[转载]gcc常用的编译选项对代码的影响

by alert7
2001-12-21

测试环境 redhat 6.2

★ 前言

本文讨论gcc的一些常用编译选项对代码的影响。当然代码变了，
它的内存布局也就会变了，随之exploit也就要做相应的变动。
gcc的编译选项实在太多，本文检了几个最常用的选项。


★ 演示程序

[alert7@redhat62 alert7]$ cat > test.c
#include
void hi(void)
{
printf("hi");
}

int main(int argc, char *argv[])
{
        hi();
        return 0;
}


★ 一般情况

[alert7@redhat62 alert7]$ gcc -o test test.c
[alert7@redhat62 alert7]$ wc -c test
  11773 test
[alert7@redhat62 alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80483e4

:       push   %ebp
0x80483e5 :     mov    %esp,%ebp
0x80483e7 :     call   0x80483d0
0x80483ec :     xor    %eax,%eax
0x80483ee :    jmp    0x80483f0
0x80483f0 :    leave
0x80483f1 :    ret
....
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80483d0 : push   %ebp
0x80483d1 :       mov    %esp,%ebp
0x80483d3 :       push   $0x8048450
0x80483d8 :       call   0x8048308
0x80483dd :      add    $0x4,%esp
0x80483e0 :      leave
0x80483e1 :      ret
0x80483e2 :      mov    %esi,%esi
End of assembler dump.

来看看部分的内存映象
                   (内存高址)
              +--------+
                              |bffffbc4| argv的地址(即argv[0]的地址)
                   0xbffffb84 +--------+
                              |00000001| argc的值
                   0xbffffb80 +--------+
                              |400309cb|main的返回地址
                   0xbffffb7c +--------+ <-- 调用main函数前的esp
                              |bffffb98| 调用main函数前的ebp
                   0xbffffb78 +--------+ <-- main函数的ebp
                              |080483ec| hi()的返回地址
                   0xbffffb74 +--------+
                              |bffffb78| 调用hi()前的esp
                   0xbffffb70 +--------+
                              |08048450| "hi"的地址
                   0xbffffb6c +--------+
                              | ...... |
                   (内存低址)

leave 指令所做的操作相当于MOV ESP,EBP 然后 POP EBP
ret 指令所做的操作相当于POP EIP


★ -O 编译选项

With `-O', the compiler tries to reduce code size and execution time.
When you specify `-O', the two options `-fthread-jumps' and
`-fdefer-pop' are turned  on
优化，减少代码大小和执行的时间

[alert7@redhat62 alert7]$ gcc -O -o test test.c
[alert7@redhat62 alert7]$ wc -c test
  11757 test
[alert7@redhat62 alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80483d8

:       push   %ebp
0x80483d9 :     mov    %esp,%ebp
0x80483db :     call   0x80483c8
0x80483e0 :     xor    %eax,%eax
0x80483e2 :    leave
0x80483e3 :    ret
0x80483e4 :    nop
...
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80483c8 : push   %ebp
0x80483c9 :       mov    %esp,%ebp
0x80483cb :       push   $0x8048440
0x80483d0 :       call   0x8048308
0x80483d5 :      leave
0x80483d6 :      ret
0x80483d7 :      nop
End of assembler dump.

在main()中，把一条jmp指令优化掉了，很显然，这条指令是可以不需要的。
在hi()中，把add    $0x4,%esp优化掉了，这会不会使stack不平衡呢？
来看看部分的内存映象
                   (内存高址)
              +--------+
                              |bffffbc4| argv的地址(即argv[0]的地址)
                   0xbffffb84 +--------+
                              |00000001| argc的值
                   0xbffffb80 +--------+
                              |400309cb|main的返回地址
                   0xbffffb7c +--------+ <-- 调用main函数前的esp
                              |bffffb98| 调用main函数前的ebp
                   0xbffffb78 +--------+ <-- main函数的ebp
                              |080483e0| hi()的返回地址
                   0xbffffb74 +--------+
                              |bffffb78| 调用hi()前的esp
                   0xbffffb70 +--------+
                              |08048440| "hi"的地址
                   0xbffffb6c +--------+
                              | ...... |
                   (内存低址)

leave 指令所做的操作相当于把MOV ESP,EBP 然后 POP EBP 
看到leave指令操作了没有，先把ebp-->esp，再pop ebp，这样即使
在过程内堆栈的esp,ebp是不平衡的，但只要返回时候碰到leave指令
就会平衡了，所以把add    $0x4,%esp优化掉也是没有问题的。


★ -O2 编译选项

-O2 Optimize  even more.  Nearly all supported optimizations that do
not involve a space-speed tradeoff are performed.  Loop unrolling
and function inlining are not done, for example.  As compared to -O,
        this option increases both compilation time and the performance of
the generated code.

