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| 关键词： 80483 assembler 地址 redhat alert7 test alert 指令 ebp 内存 |
Author: alert7Email: [email protected]://www.whitecell.org 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 test11773 test[alert7@redhat62 alert7]$ gdb -q test(gdb) disass mainDump of assembler code for function main:0x80483e4 : push %ebp0x80483e5 : mov %esp,%ebp0x80483e7 : call 0x80483d0 0x80483ec : xor %eax,%eax0x80483ee : jmp 0x80483f0 0x80483f0 : leave0x80483f1 : ret....End of assembler dump.(gdb) disass hiDump of assembler code for function hi:0x80483d0 : push %ebp0x80483d1 : mov %esp,%ebp0x80483d3 : push $0x80484500x80483d8 : call 0x8048308 0x80483dd : add $0x4,%esp0x80483e0 : leave0x80483e1 : ret0x80483e2 : mov %esi,%esiEnd of assembler dump.来看看部分的内存映象(内存高址)+--------+|bffffbc4| argv的地址(即argv[0]的地址)0xbffffb84 +--------+|00000001| argc的值0xbffffb80 +--------+|400309cb|main的返回地址0xbffffb7c +--------+ |bffffb98| 调用main函数前的ebp0xbffffb78 +--------+ |080483ec| hi()的返回地址0xbffffb74 +--------+|bffffb78| 调用hi()前的esp0xbffffb70 +--------+ |08048450| "hi"的地址0xbffffb6c +--------+ | ...... |(内存低址)leave 指令所做的操作相当于MOV ESP,EBP 然后 POP EBP ret 指令所做的操作相当于POP EIP★ -O 编译选项With `-O', the compiler tries to reduce code size and execution time.When you specify `-O', the two options `-fthread-jumps' and `-fdefer-pop' are turned on优化，减少代码大小和执行的时间[alert7@redhat62 alert7]$ gcc -O -o test test.c[alert7@redhat62 alert7]$ wc -c test11757 test[alert7@redhat62 alert7]$ gdb -q test(gdb) disass mainDump of assembler code for function main:0x80483d8 : push %ebp0x80483d9 : mov %esp,%ebp0x80483db : call 0x80483c8 0x80483e0 : xor %eax,%eax0x80483e2 : leave0x80483e3 : ret0x80483e4 : nop...End of assembler dump.(gdb) disass hiDump of assembler code for function hi:0x80483c8 : push %ebp0x80483c9 : mov %esp,%ebp0x80483cb : push $0x80484400x80483d0 : call 0x8048308 0x80483d5 : leave0x80483d6 : ret0x80483d7 : nopEnd 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 +--------+ |bffffb98| 调用main函数前的ebp0xbffffb78 +--------+ |080483e0| hi()的返回地址0xbffffb74 +--------+|bffffb78| 调用hi()前的esp0xbffffb70 +--------+ |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 $0x4,%esp优化掉也是没有问题的。★ -O2 编译选项-O2 Optimize even more. Nearly all supported optimizations that do not involve a space-speed tradeoff are performed. Loop unrollingand function inlining are not done, for example. As compared to -O,this option increases both compilation time and the performance ofthe generated code.[alert7@redhat62 alert7]$ gcc -O2 -o test test.c[alert7@redhat62 alert7]$ wc -c test11757 test[alert7@redhat62 alert7]$ gdb -q test(gdb) disass mainDump of assembler code for function main:0x80483d8 : push %ebp0x80483d9 : mov %esp,%ebp0x80483db : call 0x80483c8 0x80483e0 : xor %eax,%eax0x80483e2 : leave0x80483e3 : ret...0x80483ef : nopEnd of assembler dump.(gdb) disass hiDump of assembler code for function hi:0x80483c8 : push %ebp0x80483c9 : mov %esp,%ebp0x80483cb : push $0x80484400x80483d0 : call 0x8048308 0x80483d5 : leave0x80483d6 : ret0x80483d7 : nopEnd of assembler dump.由于程序比较简单，再优化也没有好优化的了，所以跟-O出来的一样。★ -fomit-frame-pointer 编译选项-fomit-frame-pointerDon't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on most machines.忽略帧指针。这样在程序就不需要保存，安装，和恢复ebp了。这样ebp也就是一个free的register了，在函数中就可以随便使用了。[alert7@redhat62 alert7]$ gcc -fomit-frame-pointer -o test test.c[alert7@redhat62 alert7]$ wc -c test11773 test[alert7@redhat62 alert7]$ gdb -q test(gdb) disass mainDump of assembler code for function main:0x80483e0 : call 0x80483d0 0x80483e5 : xor %eax,%eax0x80483e7 : jmp 0x80483f0 0x80483e9 : lea 0x0(%esi,1),%esi0x80483f0 : ret....End of assembler dump.