Cannot load DLL written in FASM

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abc101

Joined: 31 Jan 2011
Posts: 5

abc101 31 Jan 2011, 11:16

I'm encountering a problem. I wrote simple DLL in FASM and then tried to load it in C++.NET, but an error occured (something like "The program format is incorrect"). Can anyone help me? Here's the code:

Code:

 format PE DLL
entry DllEntryPoint

include "win32a.inc"

section ".code" code readable executable

proc DllEntryPoint hDll:DWORD, reason:DWORD, reserved:DWORD
  mov eax,TRUE
  ret
endp

proc SampleProc
  mov eax, 5
  ret
endp

section ".edata" export readable writable

export "Example.DLL",\
  SampleProc,"SampleProc"

section '.reloc' fixups data discardable

31 Jan 2011, 11:16

MSWarrior

Joined: 21 Mar 2008
Posts: 11

MSWarrior 31 Jan 2011, 11:47

I think that you must have at least one function imported for the DLL to work.
Try to create import section with just one function and see if it works.

31 Jan 2011, 11:47

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 19869
Location: In your JS exploiting you and your system

revolution 31 Jan 2011, 11:49

The DLL opens fine with OllyDbg under WinXPSP2. Perhaps something is different/wrong with C++.NET?

31 Jan 2011, 11:49

abc101

Joined: 31 Jan 2011
Posts: 5

abc101 31 Jan 2011, 11:55

Quote:

I think that you must have at least one function imported for the DLL to work.
Try to create import section with just one function and see if it works.

Well, when I added an API call, everything became OK:

Code:

format PE DLL
entry DllEntryPoint

include "win32a.inc"

section ".code" code readable executable

proc DllEntryPoint hDll:DWORD, reason:DWORD, reserved:DWORD
  mov eax,TRUE
  ret
endp

proc SampleProc
  invoke GetLastError
  mov eax, 5
  ret
endp

section ".idata" import readable writable

library kernel32,"KERNEL32.DLL",\
user32,"USER32.DLL"

include "api\kernel32.inc"
include "api\user32.inc"

section ".edata" export readable writable

export "Example.DLL",\
  SampleProc,"SampleProc"

section '.reloc' fixups data discardable

But this "invoke GetLastError" is senseless and it makes the program slow. Sad

31 Jan 2011, 11:55

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 19869
Location: In your JS exploiting you and your system

revolution 31 Jan 2011, 11:59

abc101 wrote:

But this "invoke GetLastError" is senseless ...

Agreed. Blame C++.NET.

31 Jan 2011, 11:59

abc101

Joined: 31 Jan 2011
Posts: 5

abc101 31 Jan 2011, 11:59

Quote:

The DLL opens fine with OllyDbg under WinXPSP2. Perhaps something is different/wrong with C++.NET?

C++ works correctly. I tried to write the same DLL in MASM and C++ loaded it successfully.

31 Jan 2011, 11:59

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 19869
Location: In your JS exploiting you and your system

revolution 31 Jan 2011, 12:00

abc101 wrote:

C++ works correctly. I tried to write the same DLL in MASM and C++ loaded it successfully.

Maybe MASM puts an import section in the binary? Check the binary to see.

31 Jan 2011, 12:00

abc101

Joined: 31 Jan 2011
Posts: 5

abc101 31 Jan 2011, 12:05

Quote:

Maybe MASM puts an import section in the binary? Check the binary to see.

There's 2 sections only in the MASM DLL: ".text" and ".rdata". No ".reloc", ".idata" or something else.

31 Jan 2011, 12:05

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 19869
Location: In your JS exploiting you and your system

revolution 31 Jan 2011, 12:09

I don't know the reason. But if you are worried about speed problems you can do this:

Code:

;...
proc DllEntryPoint hDll:DWORD, reason:DWORD, reserved:DWORD
  mov eax,TRUE
  ret
  invoke GetLastError
endp

proc SampleProc
  mov eax, 5
  ret
endp
;...

31 Jan 2011, 12:09

abc101

Joined: 31 Jan 2011
Posts: 5

abc101 31 Jan 2011, 12:25

Quote:

;...
proc DllEntryPoint hDll:DWORD, reason:DWORD, reserved:DWORD
mov eax,TRUE
ret
invoke GetLastError
endp

It really works. Smile

Maybe it's not exactly what I wanted, but thanks anyway. Smile

31 Jan 2011, 12:25

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