CTF-All-In-One/doc/4.3_gcc_arg.md
2018-04-26 21:27:37 +08:00

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# 4.3 GCC 编译参数解析
- [GCC](#gcc)
- [常用选择](#常用选项)
- [Address sanitizer](#address-sanitizer)
- [mcheck](#mcheck)
- [参考资料](#参考资料)
## GCC
```
$ wget -c http://www.mirrorservice.org/sites/sourceware.org/pub/gcc/releases/gcc-4.4.0/gcc-4.4.0.tar.bz2
$ tar -xjvf gcc-4.4.0.tar.bz2
$ ./configure
$ make && sudo make install
```
## 常用选项
使用 `gcc -v` 可以查看默认开启的选项:
```
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 5.4.0-6ubuntu1~16.04.9' --with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-5 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.9)
```
#### 控制标准版本的编译选项
- `-ansi`:告诉编译器遵守 C 语言的 ISO C90 标准。
- `-std=`:通过使用一个参数来设置需要的标准。
- `c89`:支持 C89 标准。
- `iso9899:1999`:支持 ISO C90 标准。
- `gnu89`:支持 C89 标准。
#### 控制标准版本的常量
这些常量(#define可以通过编译器的命令行选项来设置或者通过源代码总的 `#define` 语句来定义。
- `__STRICT_ANSI__`:强制使用 C 语言的 ISO 标准。这个常量通过命令行选项 `-ansi` 来定义。
- `_POSIX_C_SOURCE=2`:启用由 IEEE Std1003.1 和 1003.2 标准定义的特性。
- `_BSD_SOURCE`:启用 BSD 类型的特性。
- `_GNU_SOURCE`:启用大量特性,其中包括 GNU 扩展。
#### 编译器的警告选项
- `-pedantic`:除了启用用于检查代码是否遵守 C 语言标准的选项外,还关闭了一些不被标准允许的传统 C 语言结构,并且禁用所有的 GNU 扩展。
- `-Wformat`:检查 printf 系列函数所使用的参数类型是否正确。
- `Wparentheses`:检查是否总是提供了需要的圆括号。当想要检查一个复杂结构的初始化是否按照预期进行时,这个选项就很有用。
- `Wswitch-default`:检查是否所有的 switch 语句都包含一个 default case。
- `Wunused`:检查诸如声明静态函数但没有定义、未使用的参数和丢弃返回结果等情况。
- `Wall`:启用绝大多数 gcc 的警告选项,包括所有以 -W 为前缀的选项。
## Address sanitizer
Address sanitizer 是一种用于检测内存错误的技术GCC 从 4.8 版本开始支持了这一技术。ASan 在编译时插入额外指令到内存访问操作中,同时通过 Shadow memory 来记录和检测内存的有效性。ASan 其实只是 Sanitizer 一系列工具中的一员,其他工具比如 memory leak 检测在 LeakSanitizer 中uninitialized memory read 检测在 MemorySanitizer 中等等。
举个例子,很明显下面这个程序存在栈溢出:
```C
#include<stdio.h>
void main() {
int a[10] = {0};
int b = a[11];
}
```
编译时加上参数 `-fsanitize=address`,如果使用 Makefile则将参数加入到 CFLAGS 中:
```
$ gcc -fsanitize=address santest.c
```
然后运行:
```
$ ./a.out
=================================================================
==9399==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffc03f4d64c at pc 0x565515082ad6 bp 0x7ffc03f4d5e0 sp 0x7ffc03f4d5d0
READ of size 4 at 0x7ffc03f4d64c thread T0
#0 0x565515082ad5 in main (/home/firmy/a.out+0xad5)
#1 0x7fb4c04c0f69 in __libc_start_main (/usr/lib/libc.so.6+0x20f69)
#2 0x565515082899 in _start (/home/firmy/a.out+0x899)
Address 0x7ffc03f4d64c is located in stack of thread T0 at offset 76 in frame
#0 0x565515082989 in main (/home/firmy/a.out+0x989)
This frame has 1 object(s):
[32, 72) 'a' <== Memory access at offset 76 overflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
(longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow (/home/firmy/a.out+0xad5) in main
Shadow bytes around the buggy address:
0x1000007e1a70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x1000007e1a80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x1000007e1a90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x1000007e1aa0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x1000007e1ab0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x1000007e1ac0: f1 f1 f1 f1 00 00 00 00 00[f2]f2 f2 00 00 00 00
0x1000007e1ad0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x1000007e1ae0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x1000007e1af0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x1000007e1b00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x1000007e1b10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Freed heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
Container overflow: fc
Array cookie: ac
Intra object redzone: bb
ASan internal: fe
Left alloca redzone: ca
Right alloca redzone: cb
==9399==ABORTING
```
确实检测出了问题。在实战篇中,为了更好地分析软件漏洞,我们可能会经常用到这个选项。
参考https://en.wikipedia.org/wiki/AddressSanitizer
## mcheck
利用 mcheck 可以实现堆内存的一致性状态检查。其定义在 `/usr/include/mcheck.h`,是一个 GNU 扩展函数,原型如下:
```c
#include <mcheck.h>
int mcheck(void (*abortfunc)(enum mcheck_status mstatus));
```
可以看到参数是一个函数指针,但检查到堆内存异常时,通过该指针调用 abortfunc 函数,同时传入一个 mcheck_status 类型的参数。
举个例子,下面的程序存在 double-free 的问题:
```c
#include <stdlib.h>
#include <stdio.h>
void main() {
char *p;
p = malloc(1000);
fprintf(stderr, "About to free\n");
free(p);
fprintf(stderr, "About to free a second time\n");
free(p);
fprintf(stderr, "Finish\n");
}
```
通过设置参数 `-lmcheck` 来链接 mcheck 函数:
```
$ gcc -lmcheck mcheck.c
$ ./a.out
About to free
About to free a second time
block freed twice
Aborted (core dumped)
```
还可以通过设置环境变量 `MALLOC_CHECK_` 来实现,这样就不需要重新编译程序。
```
$ gcc mcheck.c
$ #检查到错误时不作任何提示
$ MALLOC_CHECK_=0 ./a.out
About to free
About to free a second time
Finish
$ #检查到错误时打印一条信息到标准输出
$ MALLOC_CHECK_=1 ./a.out
About to free
About to free a second time
*** Error in `./a.out': free(): invalid pointer: 0x0000000001fb9010 ***
Finish
$ #检查到错误时直接中止程序
$ MALLOC_CHECK_=2 ./a.out
About to free
About to free a second time
Aborted (core dumped)
```
具体参考 `man 3 mcheck``man 3 mallopt`
## 参考资料
- [GCC online documentation](https://gcc.gnu.org/onlinedocs/)