[Linux][2015-03-17] gcc/g++ 編譯選項
GCC的幾個常用選項
gcc常用的編譯選項對程式碼的影響
建立時間:2001-12-21
文章屬性:原創
文章來源:http://xfocus.org/
文章提交:alert7 (sztcww_at_sina.com)
測試環境 redhat 6.2
★ 前言
本文討論gcc的一些常用編譯選項對程式碼的影響。當然程式碼變了,
它的記憶體佈局也就會變了,隨之exploit也就要做相應的變動。
gcc的編譯選項實在太多,本文檢了幾個最常用的選項。
★ 演示程式
[[email protected] alert7]$ cat > test.c
#include <stdio.h>
void hi(void)
{
printf("hi");
}
int main(int argc, char *argv[])
{
hi();
return 0;
}
★ 一般情況
[[email protected] alert7]$ gcc -o test test.c
[[email protected] alert7]$ wc -c test
11773 test
[[email protected] alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80483e4 <main>: push %ebp
0x80483e5 <main+1>: mov %esp,%ebp
0x80483e7 <main+3>: call 0x80483d0 <hi>
0x80483ec <main+8>: xor %eax,%eax
0x80483ee <main+10>: jmp 0x80483f0 <main+12>
0x80483f0 <main+12>: leave
0x80483f1 <main+13>: ret
....
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80483d0 <hi>: push %ebp
0x80483d1 <hi+1>: mov %esp,%ebp
0x80483d3 <hi+3>: push $0x8048450
0x80483d8 <hi+8>: call 0x8048308 <printf>
0x80483dd <hi+13>: add $0x4,%esp
0x80483e0 <hi+16>: leave
0x80483e1 <hi+17>: ret
0x80483e2 <hi+18>: mov %esi,%esi
End of assembler dump.
來看看部分的記憶體映象
(記憶體高址)
+--------+
|bffffbc4| argv的地址(即argv[0]的地址)
0xbffffb84 +--------+
|00000001| argc的值
0xbffffb80 +--------+
|400309cb|main的返回地址
0xbffffb7c +--------+ <-- 呼叫main函式前的esp
|bffffb98| 呼叫main函式前的ebp
0xbffffb78 +--------+ <-- main函式的ebp
|080483ec| hi()的返回地址
0xbffffb74 +--------+
|bffffb78| 呼叫hi()前的esp
0xbffffb70 +--------+
|08048450| "hi"的地址
0xbffffb6c +--------+
| ...... |
(記憶體低址)
leave 指令所做的操作相當於MOV ESP,EBP 然後 POP EBP
ret 指令所做的操作相當於POP EIP
★ -O 編譯選項
With `-O', the compiler tries to reduce code size and execution time.
When you specify `-O', the two options `-fthread-jumps' and
`-fdefer-pop' are turned on
優化,減少程式碼大小和執行的時間
[[email protected] alert7]$ gcc -O -o test test.c
[[email protected] alert7]$ wc -c test
11757 test
[[email protected] alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80483d8 <main>: push %ebp
0x80483d9 <main+1>: mov %esp,%ebp
0x80483db <main+3>: call 0x80483c8 <hi>
0x80483e0 <main+8>: xor %eax,%eax
0x80483e2 <main+10>: leave
0x80483e3 <main+11>: ret
0x80483e4 <main+12>: nop
...
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80483c8 <hi>: push %ebp
0x80483c9 <hi+1>: mov %esp,%ebp
0x80483cb <hi+3>: push $0x8048440
0x80483d0 <hi+8>: call 0x8048308 <printf>
0x80483d5 <hi+13>: leave
0x80483d6 <hi+14>: ret
0x80483d7 <hi+15>: nop
End of assembler dump.
在main()中,把一條jmp指令優化掉了,很顯然,這條指令是可以不需要的。
在hi()中,把add $0x4,%esp優化掉了,這會不會使stack不平衡呢?
