Special Command—Unassembling code with u, ub and uf
When debugging sooner or later you will need to disassemble code to get a better understanding of that code.
By disassembling the code, you get the mnemonics translated from the 0s and 1s that constitute the binary code. It is a low level view of the code, but a higher level than seeing just numbers.
The commands syntaxes are:
u[b] [address]
u[b] [range]
u[b]
uf [options] <address>
Where options are:
/c - Displays only the call instructions in a routine.
/D - Creates linked callee names for navigation of the call graph.
/o - Sorts the display by address instead of by function offset.
/O - Creates linked call lines for accessing call information and creating breakpoints.
/i - Displays the number of instructions in a routine.
To demonstrate this command, let’s use this simple Visual C++ application that recursively calculates the Fibonacci from a specific number:
#include "stdafx.h"
using namespace std;
// Recursive function.
unsigned FiboRecursive(unsigned n, int nNum = 0)
{
if(n <= 1)
{
return n;
}
return FiboRecursive(n - 1, 1) + FiboRecursive(n - 2, 2);
}
int _tmain(int argc, _TCHAR* argv[])
{
cout << FiboRecursive(5) << endl;
return 0;
}
Let’s break the execution when the line from main() only is being executed, using a breakpoint for that.
Now let’s disassemble the eip register.
0:000> u @eip
Fibo!wmain+0x1e [c:\development\my tools\fibo\fibo\fibo.cpp @ 21]:
00a7145e 8bf4 mov esi,esp
00a71460 a19882a700 mov eax,dword ptr [Fibo!_imp_?endlstdYAAAV?$basic_ostreamDU?$char_traitsDstd (00a78298)]
00a71465 50 push eax
00a71466 6a00 push 0
00a71468 6a05 push 5
00a7146a e89bfbffff call Fibo!ILT+5(?FiboRecursiveYAIIHZ) (00a7100a)
00a7146f 83c408 add esp,8
00a71472 8bfc mov edi,esp
Using this approach we see the disassembled code starting from eip.
We can see the disassembled code that comes before eip using this approach:
0:000> ub @eip L8
Fibo!wmain+0x3 [c:\development\my tools\fibo\fibo\fibo.cpp @ 20]:
00a71443 81ecc0000000 sub esp,0C0h
00a71449 53 push ebx
00a7144a 56 push esi
00a7144b 57 push edi
00a7144c 8dbd40ffffff lea edi,[ebp-0C0h]
00a71452 b930000000 mov ecx,30h
00a71457 b8cccccccc mov eax,0CCCCCCCCh
00a7145c f3ab rep stos dword ptr es:[edi]
Keep in mind that b is for backward.
Let’s suppose you want to disassemble the entire function without having the work of finding the beginning of the function.
To accomplish that you use uf, and it automatically does that for you:
0:000> uf @eip
Fibo!wmain [c:\development\my tools\fibo\fibo\fibo.cpp @ 20]:
20 00a71440 55 push ebp
20 00a71441 8bec mov ebp,esp
20 00a71443 81ecc0000000 sub esp,0C0h
20 00a71449 53 push ebx
20 00a7144a 56 push esi
20 00a7144b 57 push edi
20 00a7144c 8dbd40ffffff lea edi,[ebp-0C0h]
20 00a71452 b930000000 mov ecx,30h
20 00a71457 b8cccccccc mov eax,0CCCCCCCCh
20 00a7145c f3ab rep stos dword ptr es:[edi]
21 00a7145e 8bf4 mov esi,esp
21 00a71460 a19882a700 mov eax,dword ptr [Fibo!_imp_?endlstdYAAAV?$basic_ostreamDU?$char_traitsDstd (00a78298)]
21 00a71465 50 push eax
21 00a71466 6a00 push 0
21 00a71468 6a05 push 5
