这是以前我学习PE的时候的一些摘要。
分析摘要:
(*1*): 写程序。a.cpp 和 foo.cpp
其中a.cpp的内容为:
extern void Foo();
void main()
{
Foo();
}
foo.cpp的内容为:
#include "stdio.h"
void Foo()
{
printf("I am Foo!");
}
编译程序产生a.obj foo.obj a.exe.
(*2*):Copy以上3个文件到..\VisualStdio\VC98\BIN目录下
用以下命令解析:
dumpbin /all a.obj >aobj.txt <Enter>
dumpbin /all foo.obj >fooobj.txt <Enter>
dumpbin /all a.exe >aexe.txt <Enter>
(*3*):
打开文件a.obj,找到代码节。内容如下:
SECTION HEADER #3
.text name
0 physical address
0 virtual address
2E size of raw data
355 file pointer to raw data //Attention!!~!~
383 file pointer to relocation table
397 file pointer to line numbers
2 number of relocations
3 number of line numbers
60501020 flags
Code
Communal; sym= _main
16 byte align
Execute Read
RAW DATA #3
00000000: 55 8B EC 83 EC 40 53 56 57 8D 7D C0 B9 10 00 00 U....@SVW.}.....
00000010: 00 B8 CC CC CC CC F3 AB E8 00 00 00 00 5F 5E 5B ............._^[
00000020: 83 C4 40 3B EC E8 00 00 00 00 8B E5 5D C3 ..@;........].
(*4*):反编译a.obj的代码节。
打开工具URSoft W32Dasm (我用的是VERSION 8.93)
在打开文件时选择所有文件，因为本软件主要是针对PE,LE,NE等文件格式的。所以对于用来
反编译OBJ文件需指定偏移量。如上Attention!处所示。即为代码节的文件偏移。
所以在打开OBJ文件的提示对话框中输入:00000355
Start Disassembly from Offset 00000355 Hex.
无需选中Check For 16 Bit Disassembly .
反编译之后的代码节内容如下:
:00000000 55 push ebp
:00000001 8BEC mov ebp, esp
:00000003 83EC40 sub esp, 00000040
:00000006 53 push ebx
:00000007 56 push esi
:00000008 57 push edi
:00000009 8D7DC0 lea edi, dword ptr [ebp-40]
:0000000C B910000000 mov ecx, 00000010
:00000011 B8CCCCCCCC mov eax, CCCCCCCC
:00000016 F3 repz
:00000017 AB stosd
:00000018 E800000000 call 0000001D //Attention!!!
:0000001D 5F pop edi
:0000001E 5E pop esi
:0000001F 5B pop ebx
:00000020 83C440 add esp, 00000040
:00000023 3BEC cmp ebp, esp
:00000025 E800000000 call 0000002A
:0000002A 8BE5 mov esp, ebp
:0000002C 5D pop ebp
:0000002D C3 ret
简要说明:
The 0xE8 is the CALL instruction opcode. The next DWORD should contain the offset to the Foo function (relative to the CALL instruction). It's pretty clear that Foo probably isn't zero bytes away from the CALL instruction. Simply put, this code wouldn't work as expected if you were to execute it. The code is broken, and needs to be fixed up.
In the above example of a call to function Foo, there will be a REL32 fixup record, and it will have the offset of the DWORD that the linker needs to overwrite with the appropriate value.
(*5*):查看紧跟代码节后的RELOCATIONS:
RELOCATIONS #3
Symbol Symbol
Offset Type Applied To Index Name
-------- ------- ------------- ------ --------------
00000019 REL32 00000000 12 ?Foo@@YAXXZ (void __cdecl Foo(void))
00000026 REL32 00000000 13 __chkesp
this(first) fixup record says that the linker needs to calculate the relative offset to
function Foo, and write that value to offset four in the section.
