Access violation at address 0000 when reading file

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Hello guys,
I have problem with reading or writing files in C++ and MASM32. The problem is when calling the ReadFile or WriteFile the program give an exception saying "Access violation at address 0000".

This is the code in MASM32:

C++

        LOCAL	Kernel32:DWORD
	LOCAL	CreateFileA:DWORD
	LOCAL	GetFileSize:DWORD
	LOCAL	ReadFile:DWORD
	LOCAL	pBytesRead:DWORD
	LOCAL	BytesToRead:DWORD
	LOCAL	hFile:DWORD

	;-----------------------------------------
	;Getting Kernel Imagebase
	;-----------------------------------------
	;kernel32.dll = 2511EF2C
	PUSH	02511EF2Ch
	CALL	find_dll
	
	MOV	Kernel32, EAX

	;-----------------------------------------
	;Find required APIs
	;-----------------------------------------
	;CreateFileA = 0CF2006EAh, GetFileSize = 06D61AB47h, ReadFile = 024EF6F37h, WriteFile = 0317EB0D7h
	
	PUSH	0CF2006EAh
	PUSH	Kernel32
	CALL	find_api

	MOV	CreateFileA, EAX

	PUSH	06D61AB47h
	PUSH	Kernel32
	CALL	find_api

	MOV	GetFileSize, EAX

	PUSH	024EF6F37h
	PUSH	Kernel32
	CALL	find_api

	MOV		ReadFile, EAX

	PUSH	NULL                                ; /hTemplateFile = NULL
	PUSH	FILE_ATTRIBUTE_NORMAL               ; |Attributes = FILE_ATTRIBUTE_NORMAL
	PUSH	OPEN_EXISTING                       ; |Mode = OPEN_EXISTING
	PUSH	0                                   ; |pSecurity = NULL
	PUSH	FILE_SHARE_READ + FILE_SHARE_WRITE  ; |ShareMode = FILE_SHARE_READ|FILE_SHARE_WRITE
	PUSH	GENERIC_ALL             ; |Access = GENERIC_ALL
	PUSH	FileName                ; |FileName = "*.exe"
	CALL	CreateFileA		; \CreateFileA

	MOV	hFile,EAX

	PUSH	0                       ; /pFileSizeHigh = NULL
	PUSH	hFile                   ; |hFile
	CALL	GetFileSize		; \GetFileSize

	CMP	EAX, 0h
	JZ	Exit

	MOV	BytesToRead, 10h

	PUSH	0			; /pOverlapped = NULL
        PUSH	pBytesRead		; |pBytesRead = ?
	PUSH	BytesToRead		; |BytesToRead
	PUSH	OFFSET Buffer		; |Buffer
	PUSH	hFile			; |hFile
	CALL	ReadFile		; \ReadFile
Exit:

In C:

C++

HANDLE	hFile;
	LPCVOID	Buffer = "Hello";	
	DWORD	BytesWritten = 0;

	hFile =	CreateFile("rw.txt",FILE_APPEND_DATA, FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

	if(!WriteFile(hFile, Buffer, 5, &BytesWritten, NULL))
		MessageBox(NULL, "Can not write!", "Error", MB_OK);

	CloseHandle(hFile);

When debug it in Ollydbg it stops right here:

C++

757F85EA   8907             MOV DWORD PTR DS:[EDI],EAX
EDI = 00000000
EAX = 00000055

And give me the error (described above) because EDI is 000000.

Why this error occurred and what is the solution to fix the problem?

Posted 5-May-13 1:36am

Rasool Ahmed

Updated 5-May-13 23:35pm

v3

Add a Solution

4 solutions

Solution 1

i guess you havent open an file. Did you check the return codes of the API.