來看看部分的記憶體映象
(記憶體高址)
+--------+
|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 +--------+ <-- 呼叫main函式前的esp
|bffffb98| 呼叫main函式前的ebp
0xbffffb78 +--------+ <-- main函式的ebp
|080483e0| hi()的返回地址
0xbffffb74 +--------+
|bffffb78| 呼叫hi()前的esp
0xbffffb70 +--------+
|08048440| "hi"的地址
0xbffffb6c +--------+
| ...... |
(記憶體低址)
leave 指令所做的操作相當於把MOV ESP,EBP 然後 POP EBP
看到leave指令操作了沒有,先把ebp-->esp,再pop ebp,這樣即使
在過程內堆疊的esp,ebp是不平衡的,但只要返回時候碰到leave指令
就會平衡了,所以把add $0x4,%esp優化掉也是沒有問題的。
★ -O2 編譯選項
-O2 Optimize even more. Nearly all supported optimizations that do
not involve a space-speed tradeoff are performed. Loop unrolling
and function inlining are not done, for example. As compared to -O,
this option increases both compilation time and the performance of
the generated code.
[[email protected] alert7]$ gcc -O2 -o test test.c
[[email protected] alert7]$ wc -c test
11757 test
[[email protected] alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80483d8 <main>: push %ebp
0x80483d9 <main+1>: mov %esp,%ebp
0x80483db <main+3>: call 0x80483c8 <hi>
0x80483e0 <main+8>: xor %eax,%eax
0x80483e2 <main+10>: leave
0x80483e3 <main+11>: ret
...
0x80483ef <main+23>: nop
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80483c8 <hi>: push %ebp
0x80483c9 <hi+1>: mov %esp,%ebp
0x80483cb <hi+3>: push $0x8048440
0x80483d0 <hi+8>: call 0x8048308 <printf>
0x80483d5 <hi+13>: leave
0x80483d6 <hi+14>: ret
0x80483d7 <hi+15>: nop
End of assembler dump.
由於程式比較簡單,再優化也沒有好優化的了,所以跟-O出來的一樣。
★ -fomit-frame-pointer 編譯選項
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions
that don't need one. This avoids the instructions to save,
set up and restore frame pointers; it also makes an extra
register available in many functions. It also makes
debugging impossible on most machines.
忽略幀指標。這樣在程式就不需要儲存,安裝,和恢復ebp了。這樣ebp也就是一個
free的register了,在函式中就可以隨便使用了。
[[email protected] alert7]$ gcc -fomit-frame-pointer -o test test.c
[[email protected] alert7]$ wc -c test
11773 test
[[email protected] alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80483e0 <main>: call 0x80483d0 <hi>
0x80483e5 <main+5>: xor %eax,%eax
0x80483e7 <main+7>: jmp 0x80483f0 <main+16>
0x80483e9 <main+9>: lea 0x0(%esi,1),%esi
0x80483f0 <main+16>: ret
....
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80483d0 <hi>: push $0x8048450
0x80483d5 <hi+5>: call 0x8048308 <printf>
0x80483da <hi+10>: add $0x4,%esp
0x80483dd <hi+13>: ret
0x80483de <hi+14>: 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//這兩條指令安裝
leave//這條指令恢復
來看看部分的記憶體映象
(記憶體高址)
+--------+
|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"字串的地址
0xbffffb74 +--------+
| ...... |
(記憶體低址)
沒有儲存上層執行環境的ebp.
★ -fomit-frame-pointer && -O2
-fomit-frame-pointer編譯選項去掉了
push %ebp
mov %esp,%ebp//這兩條指令安裝
leave//這條指令恢復
-O2編譯選項去掉了
add $0x4,%esp
兩個加起來會不會這四條指令一起去掉,從而使stack不平衡呢?
[[email protected] alert7]$ gcc -fomit-frame-pointer -O2 -o test test.c
[[email protected] alert7]$ wc -c test
11741 test
[[email protected] alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80483d8 <main>: call 0x80483c8 <hi>
0x80483dd <main+5>: xor %eax,%eax
0x80483df <main+7>: ret
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80483c8 <hi>: push $0x8048430
0x80483cd <hi+5>: call 0x8048308 <printf>
0x80483d2 <hi+10>: add $0x4,%esp
0x80483d5 <hi+13>: ret
0x80483d6 <hi+14>: mov %esi,%esi
End of assembler dump.
來看看部分的記憶體映象
(記憶體高址)
+--------+
|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"字串的地址
0xbffffb74 +--------+
| ...... |
(記憶體低址)
此時就沒有把add $0x4,%esp優化掉,如果優化掉的話,整個stack就
會變的不平衡,從而會導致程式出錯。
★ -fPIC 編譯選項
-fPIC If supported for the target machine, emit position-independent
code, suitable for dynamic linking,even if branches need large
displacements.