(*6*):实际的a.exe代码节内容:
:00401000 55 push ebp
:00401001 8BEC mov ebp, esp
:00401003 83EC40 sub esp, 00000040
:00401006 53 push ebx
:00401007 56 push esi
:00401008 57 push edi
:00401009 8D7DC0 lea edi, dword ptr [ebp-40]
:0040100C B910000000 mov ecx, 00000010
:00401011 B8CCCCCCCC mov eax, CCCCCCCC
:00401016 F3 repz
:00401017 AB stosd
:00401018 E813000000 call 00401030
:0040101D 5F pop edi
:0040101E 5E pop esi
:0040101F 5B pop ebx
:00401020 83C440 add esp, 00000040
:00401023 3BEC cmp ebp, esp
:00401025 E846000000 call 00401070
:0040102A 8BE5 mov esp, ebp
:0040102C 5D pop ebp
:0040102D C3 ret
:0040102E CC int 03
:0040102F CC int 03
//中间无内容省略。
* Referenced by a CALL at Address:
|:00401018
|
:00401030 55 push ebp
:00401031 8BEC mov ebp, esp
:00401033 83EC40 sub esp, 00000040
:00401036 53 push ebx
:00401037 56 push esi
:00401038 57 push edi
:00401039 8D7DC0 lea edi, dword ptr [ebp-40]
:0040103C B910000000 mov ecx, 00000010
:00401041 B8CCCCCCCC mov eax, CCCCCCCC
:00401046 F3 repz
:00401047 AB stosd
:00401048 68ECC04000 push 0040C0EC
:0040104D E85E000000 call 004010B0
:00401052 83C404 add esp, 00000004
:00401055 5F pop edi
:00401056 5E pop esi
:00401057 5B pop ebx
:00401058 83C440 add esp, 00000040
:0040105B 3BEC cmp ebp, esp
:0040105D E80E000000 call 00401070
:00401062 8BE5 mov esp, ebp
:00401064 5D pop ebp
:00401065 C3 ret
(*7*)看一下FOO.OBJ的内容:(由分析FOOOBJ.TXT中代码节的偏移为0x000003bf，从而用W32Dasm反编译。)
:00000000 55 push ebp
:00000001 8BEC mov ebp, esp
:00000003 83EC40 sub esp, 00000040
:00000006 53 push ebx
:00000007 56 push esi
:00000008 57 push edi
:00000009 8D7DC0 lea edi, dword ptr [ebp-40]
:0000000C B910000000 mov ecx, 00000010
:00000011 B8CCCCCCCC mov eax, CCCCCCCC
:00000016 F3 repz
:00000017 AB stosd
:00000018 6800000000 push 00000000
:0000001D E800000000 call 00000022
:00000022 83C404 add esp, 00000004
:00000025 5F pop edi
:00000026 5E pop esi
:00000027 5B pop ebx
:00000028 83C440 add esp, 00000040
:0000002B 3BEC cmp ebp, esp
:0000002D E800000000 call 00000032
:00000032 8BE5 mov esp, ebp
:00000034 5D pop ebp
:00000035 C3 ret
综上分析可知:连接器在整合各个编译单元(.obj)时，如上A.OBJ和FOO.OBJ已记录下需要调整的数据，
比如a.obj中的FOO函数位置，即
:00000018 E800000000 call 0000001D //Attention!!!
RAW DATA #3
00000000: 55 8B EC 83 EC 40 53 56 57 8D 7D C0 B9 10 00 00 U....@SVW.}.....
00000010: 00 B8 CC CC CC CC F3 AB E8 00 00 00 00 5F 5E 5B ............._^[
00000020: 83 C4 40 3B EC E8 00 00 00 00 8B E5 5D C3 ..@;........].
节后紧跟的RELOCATIONS #3
Symbol Symbol
Offset Type Applied To Index Name
-------- ------- ------------- ------ --------------
00000019 REL32 00000000 12 ?Foo@@YAXXZ (void __cdecl Foo(void))
在连接时，连接器整合代码节，将FOO.OBJ的代码节接在A.OBJ的代码节之后。如下:
:00401000 55 push ebp
....
:00401018 E813000000 call 00401030
....
:0040102D C3 ret
:0040102E CC int 03
:0040102F CC int 03
//中间无内容省略。
* Referenced by a CALL at Address:
|:00401018
|
:00401030 55 push ebp
....
:00401065 C3 ret
其中CALL 00401030中的00400000为代码优先载入基地址。
而E813000000中的13000000即为偏移值。事实上为00000013，这是INTEL CPU的特性
a peculiarity of Intel processors where numerical data is stored in
reverse order to character data.
To copy a 32 bit value (56 A7 00 FE) into the eax register, you will find the opcode, A1 (MOV EAX) followed by (FE 00 A7 56).
A1 FE 00 A7 56
从偏移00401018跳到00401030。如此可以得出指令为:E813000000
手工算法:
因为CALL指令本身占用5个字节(1个为CALL nmemonic(E8),另4个为偏移值)。
而0040101D-00401018=5所以偏移事实上应该从0040101D算起。
故00401030-0040101D=13
所以产生的CALL指令为E813000000
借助软件:
oPcodeR--由Cool McCool编写。非常好用。
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