[alert7@redhat62 alert7]$ gcc -O2 -o test test.c
[alert7@redhat62 alert7]$ wc -c test
  11757 test
[alert7@redhat62 alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80483d8

:       push   %ebp
0x80483d9 :     mov    %esp,%ebp
0x80483db :     call   0x80483c8
0x80483e0 :     xor    %eax,%eax
0x80483e2 :    leave
0x80483e3 :    ret
...
0x80483ef :    nop
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80483c8 : push   %ebp
0x80483c9 :       mov    %esp,%ebp
0x80483cb :       push   $0x8048440
0x80483d0 :       call   0x8048308
0x80483d5 :      leave
0x80483d6 :      ret
0x80483d7 :      nop
End of assembler dump.

由于程序比较简单，再优化也没有好优化的了，所以跟-O出来的一样。


★ -fomit-frame-pointer 编译选项

-fomit-frame-pointer
              Don't keep the frame pointer in a register for functions
      that don't need one.  This avoids the  instructions to save,
      set up and restore frame pointers; it also makes an extra
      register available in many functions.  It also makes
      debugging impossible on most machines.

忽略帧指针。这样在程序就不需要保存，安装，和恢复ebp了。这样ebp也就是一个
free的register了，在函数中就可以随便使用了。

[alert7@redhat62 alert7]$ gcc -fomit-frame-pointer -o test test.c
[alert7@redhat62 alert7]$ wc -c test
  11773 test
[alert7@redhat62 alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80483e0

:       call   0x80483d0
0x80483e5 :     xor    %eax,%eax
0x80483e7 :     jmp    0x80483f0
0x80483e9 :     lea    0x0(%esi,1),%esi
0x80483f0 :    ret
....
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80483d0 : push   $0x8048450
0x80483d5 :       call   0x8048308
0x80483da :      add    $0x4,%esp
0x80483dd :      ret
0x80483de :      mov    %esi,%esi
End of assembler dump.

在main()和hi()中都去掉了以下指令
push   %ebp
mov    %esp,%ebp//这两条指令安装
leave//这条指令恢复
来看看部分的内存映象
                   (内存高址)
              +--------+
                              |bffffbc4| argv的地址(即argv[0]的地址)
                   0xbffffb84 +--------+
                              |00000001| argc的值
                   0xbffffb80 +--------+
                              |400309cb|main的返回地址
                   0xbffffb7c +--------+
                              |080483e5| hi()的返回地址
                   0xbffffb78 +--------+
                              |08048450|  "hi"字符串的地址
                   0xbffffb74 +--------+
                              | ...... |
                   (内存低址)
没有保存上层执行环境的ebp.


★ -fomit-frame-pointer && -O2

-fomit-frame-pointer编译选项去掉了
push   %ebp
mov    %esp,%ebp//这两条指令安装
leave//这条指令恢复
-O2编译选项去掉了
add    $0x4,%esp

两个加起来会不会这四条指令一起去掉，从而使stack不平衡呢？

[alert7@redhat62 alert7]$ gcc -fomit-frame-pointer -O2 -o test test.c
[alert7@redhat62 alert7]$ wc -c test
  11741 test
[alert7@redhat62 alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80483d8

:       call   0x80483c8
0x80483dd :     xor    %eax,%eax
0x80483df :     ret
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80483c8 : push   $0x8048430
0x80483cd :       call   0x8048308
0x80483d2 :      add    $0x4,%esp
0x80483d5 :      ret
0x80483d6 :      mov    %esi,%esi
End of assembler dump.
来看看部分的内存映象
                   (内存高址)
              +--------+
                              |bffffbc4| argv的地址(即argv[0]的地址)
                   0xbffffb84 +--------+
                              |00000001| argc的值
                   0xbffffb80 +--------+
                              |400309cb|main的返回地址
                   0xbffffb7c +--------+
                              |080483dd| hi()的返回地址
                   0xbffffb78 +--------+
                              |08048430|  "hi"字符串的地址
                   0xbffffb74 +--------+
                              | ...... |
                   (内存低址)