21 00a7146a e89bfbffff call Fibo!ILT+5(?FiboRecursiveYAIIHZ) (00a7100a)
21 00a7146f 83c408 add esp,8
21 00a71472 8bfc mov edi,esp
21 00a71474 50 push eax
21 00a71475 8b0d9082a700 mov ecx,dword ptr [Fibo!_imp_?coutstd (00a78290)]
21 00a7147b ff159482a700 call dword ptr [Fibo!_imp_??6?$basic_ostreamDU?$char_traitsDstdstdQAEAAV01IZ (00a78294)]
21 00a71481 3bfc cmp edi,esp
21 00a71483 e8d1fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
21 00a71488 8bc8 mov ecx,eax
21 00a7148a ff159c82a700 call dword ptr [Fibo!_imp_??6?$basic_ostreamDU?$char_traitsDstdstdQAEAAV01P6AAAV01AAV01ZZ (00a7829c)]
21 00a71490 3bf4 cmp esi,esp
21 00a71492 e8c2fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
23 00a71497 33c0 xor eax,eax
24 00a71499 5f pop edi
24 00a7149a 5e pop esi
24 00a7149b 5b pop ebx
24 00a7149c 81c4c0000000 add esp,0C0h
24 00a714a2 3bec cmp ebp,esp
24 00a714a4 e8b0fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
24 00a714a9 8be5 mov esp,ebp
24 00a714ab 5d pop ebp
24 00a714ac c3 ret
Let’s see just the calls made by this function:
0:000> uf /c @eip
Fibo!wmain (00a71440) [c:\development\my tools\fibo\fibo\fibo.cpp @ 20]
Fibo!wmain+0x2a (00a7146a) [c:\development\my tools\fibo\fibo\fibo.cpp @ 21]:
call to Fibo!ILT+5(?FiboRecursiveYAIIHZ) (00a7100a)
Fibo!wmain+0x3b (00a7147b) [c:\development\my tools\fibo\fibo\fibo.cpp @ 21]:
call to MSVCP90D!std::basic_ostream<char,std::char_traits<char> >::operator<< (690fa700) [f:\dd\vctools\crt_bld\self_x86\crt\src\ostream @ 289]
Fibo!wmain+0x43 (00a71483) [c:\development\my tools\fibo\fibo\fibo.cpp @ 21]:
call to Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
Fibo!wmain+0x4a (00a7148a) [c:\development\my tools\fibo\fibo\fibo.cpp @ 21]:
call to MSVCP90D!std::basic_ostream<char,std::char_traits<char> >::operator<< (690f9f60) [f:\dd\vctools\crt_bld\self_x86\crt\src\ostream @ 171]
Fibo!wmain+0x52 (00a71492) [c:\development\my tools\fibo\fibo\fibo.cpp @ 21]:
call to Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
Fibo!wmain+0x64 (00a714a4) [c:\development\my tools\fibo\fibo\fibo.cpp @ 24]:
call to Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
Creates linked callee names from the same function:
0:000> uf /D @eip
Fibo!wmain [c:\development\my tools\fibo\fibo\fibo.cpp @ 20]:
20 00a71440 55 push ebp
20 00a71441 8bec mov ebp,esp
20 00a71443 81ecc0000000 sub esp,0C0h
20 00a71449 53 push ebx
20 00a7144a 56 push esi
20 00a7144b 57 push edi
20 00a7144c 8dbd40ffffff lea edi,[ebp-0C0h]
20 00a71452 b930000000 mov ecx,30h
20 00a71457 b8cccccccc mov eax,0CCCCCCCCh
20 00a7145c f3ab rep stos dword ptr es:[edi]
21 00a7145e 8bf4 mov esi,esp
21 00a71460 a19882a700 mov eax,dword ptr [Fibo!_imp_?endlstdYAAAV?$basic_ostreamDU?$char_traitsDstd (00a78298)]
21 00a71465 50 push eax
21 00a71466 6a00 push 0
21 00a71468 6a05 push 5
21 00a7146a e89bfbffff call Fibo!ILT+5(?FiboRecursiveYAIIHZ) (00a7100a)
21 00a7146f 83c408 add esp,8
21 00a71472 8bfc mov edi,esp
21 00a71474 50 push eax
21 00a71475 8b0d9082a700 mov ecx,dword ptr [Fibo!_imp_?coutstd (00a78290)]
21 00a7147b ff159482a700 call dword ptr [Fibo!_imp_??6?$basic_ostreamDU?$char_traitsDstdstdQAEAAV01IZ (00a78294)]
