MASM32 Overwriting a file using assembly

Question

I'm currently doing an encryption in some of my files as an exercise. However, I'm having a trouble in overwriting a .txt file and having a hard time identifying what went wrong. Below is the part of the code I've been working on which I think has the problem.

  push 0
  push FILE_ATTRIBUTE_NORMAL
  push OPEN_EXISTING
  push 0
  push 0
  push FILE_READ_DATA
  push offset fData.cFileName
  call CreateFile

  mov hndl, eax

  push 0
  push hndl
  call GetFileSize

  mov fSize, eax

  push 0
  push offset bfrLen
  push fSize
  push offset bfr
  push hndl
  call ReadFile

  push hndl
  call CloseHandle

  lea esi, bfr
  mov al, [esi]
  cmp al, 7fh
  jg skip
  encrypt:
    mov al, [esi]
    xor al, 0ffh
    mov [esi], al
    inc esi
    mov al, [esi]
    cmp al, 00h
    jne encrypt

  push 0
  push FILE_ATTRIBUTE_NORMAL
  push CREATE_ALWAYS
  push 0
  push 0
  push FILE_WRITE_DATA
  push offset file
  call CreateFile

  mov hndl, eax

  push offset bfr
  call lstrlen

  push 0
  push offset bfrLen
  push fSize
  push offset bfr
  push hndl
  call WriteFile

  push hndl
  call CloseHandle

  skip:
    ret

Review my code, guys! Thanks in advance.

Use a debugger and a syscall tracer. Add error handling to your program. Also, comment your code especially if you want others to help. Give more details than "having a trouble", describe what happens. That said, `push WriteFile` is very suspicious, didn't you mean `call WriteFile`? — Jester, Jun 08 '17 at 12:47
I didn't notice that. Thanks. However the code still doesn't work. — Pentagon, Jun 08 '17 at 12:49
Also, your `jg skip` is suspicious, it skips everything including writing out the file. Hard to tell whether you want that or not, with the limited information you provided. "doesn't work" is still not better than "having a trouble". See also [ask]. — Jester, Jun 08 '17 at 12:52
I'm sure it does "work", it just probably does not do what you did want or expect. Similar code usually doesn't stop CPU from executing it and it will do **something**. Often by using debugger you can overview what is that "something". — Ped7g, Jun 08 '17 at 12:55
_"having a hard time identifying what went wrong"_ Check every return value, and call `GetLastError` if a WinApi function returns something other than "success". — Michael, Jun 08 '17 at 14:30
`CreateFile` can fail for tons of reasons, like wrong file name, wrong path (extremely common problem), wrong permissions, already open, etc., etc. And you don't check for any of those! — Bo Persson, Jun 08 '17 at 14:43

Fifoernik · Answer 1 · 2017-06-08T13:02:30.293

1

mov al, [esi]
cmp al, 7fh
jg skip

This is a useless test! Nothing can ever be greater (signed byte) than 127.
Perhaps you meant to test for the above (unsigned byte) condition?

cmp byte [esi], 7Fh
ja  skip             ;Skip if from 128 to 255

Just a thought. Maybe this test must be repeated with each iteration? We can't possibly know these details of your task.

edited Jun 08 '17 at 13:02

answered Jun 08 '17 at 12:56

Fifoernik

9,779
1
21
27

MASM32 Overwriting a file using assembly

1 Answers1