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163 lines
6.4 KiB
Markdown
163 lines
6.4 KiB
Markdown
# 4.3 GCC 编译参数解析
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- [常用选择](#常用选项)
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- [Address sanitizer](#address-sanitizer)
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- [mcheck](#mcheck)
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- [参考资料](#参考资料)
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## 常用选项
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#### 控制标准版本的编译选项
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- `-ansi`：告诉编译器遵守 C 语言的 ISO C90 标准。
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- `-std=`：通过使用一个参数来设置需要的标准。
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- `c89`：支持 C89 标准。
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- `iso9899:1999`：支持 ISO C90 标准。
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- `gnu89`：支持 C89 标准。
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#### 控制标准版本的常量
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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。
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- `__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。
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- `_POSIX_C_SOURCE=2`：启用由 IEEE Std1003.1 和 1003.2 标准定义的特性。
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- `_BSD_SOURCE`：启用 BSD 类型的特性。
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- `_GNU_SOURCE`：启用大量特性，其中包括 GNU 扩展。
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#### 编译器的警告选项
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- `-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。
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- `-Wformat`：检查 printf 系列函数所使用的参数类型是否正确。
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- `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。
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- `Wswitch-default`：检查是否所有的 switch 语句都包含一个 default case。
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- `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。
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- `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。
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## Address sanitizer
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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。
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举个例子，很明显下面这个程序存在栈溢出：
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```C
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#include<stdio.h>
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void main() {
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int a[10] = {0};
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int b = a[11];
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}
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```
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编译时加上参数 `-fsanitize=address`，如果使用 Makefile，则将参数加入到 CFLAGS 中：
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```
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$ gcc -fsanitize=address santest.c
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```
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然后运行：
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```
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$ ./a.out
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=================================================================
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==9399==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffc03f4d64c at pc 0x565515082ad6 bp 0x7ffc03f4d5e0 sp 0x7ffc03f4d5d0
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READ of size 4 at 0x7ffc03f4d64c thread T0
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#0 0x565515082ad5 in main (/home/firmy/a.out+0xad5)
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#1 0x7fb4c04c0f69 in __libc_start_main (/usr/lib/libc.so.6+0x20f69)
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#2 0x565515082899 in _start (/home/firmy/a.out+0x899)
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Address 0x7ffc03f4d64c is located in stack of thread T0 at offset 76 in frame
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#0 0x565515082989 in main (/home/firmy/a.out+0x989)
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This frame has 1 object(s):
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[32, 72) 'a' <== Memory access at offset 76 overflows this variable
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HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
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(longjmp and C++ exceptions *are* supported)
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SUMMARY: AddressSanitizer: stack-buffer-overflow (/home/firmy/a.out+0xad5) in main
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Shadow bytes around the buggy address:
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0x1000007e1a70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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0x1000007e1a80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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0x1000007e1a90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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0x1000007e1aa0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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0x1000007e1ab0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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=>0x1000007e1ac0: f1 f1 f1 f1 00 00 00 00 00[f2]f2 f2 00 00 00 00
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0x1000007e1ad0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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0x1000007e1ae0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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0x1000007e1af0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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0x1000007e1b00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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0x1000007e1b10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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Shadow byte legend (one shadow byte represents 8 application bytes):
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Addressable: 00
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Partially addressable: 01 02 03 04 05 06 07
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Heap left redzone: fa
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Freed heap region: fd
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Stack left redzone: f1
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Stack mid redzone: f2
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Stack right redzone: f3
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Stack after return: f5
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Stack use after scope: f8
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Global redzone: f9
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Global init order: f6
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Poisoned by user: f7
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Container overflow: fc
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Array cookie: ac
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Intra object redzone: bb
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ASan internal: fe
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Left alloca redzone: ca
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Right alloca redzone: cb
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==9399==ABORTING
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```
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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。
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参考：https://en.wikipedia.org/wiki/AddressSanitizer
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## mcheck
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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：
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```c
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#include <mcheck.h>
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int mcheck(void (*abortfunc)(enum mcheck_status mstatus));
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```
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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。
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举个例子，下面的程序存在 double-free 的问题：
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```c
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#include <stdlib.h>
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#include <stdio.h>
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void main() {
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char *p;
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p = malloc(1000);
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fprintf(stderr, "About to free\n");
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free(p);
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fprintf(stderr, "About to free a second time\n");
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free(p);
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fprintf(stderr, "Finish\n");
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}
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```
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通过设置参数 `-lmcheck` 来链接 mcheck 函数：
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```
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$ gcc -lmcheck mcheck.c
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$ ./a.out
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About to free
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About to free a second time
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block freed twice
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Aborted (core dumped)
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```
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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。
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```
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$ gcc mcheck.c
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$ #检查到错误时不作任何提示
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$ MALLOC_CHECK_=0 ./a.out
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About to free
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About to free a second time
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Finish
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$ #检查到错误时打印一条信息到标准输出
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$ MALLOC_CHECK_=1 ./a.out
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About to free
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About to free a second time
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*** Error in `./a.out': free(): invalid pointer: 0x0000000001fb9010 ***
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Finish
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$ #检查到错误时直接中止程序
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$ MALLOC_CHECK_=2 ./a.out
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About to free
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About to free a second time
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Aborted (core dumped)
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```
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具体参考 `man 3 mcheck` 和 `man 3 mallopt`。
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## 参考资料
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- [GCC online documentation](https://gcc.gnu.org/onlinedocs/)
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