21 00a71481 3bfc cmp edi,esp
21 00a71483 e8d1fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
21 00a71488 8bc8 mov ecx,eax
21 00a7148a ff159c82a700 call dword ptr [Fibo!_imp_??6?$basic_ostreamDU?$char_traitsDstdstdQAEAAV01P6AAAV01AAV01ZZ (00a7829c)]
21 00a71490 3bf4 cmp esi,esp
21 00a71492 e8c2fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
23 00a71497 33c0 xor eax,eax
24 00a71499 5f pop edi
24 00a7149a 5e pop esi
24 00a7149b 5b pop ebx
24 00a7149c 81c4c0000000 add esp,0C0h
24 00a714a2 3bec cmp ebp,esp
24 00a714a4 e8b0fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
24 00a714a9 8be5 mov esp,ebp
24 00a714ab 5d pop ebp
24 00a714ac c3 ret
Sorts the output by address:
0:000> uf /o @eip
Fibo!wmain [c:\development\my tools\fibo\fibo\fibo.cpp @ 20]:
20 00a71440 55 push ebp
20 00a71441 8bec mov ebp,esp
20 00a71443 81ecc0000000 sub esp,0C0h
20 00a71449 53 push ebx
20 00a7144a 56 push esi
20 00a7144b 57 push edi
20 00a7144c 8dbd40ffffff lea edi,[ebp-0C0h]
20 00a71452 b930000000 mov ecx,30h
20 00a71457 b8cccccccc mov eax,0CCCCCCCCh
20 00a7145c f3ab rep stos dword ptr es:[edi]
21 00a7145e 8bf4 mov esi,esp
21 00a71460 a19882a700 mov eax,dword ptr [Fibo!_imp_?endlstdYAAAV?$basic_ostreamDU?$char_traitsDstd (00a78298)]
21 00a71465 50 push eax
21 00a71466 6a00 push 0
21 00a71468 6a05 push 5
21 00a7146a e89bfbffff call Fibo!ILT+5(?FiboRecursiveYAIIHZ) (00a7100a)
21 00a7146f 83c408 add esp,8
21 00a71472 8bfc mov edi,esp
21 00a71474 50 push eax
21 00a71475 8b0d9082a700 mov ecx,dword ptr [Fibo!_imp_?coutstd (00a78290)]
21 00a7147b ff159482a700 call dword ptr [Fibo!_imp_??6?$basic_ostreamDU?$char_traitsDstdstdQAEAAV01IZ (00a78294)]
21 00a71481 3bfc cmp edi,esp
21 00a71483 e8d1fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
21 00a71488 8bc8 mov ecx,eax
21 00a7148a ff159c82a700 call dword ptr [Fibo!_imp_??6?$basic_ostreamDU?$char_traitsDstdstdQAEAAV01P6AAAV01AAV01ZZ (00a7829c)]
21 00a71490 3bf4 cmp esi,esp
21 00a71492 e8c2fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
23 00a71497 33c0 xor eax,eax
24 00a71499 5f pop edi
24 00a7149a 5e pop esi
24 00a7149b 5b pop ebx
24 00a7149c 81c4c0000000 add esp,0C0h
24 00a714a2 3bec cmp ebp,esp
24 00a714a4 e8b0fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
24 00a714a9 8be5 mov esp,ebp
24 00a714ab 5d pop ebp
24 00a714ac c3 ret
Creates linked call lines for accessing call information and creating breakpoints:
0:000> uf /O @eip
Fibo!wmain [c:\development\my tools\fibo\fibo\fibo.cpp @ 20]:
20 00a71440 55 push ebp
20 00a71441 8bec mov ebp,esp
20 00a71443 81ecc0000000 sub esp,0C0h
20 00a71449 53 push ebx
20 00a7144a 56 push esi
20 00a7144b 57 push edi
20 00a7144c 8dbd40ffffff lea edi,[ebp-0C0h]
20 00a71452 b930000000 mov ecx,30h
20 00a71457 b8cccccccc mov eax,0CCCCCCCCh
20 00a7145c f3ab rep stos dword ptr es:[edi]
21 00a7145e 8bf4 mov esi,esp
21 00a71460 a19882a700 mov eax,dword ptr [Fibo!_imp_?endlstdYAAAV?$basic_ostreamDU?$char_traitsDstd (00a78298)]
21 00a71465 50 push eax
21 00a71466 6a00 push 0
21 00a71468 6a05 push 5
21 00a7146a e89bfbffff call Fibo!ILT+5(?FiboRecursiveYAIIHZ) (00a7100a)