(gdb) disass hiDump of assembler code for function hi:0x80483d0 : push $0x80484500x80483d5 : call 0x8048308 0x80483da : add $0x4,%esp0x80483dd : ret0x80483de : mov %esi,%esiEnd of assembler dump.在main()和hi()中都去掉了以下指令push %ebpmov %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 %ebpmov %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 test11741 test[alert7@redhat62 alert7]$ gdb -q test(gdb) disass mainDump of assembler code for function main:0x80483d8 : call 0x80483c8 0x80483dd : xor %eax,%eax0x80483df : retEnd of assembler dump.(gdb) disass hiDump of assembler code for function hi:0x80483c8 : push $0x80484300x80483cd : call 0x8048308 0x80483d2 : add $0x4,%esp0x80483d5 : ret0x80483d6 : mov %esi,%esiEnd 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 $0x4,%esp优化掉，如果优化掉的话，整个stack就会变的不平衡，从而会导致程序出错。★ -fPIC 编译选项-fPIC If supported for the target machine, emit position-independentcode, suitable for dynamic linking,even if branches need large displacements.产生位置无关代码（PIC），一般创建共享库时用到。在x86上，PIC的代码的符号引用都是通过ebx进行操作的。[alert7@redhat62 alert7]$ gcc -fPIC -o test test.c[alert7@redhat62 alert7]$ wc -c test11805 test[alert7@redhat62 alert7]$ gdb -q test(gdb) disass mainDump of assembler code for function main:0x80483f8 : push %ebp0x80483f9 : mov %esp,%ebp0x80483fb : push %ebx0x80483fc : 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,%eax0x804840f : jmp 0x8048411 0x8048411 : mov 0xfffffffc(%ebp),%ebx0x8048414 : leave0x8048415 : ret...End of assembler dump.(gdb) disass hiDump of assembler code for function hi:0x80483d0 : push %ebp0x80483d1 : mov %esp,%ebp0x80483d3 : push %ebx0x80483d4 : call 0x80483d9 0x80483d9 : pop %ebx0x80483da : add $0x10bb,%ebx0x80483e0 : lea 0xffffefdc(%ebx),%edx0x80483e6 : mov %edx,%eax0x80483e8 : push %eax0x80483e9 : call 0x8048308 0x80483ee : add $0x4,%esp0x80483f1 : mov 0xfffffffc(%ebp),%ebx0x80483f4 : leave0x80483f5 : ret0x80483f6 : mov %esi,%esiEnd of assembler dump.来看看部分的内存映象(内存高址)+--------+|bffffbc4| argv的地址(即argv[0]的地址)0xbffffb84 +--------+|00000001| argc的值0xbffffb80 +--------+|400309cb|main的返回地址0xbffffb7c +--------+ |bffffb98| 调用main函数前的ebp0xbffffb78 +--------+ |401081ec| 保存的ebx0xbffffb74 +--------+|0804840d| （存放过call 0x8048401的下一条指令地址）0xbffffb70 +--------+ |bffffb78| 调用hi()前的esp0xbffffb6c +--------+ |08049494| GOT表地址0xbffffb68 +--------+ |08048470|（存放过call 0x80483d9的下一条指令地址） 0xbffffb64 +--------+ | ...... |(内存低址)★ -static 编译选项-staticOn systems that support dynamic linking, this prevents linking with the shared libraries. On other systems, this option has no effect.把一些函数都静态的编译到程序中，而无需动态链接了。[alert7@redhat62 alert7]$ gcc -o test -static test.c[alert7@redhat62 alert7]$ wc -c test962808 test[alert7@redhat62 alert7]$ gdb -q test(gdb) disass mainDump of assembler code for function main:0x80481b4 : push %ebp0x80481b5 : mov %esp,%ebp0x80481b7 : call 0x80481a0 0x80481bc : xor %eax,%eax0x80481be : jmp 0x80481c0 0x80481c0 : leave0x80481c1 : ret...End of assembler dump.(gdb) disass hiDump of assembler code for function hi:0x80481a0 : push %ebp0x80481a1 : mov %esp,%ebp0x80481a3 : push $0x80715280x80481a8 : call 0x804865c 0x80481ad : add $0x4,%esp0x80481b0 : leave0x80481b1 : ret0x80481b2 : mov %esi,%esiEnd of assembler dump.[alert7@redhat62 alert7]$ ldd testnot 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。 |
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