa -globalopt -mem2reg -instcombine -simplifycfg -basiccg -domtree -early-cse -lazy-value-info -jump-threading -correlated-propagation -simplifycfg -instcombine -simplifycfg -reassociate -domtree -loops -loop-simplify -lcssa -loop-rotate -licm -lcssa -instcombine -scalar-evolution -loop-simplifycfg -loop-simplify -aa -loop-accesses -loop-load-elim -lcssa -indvars -loop-idiom -loop-deletion -memdep -gvn -memdep -sccp -instcombine -lazy-value-info -jump-threading -correlated-propagation -domtree -memdep -dse -dce -bdce -adce -die -simplifycfg -instcombine -strip-dead-prototypes -globaldce -constmerge -constprop -instnamer -domtree -instcombine -simple-types -stack-ptr-op-remove -type-conversions -idioms -instcombine -global-to-local -dead-global-assign -instcombine -stack-protect -phi2seq -o /home/firmy/test/a.out.c.backend.bc
##### Decompiling /home/firmy/test/a.out.c.backend.bc into /home/firmy/test/a.out.c...
RUN: /usr/local/retdec/bin/llvmir2hll -target-hll=c -var-renamer=readable -var-name-gen=fruit -var-name-gen-prefix= -call-info-obtainer=optim -arithm-expr-evaluator=c -validate-module -llvmir2bir-converter=orig -o /home/firmy/test/a.out.c /home/firmy/test/a.out.c.backend.bc -enable-debug -emit-debug-comments -config-path=/home/firmy/test/a.out.c.json
##### Done!
```
总共输出下面几个文件:
```
$ ls
a.out a.out.c a.out.c.backend.bc a.out.c.backend.ll a.out.c.frontend.dsm a.out.c.json
```
可以看到 RetDec 可以分为三个阶段:
- 预处理阶段:首先检查文件类型是否为可执行文件,然后调用 `fileinfo` 获取文件信息生成 `a.out.c.json`,然后调用 `unpacker` 查壳和脱壳等操作
- 核心阶段:接下来才是重头戏,调用 `bin2llvmir` 将二进制文件转换成 LLVM IR并输出 `a.out.c.frontend.dsm`、`a.out.c.backend.ll` 和 `a.out.c.backend.bc`
- 后端阶段:这个阶段通过一系列代码优化和生成等操作,将 LLVM IR 反编译成 C 代码 `a.out.c`,还有 CFG 等。
`decompile.sh` 有很多选项,使用 `decompile.sh -h` 查看。
比如反编译指定函数:
```
$ /usr/local/retdec/bin/decompile.sh --select-functions main a.out
```
反编译指定的一段地址:
```
$ /usr/local/retdec/bin/decompile.sh --select-ranges 0x51d-0x558 a.out
```
生成函数 CFG 图(.dot格式
```
$ /usr/local/retdec/bin/decompile.sh --backend-emit-cfg a.out
```
## r2pipe decompiler
radare2 通过 r2pipe 脚本,利用 retdec.com 的 REST API 提供了反编译的功能,所以你首先要到网站上注册,拿到免费的 API key。
安装上该模块,当然你可能需要先安装上 npm它是 JavaScript 的包管理器:
```
$ git clone https://github.com/jpenalbae/r2-scripts.git
$ cd r2-scripts/decompiler/
$ npm install
```
将 API key 写入到 `~/.config/radare2/retdec.key` 中,然后就可以开心地反编译了。
还是 helloworld 的例子,用 r2 打开,反编译 main 函数。
```c
[0x000003e0]> #!pipe node /home/firmy/r2-scripts/decompiler/decompile.js @ main
Start: 0x51d
End: 0x558
Uploading binary to retdec.com
Please wait for decompilation to finish....
//
// This file was generated by the Retargetable Decompiler
// Website: https://retdec.com
// Copyright (c) 2017 Retargetable Decompiler <info@retdec.com>
//
#include <stdint.h>
#include <stdio.h>
// ------------------------ Functions -------------------------
// Address range: 0x51d - 0x558
int main() {
int32_t v1;
int32_t v2 = __x86_get_pc_thunk_ax((int32_t)&v1, 0);
puts((char *)(v2 + 175));
return 0;
}
// --------------- Dynamically Linked Functions ---------------
// int puts(const char * s);
// --------------------- Meta-Information ---------------------
// Detected compiler/packer: gcc (7.2.0)
// Detected functions: 1
// Decompiler release: v2.2.1 (2016-09-07)
// Decompilation date: 2017-12-15 07:48:04
```
每次输入反编译器路径是不是有点烦,在文件 `~/.config/radare2/radare2rc` 里配置一下 alias 就好了,用 `$decompile` 替代:
```sh
# Alias
$decompile=#!pipe node /home/user/r2-scripts/decompiler/decompile.js
```
```
[0x000003e0]> $decompile -h
Usage: $decompile [-acChps] [-n naming] @ addr
-a: disable selective decompilation (decompile the hole file)
-c: clear comments
-C: save decompilation results in r2 as a comment
-p: produce python code instead of C
-s: silent. Do not display messages
-h: displays this help menu
-n naming: select variable naming
Where valid variable namings are:
readable: Tries to produce as meaningful variable names as possible
address: Variables are named by their addresses in the binary file
hungarian: Prefix variables with their type
simple: Name variables simply by assigning fruit names
unified: Globals, locals and parameters are named just gX, vX and aX
**********************************************************************
This will upload the binary being analyzed to retdec.com !!!
You have been warned...
**********************************************************************
```
## 参考资料
- [retdec github](https://github.com/avast-tl/retdec)
- [RetDec: An Open-Source Machine-Code Decompiler](https://retdec.com/web/files/publications/retdec-slides-botconf-2017.pdf)
- [radare r2pipe decompiler](https://github.com/jpenalbae/r2-scripts/tree/master/decompiler)