Understanding and Implementing Microsoft Detours for Function Hooking

Detours is a library developed by Microsoft Research for intercepting and modifying the behavior of arbitrary Win32 functions on various Windows-compatible processors. Unlike static redirection, Detours operates at runtime, making it incredibly versatile for debugging, instrumentation, and extending application functionality without altering the source code or executable files on disk.

Why Use Detours?

• Dynamic Interception: Modify function calls at runtime without altering the binary on disk.
• Flexibility: Works with both 32-bit and 64-bit processes on multiple architectures.
• Granularity: Allows for fine-grained control over function calls, enabling developers to extend or completely replace function behavior.

Basic Concepts

• Target Function: The original function you want to intercept.
• Detour Function: Your custom function that will handle the intercepted call.
• Trampoline: A small piece of code that allows the original function to be called from the detour function if needed.

Setting Up Detours

To use Detours:

1. Include the Detours Library: Ensure detours.h is included in your project, and link against detours.lib.

2. Define Your Detour Function: This function should match the signature of the target function.

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

// Example target function
void (WINAPI * TrueSleep)(DWORD dwMilliseconds) = Sleep;

// Our detour function
void WINAPI MySleep(DWORD dwMilliseconds) {
    printf("Sleeping for %lu milliseconds.\n", dwMilliseconds);
    TrueSleep(dwMilliseconds);
}

3. Attach and Detach the Detour:

int main() {
    // Start the detour transaction
    DetourTransactionBegin();
    DetourUpdateThread(GetCurrentThread());
    
    // Attach our detour to the Sleep function
    DetourAttach(&(PVOID&)TrueSleep, MySleep);
    
    // Commit the transaction
    if (DetourTransactionCommit() == NO_ERROR) {
        printf("Successfully detoured Sleep function.\n");
    } else {
        printf("Failed to detour.\n");
        return 1;
    }

    // Using the detoured function
    Sleep(1000); // This will call MySleep

    // Later, to remove the detour
    DetourTransactionBegin();
    DetourUpdateThread(GetCurrentThread());
    DetourDetach(&(PVOID&)TrueSleep, MySleep);
    DetourTransactionCommit();

    return 0;
}

int main() {
// Start the detour transaction
DetourTransactionBegin();
DetourUpdateThread(GetCurrentThread());

// Attach our detour to the Sleep function
DetourAttach(&(PVOID&)TrueSleep, MySleep);

// Commit the transaction
if (DetourTransactionCommit() == NO_ERROR) {
printf(“Successfully detoured Sleep function.\n”);
} else {
printf(“Failed to detour.\n”);
return 1;
}

// Using the detoured function
Sleep(1000); // This will call MySleep

// Later, to remove the detour
DetourTransactionBegin();
DetourUpdateThread(GetCurrentThread());
DetourDetach(&(PVOID&)TrueSleep, MySleep);
DetourTransactionCommit();

return 0;
}

Technical Details and Considerations

• Calling Conventions: Ensure your detour function uses the same calling convention as the target to avoid stack corruption or incorrect parameter passing.
• Thread Safety: Detours operations should be thread-safe. Use DetourUpdateThread for the current thread or manually manage for other threads.
• Error Handling: Always check the return values from Detour functions to handle potential failures in attaching or detaching.
• System-Wide Hooking: While Detours primarily targets specific processes, system-wide hooks can be implemented using mechanisms like AppInit_DLLs or through a kernel driver, though these methods are more complex and have their own set of limitations and security implications.
• Performance: Since Detours modifies in-memory code, there’s minimal performance impact, but always consider the overhead of your detour function itself.

Advanced Usage

• Using Trampolines: If you need to call the original function from your detour, use the trampoline:

LONG dwSlept = 0;
void WINAPI MySleepEx(DWORD dwMilliseconds) {
    // Do something before calling the original function
    dwSlept += dwMilliseconds;
    TrueSleep(dwMilliseconds); // Call original Sleep via trampoline
    // Post-call logic can go here
}

• Hooking Multiple Functions: You can set up multiple detours within the same transaction for cleaner code management.

Conclusion

Detours provides a powerful way to extend or modify software behavior at runtime, making it invaluable for developers needing to debug, extend, or secure applications without altering their source code. Its ability to work dynamically makes it a go-to solution for real-time function interception on Windows platforms. However, with great power comes responsibility; ensure your use of Detours respects system stability and security.