21 00a7146f 83c408 add esp,8
21 00a71472 8bfc mov edi,esp
21 00a71474 50 push eax
21 00a71475 8b0d9082a700 mov ecx,dword ptr [Fibo!_imp_?coutstd (00a78290)]
21 00a7147b ff159482a700 call dword ptr [Fibo!_imp_??6?$basic_ostreamDU?$char_traitsDstdstdQAEAAV01IZ (00a78294)]
21 00a71481 3bfc cmp edi,esp
21 00a71483 e8d1fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
21 00a71488 8bc8 mov ecx,eax
21 00a7148a ff159c82a700 call dword ptr [Fibo!_imp_??6?$basic_ostreamDU?$char_traitsDstdstdQAEAAV01P6AAAV01AAV01ZZ (00a7829c)]
21 00a71490 3bf4 cmp esi,esp
21 00a71492 e8c2fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
23 00a71497 33c0 xor eax,eax
24 00a71499 5f pop edi
24 00a7149a 5e pop esi
24 00a7149b 5b pop ebx
24 00a7149c 81c4c0000000 add esp,0C0h
24 00a714a2 3bec cmp ebp,esp
24 00a714a4 e8b0fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
24 00a714a9 8be5 mov esp,ebp
24 00a714ab 5d pop ebp
24 00a714ac c3 ret
Displays the number of instructions in a routine:
0:000> uf /i @eip
37 instructions scanned
Fibo!wmain [c:\development\my tools\fibo\fibo\fibo.cpp @ 20]:
20 00a71440 55 push ebp
20 00a71441 8bec mov ebp,esp
20 00a71443 81ecc0000000 sub esp,0C0h
20 00a71449 53 push ebx
20 00a7144a 56 push esi
20 00a7144b 57 push edi
20 00a7144c 8dbd40ffffff lea edi,[ebp-0C0h]
20 00a71452 b930000000 mov ecx,30h
20 00a71457 b8cccccccc mov eax,0CCCCCCCCh
20 00a7145c f3ab rep stos dword ptr es:[edi]
21 00a7145e 8bf4 mov esi,esp
21 00a71460 a19882a700 mov eax,dword ptr [Fibo!_imp_?endlstdYAAAV?$basic_ostreamDU?$char_traitsDstd (00a78298)]
21 00a71465 50 push eax
21 00a71466 6a00 push 0
21 00a71468 6a05 push 5
21 00a7146a e89bfbffff call Fibo!ILT+5(?FiboRecursiveYAIIHZ) (00a7100a)
21 00a7146f 83c408 add esp,8
21 00a71472 8bfc mov edi,esp
21 00a71474 50 push eax
21 00a71475 8b0d9082a700 mov ecx,dword ptr [Fibo!_imp_?coutstd (00a78290)]
21 00a7147b ff159482a700 call dword ptr [Fibo!_imp_??6?$basic_ostreamDU?$char_traitsDstdstdQAEAAV01IZ (00a78294)]
21 00a71481 3bfc cmp edi,esp
21 00a71483 e8d1fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
21 00a71488 8bc8 mov ecx,eax
21 00a7148a ff159c82a700 call dword ptr [Fibo!_imp_??6?$basic_ostreamDU?$char_traitsDstdstdQAEAAV01P6AAAV01AAV01ZZ (00a7829c)]
21 00a71490 3bf4 cmp esi,esp
21 00a71492 e8c2fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
23 00a71497 33c0 xor eax,eax
24 00a71499 5f pop edi
24 00a7149a 5e pop esi
24 00a7149b 5b pop ebx
24 00a7149c 81c4c0000000 add esp,0C0h
24 00a714a2 3bec cmp ebp,esp
24 00a714a4 e8b0fcffff call Fibo!ILT+340(__RTC_CheckEsp) (00a71159)
24 00a714a9 8be5 mov esp,ebp
24 00a714ab 5d pop ebp
24 00a714ac c3 ret
Comments
Anonymous
November 09, 2009
Hi, The call stacks for uf @eip uf /D @eip uf /O @eip all look identical to me (I'm too lazy to do actual diffs). Am I missing something? thanks, MarcAnonymous
November 09, 2009
Marc you're right and thanks for mentioning this! The problem is that the output has hyperlinks (DML) so when I copied and pasted it, it didn't work! If you try the commands you're going to see that some of them use hyperlinks.