StackOverflow2013

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copied!<blockquote> <p>Was just thinking that if I can turn somthing into a jmp somehow then I can put my code somewhere else, do whatever I overwrote and jump back to the next instruction after? </p> </blockquote> <p>...this is a good approach for patching code at run-time, yes. But there may be fiddly details to consider depending on exactly what the instructions that are overwritten do. And code injection may be harder than you expect anyway: for example, the code may well be mapped read-only.</p> <blockquote> <p>the entire code for this is very very long since it is very generall being able to call any function with upto 16 arguments plus relevant memory management for strings, and supporting either __stdcall or __cdecl)</p> </blockquote> <p>So: this code calls back into your code, and can pass arguments? Can you register at least one pointer-sized argument along with the function that will be called back?</p> <p>If so, you can pass the <code>this</code> pointer yourself, with a bit of fiddling. You will need to register a function that does <em>not</em> require a <code>this</code> pointer (such as a static member function) as the callback, along with the <code>this</code> pointer as an argument for the callback; that callback can then unwrap the argument in order to call the member function that you really want to call.</p> <p>Something like this (with apologies if it's obvious that I spend most of my life writing C rather than C++):</p> <pre><code>#include <iostream> // Simple C callback interface extern "C" { typedef void (*tCallbackFn)(void *); static struct { tCallbackFn fn; void *context; } callback; static void set_callback(tCallbackFn fn, void *context) { callback.fn = fn; callback.context = context; } static void call_callback(void) { (callback.fn)(callback.context); } } // Simple test class class MyClass { public: // We'll use this for test purposes int id; // This is the actual member function we want to invoke void DoIt(void) { std::cout << "Callback - id = " << id << std::endl; } // Static wrapper which we can pass as the function in the callback interface static void CallbackWrapper(void *context) { static_cast<MyClass *>(context)->DoIt(); } // Register wrapper as callback function, with "this" pointer as context void Test(void) { set_callback(CallbackWrapper, static_cast<void *>(this)); call_callback(); } }; // Test it int main(void) { MyClass m1, m2; m1.id = 1; m2.id = 2; m1.Test(); m2.Test(); return 0; } </code></pre>

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