Special Command—Using # to Find Patterns of Assembly Instructions
Sometimes you need to look for patterns of disassembled code. You can browse the disassembled code and manually look for a specific pattern, or you can use a command to automate it. The # command does that.
# [Pattern] [Address [L Size ]]
Parameters:
Pattern - Specifies the pattern to search for in the disassembly code. If you have previously used the # command and you omit Pattern, the command reuses the most recently used pattern.
Address - Specifies the address where the search begins.
Size - Specifies the number of instructions to search. If you omit Size, the search continues until the first match occurs.
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 only line from main() is being executed, using a breakpoint for that.
Now let’s disassemble the eip register.
0:000> uf @eip
Fibo!wmain [c:\development\my tools\book\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
Using the command below we’re going to display the first occurrence of ret.
0:000> # ret 00a71440
Fibo!wmain+0x6c [c:\development\my tools\book\fibo\fibo\fibo.cpp @ 24]:
00a714ac c3 ret
Looking for another pattern:
0:000> # call*Fibo!ILT 00a71440
Fibo!wmain+0x2a [c:\development\my tools\book\fibo\fibo\fibo.cpp @ 21]:
00a7146a e89bfbffff call Fibo!ILT+5(?FiboRecursiveYAIIHZ) (00a7100a)
Now let’s look for push instructions in a specific module/executable:
0:000> lm
start end module name
00a60000 00a7b000 Fibo C (private pdb symbols) C:\development\My Tools\Book\Fibo\Debug\Fibo.pdb
67350000 67473000 MSVCR90D (deferred)
690c0000 69197000 MSVCP90D (private pdb symbols) c:\publicsymbols\msvcp90d.i386.pdb\7B1C9137C0074A0E921BE874ADF944191\msvcp90d.i386.pdb
75e00000 75e44000 KERNELBASE (deferred)
75eb0000 75fb0000 kernel32 (deferred)
776c0000 77840000 ntdll (pdb symbols) c:\publicsymbols\wntdll.pdb\E06BEA155E9748BEA818E2D0DD2FED952\wntdll.pdb
0:000> # push 00a60000
Fibo!__ImageBase+0x40:
00a60040 0e push cs
As you can see, when looking for patterns in disassembled code “ # ” is the way to go! Here is an script that automates the process.
Tip: Most commands mentioned in the Special Commands section of this blog have a script that uses it and could serve as another example of using the command. Also there’re scripts that use commands from the SOS.DLL extension to debug managed code.
Check out the WinDbg Scripts and PowerDbg Scripts section.