FIX: DAO 3.6 overruns buffers in the GetRowsEx method (236056)

MORE INFORMATION

Steps to reproduce the behavior

1. Open the GetRows sample from the DAOSDK, which comes with Windows platform SDK.
2. Go to Project Settings under the Link tab and change the .lib file used from Ddao35d.lib to Ddao36d.lib to utilize DAO version 3.6 instead of DAO version 3.5.
   
   Note In order to get Ddao36d.lib and Ddao36d.dll, you will have to compile and create them by using DAO workspace for DAO 3.6. This workspace comes with Visual Studio Service Pack 3. Information on how to get Visual Studio SP3 is given at the end of this article.
3. In the Getrdlg.h file change the number of records fetched from ten to four:

   #define MAX_EMP_REC		4 // Arbitrary maximum
   					

4. Go to the Getrdlg.cpp file. The EMP typedef is located just after the definition of the DoGetRows() function. In EMP typedef structure, change the size of array strFirstName from twenty to eight. The correct code is as follows:

   typedef struct
   	{
   	LONG	lEmpId;
   	TCHAR	*strLastName;
   	TCHAR	strFirstName[8];
   	} EMP, *LPEMP ;
   					

5. Also, make a change in the Bindings structure, which is located right after EMP structure. Change the size of strFirstName from twenty to eight. This is the correct code:

   // Employee table binding 
   DAORSETBINDING	Bindings[] = 
   {
   //Index Type		Column		Type			Offset						Size
   {dbBindIndexINT,	EMP_ID,		dbBindI4,		offsetof(EMP,lEmpId),		sizeof(LONG)},
   {dbBindIndexINT,	EMP_LNAME,	dbBindLPSTRING,	offsetof(EMP,strLastName),	sizeof(TCHAR *)},
   {dbBindIndexINT,	EMP_FNAME,	dbBindSTRING,	offsetof(EMP,strFirstName),	sizeof(TCHAR) * 8}
   };
   					

6. In the Getrdlg.cpp file change the size of the character buffer that will hold the variable size data from MAX_EMP_REC * 15 to 33. The buffer is declared in the beginning of the DoGetRowsEx() function.

   TCHAR			pBuf[33];		// allow average of 15 chars/name
   					

7. Compile and run the code. Select the Employee.mdb file (located in \mssdk\samples\DBMsg\DataAccess\DAO) after clicking Execute. A Debug error dialog will appear with the above described message.
8. To avoid the debug error, comment out the line that deletes the pEmpRows array at the end of the DoGetRowsEx function:

   //delete [] pEmpRows;
   					

9. Recompile the code and run it again. The fourth name will be truncated in the dialog box.

Workaround

When retrieving the character data directly with the dbBindSTRING type, allocate twice as much space than the maximum size of the field in the custom structure. Change the size of strFirstName to twenty in both structures as follows:

typedef struct
	{
	LONG	lEmpId;
	TCHAR	*strLastName;
	TCHAR	strFirstName[20];
	} EMP, *LPEMP ;

// Employee table binding 
DAORSETBINDING	Bindings[] = 
{
//Index Type		Column		Type			Offset						Size
{dbBindIndexINT,	EMP_ID,		dbBindI4,		offsetof(EMP,lEmpId),		sizeof(LONG)},
{dbBindIndexINT,	EMP_LNAME,	dbBindLPSTRING,	offsetof(EMP,strLastName),	sizeof(TCHAR *)},
{dbBindIndexINT,	EMP_FNAME,	dbBindSTRING,	offsetof(EMP,strFirstName),	sizeof(TCHAR) * 20}
};
				

When retrieving the character data indirectly into a buffer for variable length data with dbBindLPSTRING, allocate the buffer big enough to hold an extra record and two extra Unicode null characters (4 bytes). So make this change to the size of pBuf character buffer:

TCHAR			pBuf[(MAX_EMP_REC + 1) * 20 + 4];// allow average of 15 chars/name
				

Recompile and run the code.