Mastering Function Interception with Microsoft Detours

Microsoft Detours is a library for intercepting, monitoring, and extending the functionality of binary functions on Windows. This guide dives deep into how you can leverage Detours for advanced software development, with numerous code snippets to illustrate key concepts.

Introduction to Microsoft Detours

Detours allows developers to insert custom code into existing functions without modifying the original source. This technique is invaluable for:

• Debugging: To trace or log function calls.
• Security: To implement runtime security checks.
• Software Extension: Adding features to existing applications.

Setting Up Your Environment

Before diving into any examples, ensure you have:

• Microsoft Visual Studio installed.
• Microsoft Detours library downloaded and linked to your project.

Basic Detour Example

Let’s start with intercepting the printf function:

#include <windows.h>
#include <detours.h>
#include <stdio.h>

// Original printf function
static int (*TruePrintf)(const char *format, ...) = printf;

// Our detour function
int MyPrintf(const char *format, ...) {
    va_list args;
    va_start(args, format);
    int ret = vprintf("[Intercepted] ", args);
    va_end(args);
    return ret + TruePrintf(format, args);
}

int main() {
    DetourTransactionBegin();
    DetourUpdateThread(GetCurrentThread());
    DetourAttach(&(PVOID&)TruePrintf, MyPrintf);
    DetourTransactionCommit();

    printf("Hello, Detours!\n");

    // Cleanup
    DetourTransactionBegin();
    DetourUpdateThread(GetCurrentThread());
    DetourDetach(&(PVOID&)TruePrintf, MyPrintf);
    DetourTransactionCommit();

    return 0;
}

This example shows how to prepend text to every printf output, demonstrating basic function interception.

Here’s how to intercept MessageBoxW:

#include <windows.h>
#include <detours.h>
#include <iostream>

// Original function pointer
static int (WINAPI *TrueMessageBox)(HWND, LPCWSTR, LPCWSTR, UINT) = MessageBoxW;

// Our detour function
int WINAPI HookedMessageBox(HWND hWnd, LPCWSTR lpText, LPCWSTR lpCaption, UINT uType) {
    std::wcout << L"Intercepted: " << lpText << std::endl;
    return TrueMessageBox(hWnd, L"This is intercepted!", lpCaption, uType);
}

int main() {
    DetourTransactionBegin();
    DetourUpdateThread(GetCurrentThread());
    DetourAttach(&(PVOID&)TrueMessageBox, HookedMessageBox);
    DetourTransactionCommit();

    MessageBoxW(NULL, L"Hello, world!", L"Test", MB_OK);

    // Cleanup
    DetourTransactionBegin();
    DetourUpdateThread(GetCurrentThread());
    DetourDetach(&(PVOID&)TrueMessageBox, HookedMessageBox);
    DetourTransactionCommit();

    return 0;
}

Advanced Usage: Monitoring System Calls

Here’s how you might monitor CreateFileW to log file operations:

#include <windows.h>
#include <detours.h>
#include <fstream>

static HANDLE (WINAPI *RealCreateFileW)(LPCWSTR lpFileName, DWORD dwDesiredAccess, DWORD dwShareMode, LPSECURITY_ATTRIBUTES lpSecurityAttributes, DWORD dwCreationDisposition, DWORD dwFlagsAndAttributes, HANDLE hTemplateFile) = CreateFileW;

HANDLE WINAPI DetourCreateFileW(LPCWSTR lpFileName, DWORD dwDesiredAccess, DWORD dwShareMode, LPSECURITY_ATTRIBUTES lpSecurityAttributes, DWORD dwCreationDisposition, DWORD dwFlagsAndAttributes, HANDLE hTemplateFile) {
    std::wofstream logFile(L"C:\\file_operations.log", std::ios::app);
    if (logFile.is_open()) {
        logFile << L"Attempt to create file: " << lpFileName << std::endl;
        logFile.close();
    }
    return RealCreateFileW(lpFileName, dwDesiredAccess, dwShareMode, lpSecurityAttributes, dwCreationDisposition, dwFlagsAndAttributes, hTemplateFile);
}

void SetupFileMonitoring() {
    DetourTransactionBegin();
    DetourUpdateThread(GetCurrentThread());
    DetourAttach(&(PVOID&)RealCreateFileW, DetourCreateFileW);
    DetourTransactionCommit();
}

This code logs all file creation attempts, showcasing Detours’ utility in security applications.

Performance Considerations

When using Detours, performance can be impacted. Here’s how you might measure this:

#include <windows.h>
#include <detours.h>
#include <chrono>

static void (WINAPI *TrueSleep)(DWORD) = Sleep;

void WINAPI MySleep(DWORD dwMilliseconds) {
    auto start = std::chrono::high_resolution_clock::now();
    TrueSleep(dwMilliseconds);
    auto end = std::chrono::high_resolution_clock::now();
    auto duration = std::chrono::duration_cast<std::chrono::microseconds>(end - start);
    std::cout << "Sleep duration: " << duration.count() << " microseconds" << std::endl;
}

void SetupPerformanceMonitor() {
    DetourTransactionBegin();
    DetourUpdateThread(GetCurrentThread());
    DetourAttach(&(PVOID&)TrueSleep, MySleep);
    DetourTransactionCommit();
}

int main() {
    SetupPerformanceMonitor();
    Sleep(1000); // This will now be intercepted by MySleep
    return 0;
}

This example measures how long the Sleep function actually sleeps, which can be useful for profiling.

Conclusion

Microsoft Detours opens up a realm of possibilities for developers to extend, debug, and secure applications at the binary level. Through these examples, we’ve seen how to intercept and modify behaviors, monitor system interactions, and even profile application performance. Remember, while Detours is powerful, it should be used judiciously due to potential performance implications and the complexity it adds to software maintenance.