產生位置無關程式碼(PIC),一般建立共享庫時用到。
在x86上,PIC的程式碼的符號引用都是通過ebx進行操作的。
[[email protected] alert7]$ gcc -fPIC -o test test.c
[[email protected] alert7]$ wc -c test
11805 test
[[email protected] alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80483f8 <main>: push %ebp
0x80483f9 <main+1>: mov %esp,%ebp
0x80483fb <main+3>: push %ebx
0x80483fc <main+4>: call 0x8048401 <main+9>
0x8048401 <main+9>: pop %ebx//取得該指令的地址
0x8048402 <main+10>: add $0x1093,%ebx//此時ebx裡面存放著是GOT表的地址
0x8048408 <main+16>: call 0x80483d0 <hi>
0x804840d <main+21>: xor %eax,%eax
0x804840f <main+23>: jmp 0x8048411 <main+25>
0x8048411 <main+25>: mov 0xfffffffc(%ebp),%ebx
0x8048414 <main+28>: leave
0x8048415 <main+29>: ret
...
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80483d0 <hi>: push %ebp
0x80483d1 <hi+1>: mov %esp,%ebp
0x80483d3 <hi+3>: push %ebx
0x80483d4 <hi+4>: call 0x80483d9 <hi+9>
0x80483d9 <hi+9>: pop %ebx
0x80483da <hi+10>: add $0x10bb,%ebx
0x80483e0 <hi+16>: lea 0xffffefdc(%ebx),%edx
0x80483e6 <hi+22>: mov %edx,%eax
0x80483e8 <hi+24>: push %eax
0x80483e9 <hi+25>: call 0x8048308 <printf>
0x80483ee <hi+30>: add $0x4,%esp
0x80483f1 <hi+33>: mov 0xfffffffc(%ebp),%ebx
0x80483f4 <hi+36>: leave
0x80483f5 <hi+37>: ret
0x80483f6 <hi+38>: mov %esi,%esi
End of assembler dump.
來看看部分的記憶體映象
(記憶體高址)
+--------+
|bffffbc4| argv的地址(即argv[0]的地址)
0xbffffb84 +--------+
|00000001| argc的值
0xbffffb80 +--------+
|400309cb|main的返回地址
0xbffffb7c +--------+ <-- 呼叫main函式前的esp
|bffffb98| 呼叫main函式前的ebp
0xbffffb78 +--------+ <-- main函式的ebp
|401081ec| 儲存的ebx
0xbffffb74 +--------+
|0804840d| (存放過call 0x8048401的下一條指令地址)
0xbffffb70 +--------+
|bffffb78| 呼叫hi()前的esp
0xbffffb6c +--------+
|08049494| GOT表地址
0xbffffb68 +--------+
|08048470|(存放過call 0x80483d9的下一條指令地址)
0xbffffb64 +--------+
| ...... |
(記憶體低址)
★ -static 編譯選項
-static
On systems that support dynamic linking, this prevents
linking with the shared libraries. On other systems,
this option has no effect.
把一些函式都靜態的編譯到程式中,而無需動態連結了。
[[email protected] alert7]$ gcc -o test -static test.c
[[email protected] alert7]$ wc -c test
962808 test
[[email protected] alert7]$ gdb -q test
(gdb) disass main
Dump of assembler code for function main:
0x80481b4 <main>: push %ebp
0x80481b5 <main+1>: mov %esp,%ebp
0x80481b7 <main+3>: call 0x80481a0 <hi>
0x80481bc <main+8>: xor %eax,%eax
0x80481be <main+10>: jmp 0x80481c0 <main+12>
0x80481c0 <main+12>: leave
0x80481c1 <main+13>: ret
...
End of assembler dump.
(gdb) disass hi
Dump of assembler code for function hi:
0x80481a0 <hi>: push %ebp
0x80481a1 <hi+1>: mov %esp,%ebp
0x80481a3 <hi+3>: push $0x8071528
0x80481a8 <hi+8>: call 0x804865c <printf>
0x80481ad <hi+13>: add $0x4,%esp
0x80481b0 <hi+16>: leave
0x80481b1 <hi+17>: ret
0x80481b2 <hi+18>: mov %esi,%esi
End of assembler dump.
[[email protected] alert7]$ ldd test
not a dynamic executable
-static出來的程式碼已經沒有PLT了,GOT雖然有,已經全部為0了。
★ 小節
拋磚引玉般簡單的例項描述了下gcc常用的編譯選項對程式碼的影響。
不正之處,還請斧正。謝謝。