Try it first in C and "show your code" ;-)

Posted 5-May-13 21:13pm

KarstenK

Comments

Rasool Ahmed 6-May-13 3:16am

I forgot to tell you that the writefile succeeded, the file have data after calling writefile but stops on that exception. I tried on C too and the problem is same. here is the code of the C:
HANDLE hFile;
LPCVOID Buffer = "Hello";
DWORD BytesWritten = 0;

hFile = CreateFile("rw.txt",FILE_APPEND_DATA, FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

if(WriteFile(hFile, Buffer, 5, &BytesWritten, NULL))
MessageBox(NULL, "Can not write!", "Error", MB_OK);

CloseHandle(hFile);

Volynsky Alex 6-May-13 3:47am

An access violation at anywhere near adress '00000000' indicates a null pointer access. You're using something before it's ever been created....

Rasool Ahmed 6-May-13 3:57am

May be the problem in BytesWritten!!! what you think?

KarstenK 6-May-13 4:05am

NO WAY.

The problem ist your buggy code ;-)

Rasool Ahmed 6-May-13 4:07am

OllyDbg shows me that EDI is NULL, why is that??

I searched alot in google but nothing found.

KarstenK 6-May-13 4:13am

I read this PUSH FileName ; |FileName = "*.exe"

:mad:

Before you have written clear code in C it is useless to ask me again.

"Have a lot of fun" ;-)

Rasool Ahmed 6-May-13 4:20am

What you mean??
Please explain to me if you have.

Volynsky Alex 6-May-13 4:33am

Maybe try to do initialization the variable, i.e. :
DWORD BytesWritten = 0;

Volynsky Alex 6-May-13 5:08am

Sorry,
I did not notice that you already did initialization

Rasool Ahmed 6-May-13 4:08am

This is rare.

Solution 2

at first i would use a complete path for opening the file and think about all flags.

What is the value of BytesWritten?

AND CHECK ALL API-RETURN VALUES :mad:

Posted 5-May-13 21:47pm

KarstenK

Comments

Rasool Ahmed 6-May-13 4:17am

The function won't return value because it stops before done its work.

KarstenK 6-May-13 4:29am

If this code isnt returning, so the error is in the input values :mad:

Is it your first piece of code you are writing?

Rasool Ahmed 6-May-13 5:07am

your saying is like Volynsky Alex saied. I will try and if I found the solution I will post it here. Thanx KarstenK

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Volynsky Alex · Accepted Answer · 2013-05-05T23:15:00

Try to use something like:

C++

TCHAR Buffer[256] = "Hello";
    ....
    hFile = CreateFile("rw.txt", GENERIC_WRITE, 0, NULL,CREATE_ALWAYS, 
                       FILE_ATTRIBUTE_NORMAL, NULL);
 
    if (hFile == INVALID_HANDLE_VALUE) {
        printf("ERROR %x \n",GetLastError());
        return 2;
    }
    

 
   if(WriteFile(hFile, Buffer, 256, NULL, NULL ))
   {
   }
   else
   {
     printf("ERROR %x \n",GetLastError());
   }

Rasool Ahmed · Accepted Answer · 2013-05-05T23:36:00

I finally found the problem, the problem was me :)
The problem is when I call WriteFile or ReadFile the function will write on BytesRead or BytesWritten which means that I must give the effective address of the variable.
In MASM32:

C++

    PUSH    0           ; /pOverlapped = NULL
    PUSH    pBytesRead      ; |pBytesRead = ? //I gave the value of pBytesRead which is NULL
PUSH    BytesToRead     ; |BytesToRead
PUSH    OFFSET Buffer       ; |Buffer
PUSH    hFile           ; |hFile
CALL    ReadFile        ; \ReadFile

The correct code is:

C++

    PUSH    0           ; /pOverlapped = NULL
    LEA EAX, pBytesRead
    PUSH    EAX             ;|pBytesRead = ?
    PUSH    BytesToRead     ; |BytesToRead
PUSH    OFFSET Buffer       ; |Buffer
PUSH    hFile           ; |hFile
CALL    ReadFile        ; \ReadFile

In C code:

C++

HANDLE  hFile;
    LPCVOID Buffer = "Hello";
    DWORD   BytesWritten = 0;

    hFile = CreateFile("rw.txt",FILE_APPEND_DATA, FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

    if(!WriteFile(hFile, Buffer, 5, &BytesWritten, NULL))
        MessageBox(NULL, "Can not write!", "Error", MB_OK);

    CloseHandle(hFile)