此时就没有把add    $0x4,%esp优化掉，如果优化掉的话，整个stack就
会变的不平衡，从而会导致程序出错。


★ -fPIC 编译选项

-fPIC If  supported for the target machine, emit position-independent
code, suitable for dynamic linking,even if branches need large
displacements.
产生位置无关代码（PIC），一般创建共享库时用到。
在x86上，PIC的代码的符号引用都是通过ebx进行操作的。

[alert7@redhat62 alert7]$ gcc -fPIC -o test test.c
[alert7@redhat62 alert7]$ wc -c test
  11805 test
[alert7@redhat62 alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80483f8

:       push   %ebp
0x80483f9 :     mov    %esp,%ebp
0x80483fb :     push   %ebx
0x80483fc :     call   0x8048401
0x8048401 :     pop    %ebx//取得该指令的地址
0x8048402 :    add    $0x1093,%ebx//此时ebx里面存放着是GOT表的地址
0x8048408 :    call   0x80483d0
0x804840d :    xor    %eax,%eax
0x804840f :    jmp    0x8048411
0x8048411 :    mov    0xfffffffc(%ebp),%ebx
0x8048414 :    leave
0x8048415 :    ret
...
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80483d0 : push   %ebp
0x80483d1 :       mov    %esp,%ebp
0x80483d3 :       push   %ebx
0x80483d4 :       call   0x80483d9
0x80483d9 :       pop    %ebx
0x80483da :      add    $0x10bb,%ebx
0x80483e0 :      lea    0xffffefdc(%ebx),%edx
0x80483e6 :      mov    %edx,%eax
0x80483e8 :      push   %eax
0x80483e9 :      call   0x8048308
0x80483ee :      add    $0x4,%esp
0x80483f1 :      mov    0xfffffffc(%ebp),%ebx
0x80483f4 :      leave
0x80483f5 :      ret
0x80483f6 :      mov    %esi,%esi
End of assembler dump.
来看看部分的内存映象
 
    (内存高址)
              +--------+
              |bffffbc4| argv的地址(即argv[0]的地址)
   0xbffffb84 +--------+
              |00000001| argc的值
   0xbffffb80 +--------+
              |400309cb|main的返回地址
   0xbffffb7c +--------+ <-- 调用main函数前的esp
              |bffffb98| 调用main函数前的ebp
   0xbffffb78 +--------+ <-- main函数的ebp
              |401081ec| 保存的ebx
   0xbffffb74 +--------+
              |0804840d| （存放过call 0x8048401的下一条指令地址）
   0xbffffb70 +--------+
              |bffffb78| 调用hi()前的esp
   0xbffffb6c +--------+
              |08049494| GOT表地址
   0xbffffb68 +--------+
              |08048470|（存放过call 0x80483d9的下一条指令地址）
   0xbffffb64 +--------+
              | ...... |
     (内存低址)


★ -static 编译选项

-static
On systems that support dynamic linking, this prevents
linking with the shared libraries.  On other  systems,
this option has no effect.
把一些函数都静态的编译到程序中，而无需动态链接了。

[alert7@redhat62 alert7]$ gcc -o test -static test.c
[alert7@redhat62 alert7]$ wc -c test
962808 test
[alert7@redhat62 alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80481b4

:       push   %ebp
0x80481b5 :     mov    %esp,%ebp
0x80481b7 :     call   0x80481a0
0x80481bc :     xor    %eax,%eax
0x80481be :    jmp    0x80481c0
0x80481c0 :    leave
0x80481c1 :    ret
...
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80481a0 : push   %ebp
0x80481a1 :       mov    %esp,%ebp
0x80481a3 :       push   $0x8071528
0x80481a8 :       call   0x804865c
0x80481ad :      add    $0x4,%esp
0x80481b0 :      leave
0x80481b1 :      ret
0x80481b2 :      mov    %esi,%esi
End of assembler dump.
[alert7@redhat62 alert7]$ ldd test
        not a dynamic executable
-static出来的代码已经没有PLT了，GOT虽然有，已经全部为0了。


★ 小节

抛砖引玉般简单的实例描述了下gcc常用的编译选项对代码的影响。
不正之处，还请斧正。谢谢。