In this article I will describe how to programmatically translate an instruction address from a Win32 executable to a source line. The described approach uses the Microsoft debugger engine dbgeng.dll. Therefore you should have it installed. If you don’t have it, you can download and install the Windows debugger WinDbg which will install dbgeng.dll.

Requirements

To use this approach you will need to have installed windows debugger engine – dbgeng.dll. If you don’t have it, install WinDbg debugger. This will install dbgeng.dll

Approach

Many debuggers support debugging at source code level. The debugger is capable to provide debugging at the source level by translating instruction addresses to lines in source code files. In our approach we use the same mechanisms that debuggers use. This particular solution, to translate an instruction address to a source line, uses functionality provided by the windows debugger engine dbgeng.dll.

In overview, we create a process from the Win32 executable file from which we want to translate instruction addresses. Then we attach the debugger engine to the created process. To create a process and attach the debugger to it we use CreateProcessAndAttach function. When the debugger is attached, we call function GetLineByOffset to do the actual translation. After we finish with the translation task we terminate the process with the function TerminateCurrentProcess. Besides these procedures there are few more things that are necessary to do which I explain in the implementation.

Implementation

First we create objects that implement the following COM interfaces: IDebugControl , IDebugClient, IDebugSymbols. To create these objects we use function DebugCreate.

static IDebugClient5 *dbgClient5 = NULL;
static IDebugSymbols *dbgSymbols = NULL;
static IDebugControl *dbgControl = NULL;
...
DebugCreate(__uuidof(IDebugControl), (void**) & dbgControl);
DebugCreate(__uuidof(IDebugClient), (void **) & dbgClient5);
DebugCreate(__uuidof(IDebugSymbols), (void **) & dbgSymbols);

Before crating the process from the executable we set the debugger engine filters so that the target process breaks into the debugger immediately after it is created. To set the debugger filters we use function SetSpecificFilterParameters.

DEBUG_SPECIFIC_FILTER_PARAMETERS filter[10];
for (int i = 0; i < 10; i++) {
	filter[i].ExecutionOption = DEBUG_FILTER_BREAK;
	filter[i].ContinueOption = DEBUG_FILTER_GO_HANDLED;
	filter[i].TextSize = 0;
	filter[i].CommandSize = 0;
	filter[i].ArgumentSize = 0;
}

dbgControl->SetSpecificFilterParameters(0, 10, filter);

Using function CreateProcessAndAttach we create a windows debug process and attach the debugger to it.

ULONG64 server            = 0;
PSTR    commandLine       = executableName;
ULONG   processId         = 0;
ULONG   attachFlags       = 0; 

dbgClient5->CreateProcessAndAttach(
	server,
	commandLine,
	DEBUG_PROCESS,
	processId,
	attachFlags);

To translate the instruction address to a source line we call function GetLineByOffset.

ULONG64 offset = instrAddress;
ULONG fileNameBufferSize = MAX_FILE_NAME_SIZE;
memset(sourceInfo->fileName, 0, fileNameBufferSize);
sourceInfo->lineNo = 0;
sourceInfo->fileSize = 0;
sourceInfo->displacement = 0;

HRESULT isOk = dbgSymbols->GetLineByOffset(
	offset,
	&sourceInfo->lineNo,
	sourceInfo->fileName,
	fileNameBufferSize,
	&sourceInfo->fileSize,
	&sourceInfo->displacement);

If we want to translate multiple addresses, we have to call function GetLineByOffset multiple times without repeating the earlier operations. After we finish with the translation we detatch the debugger from the process by terminating the process with function TerminateCurrentProcess.

HRESULT isOk = dbgClient5->TerminateCurrentProcess();

Download

The code complete source code is available for download from InstructionToSourceLine.zip. Note that the instruction address provided as input should be a decimal number but not hexadecimal.

C# Version

There is also a C# implementation of this tool that you can download from InstructionToSourceLineCSharp.zip. This C# implementation consists of 2 parts – DbgEngManaged and InstructionToSourceLineCSharp. DbgEngManaged is a managed c++ library wrapper for the debugger engine DbgEng.dll. InstructionToSourceLineCSharp is a C# console application that references DbgEngManaged library. Note that the error handling here is not done properly so don't judge me for it The provided code here is something that I have quickly prototyped and decided to share for those who for some reason need to use the debugger engine. It can also serve as an example of using native and managed code together. 

Programming, windows address, c++, csharp, DbgEng, instruction, interop, managed, native, source, source line, translate, win32, WinDbg, windows

In this post I would like to explain with a working example the sematics of the GetOffsetByName from IDebugSymbols interface in WinDbg Extension API.

1. You can use this method to return the offset of a symbol that is in the data segment but not on the heap. Data segment is the place in the program where global variables are stored. If you try to get the offset of a field of a class instance (object) this will not work (e.g. instanceOfMyClass->fieldName).
2. The value that this method returns as offset is address of the global variable i.e. &var. Lets explain with an example.

On the running example I assuming

class MyClass {

static int staticField.

};

myext.getoffsetbyname is the custom extension that implements GetOffsetByName

Then if we un the following commands in WinDbg command window

0:000:x86> x simple!MyClass::staticField

0×004d81e4 simple!MyClass::staticField = 0×00000005

–

0:000:x86> ?? &(MyClass::staticField)

class MyClass[] ** 0×004d81e4

–

0:000:x86> !tm.rset simple!MyClass::staticField

Symbol Offset: 0×004d81e4

–

0:000:x86> dd 0×004d81e4 L1

0×004d81e4 0×00000005

And the file implementing the extension is given below. To build the code refer to Building DbgEng Extensions.

Programming Extension, GetOffsetByName, WinDbg